Amino Acids: Biochemistry and Nutrition [2 ed.] 0367552787, 9780367552787

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Amino Acids

Biochemistry and Nutrition Second Edition

Amino Acids

Biochemistry and Nutrition Second Edition

Guoyao Wu Texas A&M University College Station, Texas, USA

Second edition published 2022 by CRC Press 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487-2742 and by CRC Press 2 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN © 2022 Taylor & Francis Group, LLC First edition published by CRC Press 2013 CRC Press is an imprint of Taylor & Francis Group, LLC Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and p ­ ublishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to ­copyright ­holders if permission to publish in this form has not been obtained. If any copyright material has not been ­acknowledged, please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, ­including ­photocopying, microfilming, and recording, or in any information storage or retrieval system, without written ­permission from the publishers. For permission to photocopy or use material electronically from this work, access www.copyright.com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. For works that are not available on CCC, please contact [email protected] Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Names: Wu, Guoyao, 1962- author. Title: Amino acids : biochemistry and nutrition / Guoyao Wu. Description: 2nd edition. | Boca Raton : CRC Press, 2021. | Includes bibliographical references and index. | Summary: “Following its predecessor, the second edition of Amino Acids: Biochemistry and Nutrition presents exhaustive coverage of amino acids in the nutrition, metabolism and health of humans and other animals. Substantially revised, expanded and updated to reflect scientific advances, this book introduces the basic principles of amino acid biochemistry and nutrition, while highlighting the current knowledge of the field and its future possibilities. The book begins with the basic chemical concepts of amnio acids, peptides and proteins, and their digestion and absorption. Subsequent chapters cover cell-, tissue-, and species-specific synthesis and catabolism of amino acids and related bioactive metabolites, and the use of isotopes to study amino acids metabolism in cells and the body. The book details protein turnover, physiological functions of amino acids, as well as both the regulation and inborn errors of amino acid metabolism. The book concludes with a presentation on human and animal dietary requirements of amino acids and evaluates dietary protein quality”— Provided by publisher. Identifiers: LCCN 2021011176 (print) | LCCN 2021011177 (ebook) | ISBN 9780367552787 (hardback) | ISBN 9781032030890 (paperback) | ISBN 9781003092742 (ebook) Subjects: MESH: Amino Acids Classification: LCC QP551 (print) | LCC QP551 (ebook) | NLM QU 60 | DDC 572/.65—dc23 LC record available at https://lccn.loc.gov/2021011176 LC ebook record available at https://lccn.loc.gov/2021011177 ISBN: 9780367552787 (hbk) ISBN: 9781032030890 (pbk) ISBN: 9781003092742 (ebk) Typeset in Times by codeMantra

Contents Preface.......................................................................................................................................... xxiii Acknowledgments...........................................................................................................................xxv Author...........................................................................................................................................xxvii Chapter 1 Discovery and Chemistry of Amino Acids...................................................................1 1.1 D  efinition and Nomenclature of AAs........................................................................................ 3 1.1.1  Definition of AAs..........................................................................................................3 1.1.2  Definition of Imino Acids.............................................................................................. 5 1.1.3  Isomers of AAs.............................................................................................................. 5 1.1.3.1  L- and D-AAs................................................................................................. 5 1.1.3.2 R- and S-AAs..................................................................................................9 1.1.3.3 cis- and trans-AAs........................................................................................ 11 1.1.4  Proteinogenic and Non-Proteinogenic AAs................................................................ 12 1.1.5  Free AAs and Peptide (or Protein)-Bound AAs.......................................................... 12 1.2  Discovery of AAs.................................................................................................................... 16 1.2.1  L-AAs and Glycine...................................................................................................... 16 1.2.2 β- and γ-AAs with Physiological Significance............................................................ 27 1.2.3  D-AAs..........................................................................................................................28 1.2.3.1 Presence of D-AAs in Plant- and Animal-Sourced Foods...........................28 1.2.3.2  Presence of D-AAs in the Animal Kingdom................................................ 29 1.2.3.3  Presence of D-AAs in Microbes................................................................... 30 1.2.4  Other AAs.................................................................................................................... 31 1.2.4.1  Other AAs in the Animal Kingdom............................................................. 31 1.2.4.2  Other AAs in Spoiled Animal Products....................................................... 33 1.2.4.3  Other AAs in Plants...................................................................................... 33 1.2.4.4  Other AAs in Microbes................................................................................34 1.2.4.5  Other AAs in Processed Foods..................................................................... 35 1.3  Chemical Properties of AAs.................................................................................................... 35 1.3.1  Physical Appearance, Fluorescence, and Melting Points of Crystalline AAs............ 35 1.3.2  Tastes of Crystalline AAs............................................................................................ 35 1.3.3  Solubility of AAs in Water and Organic Solvents....................................................... 37 1.3.4  The Zwitterionic Form of AAs.................................................................................... 38 1.3.5  Chemical Stability of AAs.......................................................................................... 42 1.3.5.1  Stability of Crystalline AAs......................................................................... 42 1.3.5.2  Stability of Free AAs in Water and Buffered Solutions at 27.5% crude protein); PP, potato protein; SPC, soy protein concentrate; SWP, sweet potato; WF, wheat flour; WPC, whey protein concentrate. a Cooked. The true digestibility of protein was calculated as the average value for its constituent amino acids.

Barley

Amino Acid

TABLE 2.7 True Ileal Digestibilities (%) of Dietary Amino Acids and Nitrogen in Healthy Adult Humans 97 99 98 99 98 98 89 99 99 97 99 99 95 93 93 --99 98 97

WPC

Protein Digestion and Absorption 85

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Amino Acids

In the ileal digesta, the content of an AA consists primarily of three fractions: the undigested dietary AA, the basal EIAA (EIAAb), and the diet-specific EIAA (EIAAs). The basal ileal losses of AAs are influenced by the physiological state of animals but not by feed ingredient composition. By contrast, diet-specific ileal losses of AAs are induced by feed ingredient composition (e.g., the type and amount of protein, lipids, fiber, and anti-nutritional factors) and by interactions between the diet and the small intestine (intestinal secretion, integrity, local immune responses, and health). EIAAb can be measured by using (1) a cornstarch-based nitrogen-free diet; (2) the regression method (EIAAb is obtained when the intake of dietary protein in animals fed graded levels of protein is extrapolated to 0.0); or (3) the peptide alimentation technique (animals are fed a diet containing enzymehydrolyzed casein). If a nitrogen-free diet is used, EIAAb is usually estimated by determining the AA in ileal digesta and using an indigestible marker (e.g., 0.3% chromium oxide, Cr2O3) in the diet. EIAA b (g/kg of DM intake) = AA in ileal digesta (g/kg DM) × (Marker in diet/Marker in digesta) There are currently no reliable routine methods for directly determining EIAAs in animals. However, EIAAs may be estimated by subtracting EIAAb from total EIAA (EIAAt). In this case, EIAAt can be estimated by using the homoarginine (hArg; Yin et al. 2015) or isotope tracer dilution technique (animals receive either a diet containing [15N]AA-labeled protein or intravenous infusion  of an [15N]AA (e.g., [15N]leucine; de Lange et al. 1990). The hArg method involves (1) the generation of hArg from lysine (nearly all peptide-bound lysine) in diets through the guanidination reaction of its ε-amino group with o-methylisourea (e.g., 0.3–0.6 mM) at 95°C under alkaline conditions; (2)  ­consumption of the modified diet (containing hArg residues) by test animals (e.g., pigs); and (3) the analysis of hArg in the ileal digesta of the test animals.

Guanidination H2N–CH2–CH2–CH2–CH2–CH–COOH HN–CH2–CH2–CH2–CH2–CH–COOH | | | NH2 NH2 H2N – C=NH H2N – C=NH L-Lysine L-Homoarginine O – CH3 O-Methylisourea

The hArg method for the determination of endogenous AA flows in animals is based on the following principles: (1) a small but representative proportion of lysine in the diet can be converted into hArg in a guanidination reaction with methylisourea; (2) hArg is neither formed nor degraded in mammalian cells or microorganisms; (3) hArg is not a substrate for protein synthesis; (4) after digestion and absorption, hArg does not return to the small intestine; and (5) diet-derived hArg can be differentiated from endogenous AAs. The validity of these assumptions should be re-evaluated in view of the recent finding that hArg is synthesized from arginine and lysine in the whole bodies of animals (e.g., pigs and rats; Hou et al. 2016a), including their liver, kidneys, brain, and small intestine (Hou et al. 2015; Wu et al. 2016a). In addition, because leucine is catabolized by the small intestine (Chen et al. 2009), the appearance of intravenously administered 15N-leucine in the lumen of the small intestine (e.g., distal ileum) may not accurately reflect the endogenous flow of leucine in the small intestine. This view is supported by the finding that the current 15N-leucine infusion technique is not suitable for quantitative measurements of ileal endogenous AA flows in growing pigs (Leterme et al. 1998). True ileal digestibility of lysine (%) = (hArg in diet – hArg in ileal digesta) hArg in diet  × 100. Total EIAA for lysine (EIAATLys) is estimated as the total flow of lysine in distal ileal digesta minus the flow of undigested dietary lysine.

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Protein Digestion and Absorption



EIAA TLys = Total Lys in ileal digesta − [ Lys in diet × (1 − True ileal digestibility of Lys (%) ]

EIAAt for the other AAs is calculated on the basis of EIAATLys and the reported ratios of other AAs to Lys in endogenous secretions (mainly proteins), which are relatively constant in swine.

EIAA t for the other AAs = EIAA TLys × AA Literature LysLiterature in endogenous proteins

For growing pigs, EIAAb values obtained with feeding a nitrogen-free diet or the regression method are similar (approximately 11 g crude protein/kg of DM intake) but 35% lower than those (17 g crude protein/kg of DM intake) obtained with the peptide alimentation technique. It appears that feeding either certain peptides or protein to humans and other animals can stimulate the flow of endogenous AAs into the lumen of the ileum (Moughan et al. 1992, 2007; Starck et al 2018), likely due to increases in (1) the secretion of pancreatic juice into the lumen of the small intestine; (2) the release of proteins, peptides, AAs, urea, and ammonia from enterocytes into the lumen of the small intestine; and (3) the synthesis of proteins, peptides, or AAs by microorganisms in the lumen of the small intestine. For consistency, feeding a nitrogen-free diet may be preferred over the other two methods to estimate EIAAb. Normally, EIAAb is relatively constant but EIAAs is markedly affected by the composition of dietary ingredients. For example, EIAAs is minimal in growing pigs fed a diet containing highly digestible protein (e.g., casein or egg protein) but can account for >50% of EIAAt in response to a diet containing a high proportion of fiber or anti-nutritional factors (Stein et al. 2007). As noted previously, the amount of nitrogen recovered at the distal ileum is influenced by many dynamic factors, including food matrix, protein digestibility, the absorption of peptides and AAs, secretions, and cell turnover. The ileal endogenous nitrogen consists of primarily protein and large polypeptide nitrogen, such as 60%−80% in pigs fed nitrogen-free diets (Moughan and Schuttert 1991) and rats fed nitrogen-free or low-protein diets (Butts et al. 1992; Moughan and Schuttert 1991), as well as 79% in humans fed a rapeseed protein isolate (Bos et al., 2007). Most (80%–90%) of the endogenous nitrogen that flows into the small intestine has been absorbed in the form of di- and tripeptides, as well as free AAs, urea, ammonia, and related metabolites by the time the digesta reaches the terminal ileum in pigs, rats, and humans (Moughan 2012). Besides the small intestine, there is also a substantial amount of endogenous nitrogen in the large intestine of humans and other animals (Starck et al. 2018), and typical ­values are presented in Table 2.6. Recently, a concept of standardized ileal AA digestibility has been proposed to provide for some consistency in digestibility values. Previously determined values of the basal ileal endogenous flows of AAs are used to calculate the standardized ileal digestibilities of dietary AAs in animals. Accurate measurements of the basal ileal endogenous losses of AAs (including proper animal and AA analysis procedures) are crucial for the establishment of a reliable database of standardized ileal AA digestibilities for food ingredients. Values of standardized ileal AA digestibility are usually intermediate between apparent and true ileal AA digestibilities. As for true ileal AA digestibility, the proportional quantities of standardized ileal AA digestibility from each feed ingredient in a diet are generally additive (Fan et al. 1994; Moughan 2003). It has been recommended that standardized ileal AA digestibility values be used for feed formulation until more data on the true ileal AA digestibilities of feedstuffs are available in the literature (Stein et al. 2007). Note that the standardized ileal AA digestibility differs from the true ileal AA digestibility in that the former refers to the apparent ileal digestibility corrected for the basal ileal endogenous AA losses that have been reported for a given species. Standardized ileal AA digestibility (%)

=  AA intake – ( AA in ileal digesta − EIAA b )  AA intake × 100



88

Amino Acids

Measurements of fecal or ileal nitrogen and AA digestibility, as well as apparent and true nitrogen and AA digestibility, have very different nutritional significance, and therefore, caution should be exercised in data interpretation and use (Fuller and Tomé 2005). Measurements at the distal ileal can reflect losses of AAs from both dietary and endogenous origin, whereas measurements of AAs in feces can be useful for assessing whole-body nitrogen losses. In humans and other animals, there is active recycling of nitrogen (including AAs) between the gut (both the small and large intestines) and the systemic blood circulation. Because the measurements of endogenous AA flows and true digestibility require invasive surgical techniques, it is a challenge to directly assess either true ileal AA digestibility or AA bioavailability in humans. This difficulty can be overcome by using animal models, such as rats (FAO 1991, 2007, 2013) and swine (FAO 2013; Starck et al. 2018).

2.3.5 Absorption of Free AAs and Small Peptides by the Small Intestine 2.3.5.1  Transport of Free AAs via Transmembrane Transporters The jejunum is the major site for the absorption of small peptides and free AAs, followed by the ileum and duodenum (Wu 2018). Major AA transporter systems and their proteins (many of which are not expressed by intestinal epithelia) are summarized in Table 2.8. In essence, free AAs in the lumen of the small intestine are taken up by enterocytes via several mechanisms: (1) simple diffusion (passive, non-saturable); (2) Na+-independent systems (facilitated diffusion); and (3) Na+-dependent systems (active transport) (Malandro and Kilberg 1996). Some transport proteins can use lithium to replace sodium, and a few of the transport proteins are H+-driven. The Na+-dependent AA transport requires energy (1 mol ATP/mol AA), as Na+ that enters the cell is pumped out of the cell by Na-K ATPase in exchange for the entry of K+ to maintain intracellular ion homeostasis. In animal cells, ATP binds Mg2+ to form the biologically active Mg-ATP complex for enzyme-catalyzed ATP-dependent reactions. AA transporters are highly expressed in the apical membranes of the enterocytes of humans and other animals, including sheep and cattle (Blachier et al. 2009; Howell et al. 2001). Based on sequence similarity, AA transporters are grouped into solute carrier (SLC) families. Eleven of them have been identified to date for animal cells (Closs et al. 2006; Kandasamy et al. 2018; Kantipudi et al. 2020). Interestingly, a lysosomal cystine transporter (cystinosin) has now been identified (SLC 66A4; NCBI 2020). Traditionally, AA transport systems have been classified according to substrate preference and Na+ dependence. Uptake of ~60% of free AAs from the lumen of the small intestine into enterocytes is performed by Na+-dependent AA transport systems (Ganapathy et al. 2006). 3 Na+ (from inside the cell to outside the cell) + ATP4− + H2O ↔ 2 K+ (from outside the cell to inside the cell) + ADP3− + HPO42− + H+ (Na-K ATPase) As a result of molecular cloning, contemporary classifications are usually based on gene family and Na+ or H+ dependence. Note that (1) more than one transporter can transport an AA; (2) Systems A and IMINO can transport aminoisobutyrate (AIB) and N-methylaminoisobutyrate (MeAIB), but Systems N, ASC, and NBB do not transport N-methylated substrates; (3) System N can use lithium (Li+) to substitute Na+; (4) Systems A and N are sensitive to inhibition by a low extracellular pH; and (5) System ASC is relatively insensitive to low pH. In recent years, some AA transporters, including System L transporters (LAT1 and LAT2), System N transporter (SNAT3), and a bidirectional Gln transporter (SLC1A5; in epithelial cells), may serve as transceptors, which are capable of sensing and signaling AA availability to a regulatory machinery [e.g., the mechanistic target of rapamycin (MTOR) pathway)] in cells. 2.3.5.2  Transport of AAs via the γ-Glutamyl Cycle In 1973, A. Meister proposed that the γ-glutamyl cycle could function as a mechanism for transport of AAs across biological membranes (Figure 2.7). Note that the γ-carboxyl group of glutamate forms a peptide bond with the amino group of cysteine. This cycle has the following biochemical features: (1) the requirement of 3 mol ATP for the transport of 1 mol AA; (2) a 1:1 stoichiometry between GSH turnover and AA transport; (3) dependence of a membrane-bound enzyme, instead of a transmembrane protein channel, for AA transport; (4) no requirement of Na+ for cotransport of AAs;

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Protein Digestion and Absorption

TABLE 2.8 Transporters of Amino Acids and Related Substances in Animal Cells Gene

Protein

Substrate(s)

System (Comment)

(1) Na -Independent Systems for AA Transport Basic L-AAs (Arg, His, Lys, Orn, y+ (uniporter; both AM and BM; UB) HA, ADMA) Basic L-AAs (Arg, His, Lys, Orn, y+ (uniporter; liver, pancreatic islets) HA, ADMA, SDMA) Basic L-AAs (Arg, His, Lys, Orn) y+ (uniporter; brain, thymus, ovary, testes) Large neutral L-AAs (BCAAs, Gln, L1 (transport large neutral L- and D-AAs; Met, Phe, Tyr, Trp); large neutral localized to the BM of intestine and kidneys; D-AAs participates in the efflux of large neutral L- and D-AAs); occurs in tissues Basic L-AAs (Arg, Lys, His, Orn); y+L (Na+-dependent for some neutral AAs); BCAAs, Gln, Met an arginine/glutamine exchanger Basic L-AAs (Arg, Lys, His, Orn); y+L (lysinuria, if defected; see above); occurs some neutral L-AAs (Ala, Cys, mainly in the BM of epithelial cells in the Gln, Leu, Met); Na+-dependent for small intestine and kidney tubules some neutral AAs; mediates ADMA efflux from cells All neutral L-AA (except proline); L2 (transport large neutral L; mainly localized Gly to the BM of intestine and kidneys); does not transport D-Ser; also occurs in many tissues L-Arg, L-Lys, L-Orn, L-Cystine; b0,+ (brain, kidney, small intestine; NIENB); exchanges extracellular basic AAs on the AM of intestine and kidneys; does and cystine with intracellular not transport D-Ser neutral AAs Gly, small L-AAs (Ala, Ser, Cys, asc1 (localized to the BM of intestine and Thr); small D-AAs (e.g., Ala, Ser, kidneys; participates in the efflux of Gly, as Cys, Thr) well as small L- and D-AAs) L-Asp, L-Glu, and L-cystine x−c; extracellular cystine/intracellular Glu; exchanger extracellular cystine/intracellular Asp Gly, L-Ala, L-Ser, L-Cys, L-Thr Asc Aromatic L-AAs (Trp, Tyr, Phe); T; the BA of intestine, kidney, and placenta L-DOPA Pro, Gly, Ala, GABA, MeAIB, Imino (proton AAT); the AM of intestine, β-Ala, Tau, Hyp kidney, lung, and liver; other tissues Proton AAT; the AM of kidney and lung Pro, Gly, Ala, GABA, MeAIB, β-Ala Large neutral L-AAs (BCAAs, Met, L Phe) Large neutral L-AAs (BCAAs, Met, L Phe) +

SLC7A1

CAT-1

SLC7A2

CAT-2 (2A and 2B) CAT-3 LAT1/4F2hca

SLC7A3 SLC7A5/ SLC3A2

SLC7A6/ SLC3A2 SLC7A7/ SLC3A2

y+LAT2/4F2hcb

SLC7A8/ SLC3A2

LAT2/4F2hcc

SLC7A9/ SLC3A1

b0,+AT/rBAT

SLC7A10/ SLC3A2

asc-AT1/4F2hc

SLC7A11/ SLC3A2 SLC7A12 SLC16A10

xCT/4F2hcc

SLC36A1

PAT1/LYAAT1

SLC36A2 SLC43A1

PAT2/LYAAT2 LAT3

SLC43A2

LAT4

y+LAT1/4F2hcb

asc-AT2 TAT1

SLC1A1

EAAT3

SLC1A2

EAAT2

(2) Na+-Dependent Systems for AA Transport D-Asp, L-Asp, L-Glu, L-Cys, X−AG (the AM of intestine, kidney, liver) L-cystine D-Asp, L-Asp, L-Glu, L-Cys, X−AG (the AM of liver) L-cystine (Continued)

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Amino Acids

TABLE 2.8 (Continued) Transporters of Amino Acids and Related Substances in Animal Cells Gene

Protein

Substrate(s)

SLC1A3

EAAT1

D-Asp, L-Asp, L-Glu

SLC1A4

ASCT1

L-Ala, L-Ser, L-Cys, L-Thr; D-Ser

SLC1A5

ASCT2 (ATB0)

L-AAs (Ala, Ser, Cys, Thr, Gln); D-Ser

SLC1A6 SLC1A7 SLC3A1 SLC3A2 SLC6A1 SLC6A5 SLC6A6

EAAT4 EAAT5 rBAT 4F2hc GAT1 GLYT2 TAUT

SLC6A7 SLC6A9 SLC6A11 SLC6A12 SLC6A13 SLC6A14

PROT GLYT1 GAT3 BGT1 GAT2 ATB0,+

D-Asp, L-Asp, L-Glu D-Asp, L-Asp, L-Glu Trafficking subunit for HATs Trafficking subunit for HATs GABA Gly, sarcosine Taurine, GABA, β-Ala Pro (in central nervous system) Gly, sarcosine GABA, betaine, taurine GABA, betaine

SLC6A15 SLC6A17

B0AT2 NTT4/B0AT3

SLC6A18 SLC6A19

XT2/B0AT3 B0AT1

SLC6A20

IMINO

SLC7A13 SLC17A6 SLC17A7 SLC17A8 SLC25A2

AGT1 VGLUT2 VGLUT1 VGLUT3 ORC2 (ORNT2)

SLC25A12

AGC1

GABA, betaine, Pro, β-Ala, Tau All neutral and cationic L-AAs, β-Ala; small and neutral D-AAs (e.g., D-Ala, D-Ser, D-Met, D-Leu, D-Trp)d L-Pro, L-BCAAs, L-Met L-AAs (Leu, Met, Pro, Cys, Ala, Gln, Ser, His), Gly; possibly other neutral AAs Gly and L-Ala All neutral L-AAs (including Gln, Trp, The, Phe, Tyr Met, BCAAs; brush border of small intestine) Pro, OH-Pro; L-Cys, L-Ala, L-Leu, L-Met, L-Phe, Gly; sarcosine, pipecolate L-Asp, L-Glu L-Glu L-Glu L-Glu Basic L-AAs (Arg, Lys, His, Orn), and Cit; an exchanger of Orn/Cit L-Asp, L-Glu

System (Comment) X−AG (glia, neurons, retina, fibroblasts, myocytes; inner mitochondrial membrane in tissues as part of the malate shuttle) ASC (the AM of stomach, intestine, kidney, lung, and cornea); brain ASC (the AM of intestine, kidney, lung, and other epithelial tissues); Na+- and Cl−coupled transporter X−AG X−AG (the AM of retina) HC-HAAT (cystinuria, if defected) HC-HAAT BETA (β); also requires Cl− Gly BETA (β), TauT; also requires Cl− Proline transporter Gly BETA (β), TauT; also requires Cl− BETA (β); also requires Cl− BETA (β); also requires Cl− B0,+ (Na+- and Cl−-coupled transporter); localized to the AM of intestine and possibly kidneys; participates in the absorption of some L- and D-AAs B0 B0 (brain, heart, skeletal muscle)

Gly (the AM of kidney); also requires Cl− NBB (B0); small intestine, kidney, and placenta; Hartnup disease, if defected IMINO; the AM of intestine, kidney, stomach, and choroid plexus; other tissues Asp, Glu transporter VGT VGT VGT Orn/Cit carrier; Basic AA carrier in the mitochondria; also transport carnitine Asp/Glu carrier; expressed in the skeletal muscle, heart, kidneys, and brain, but not in the liver (Continued)

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Protein Digestion and Absorption

TABLE 2.8 (Continued) Transporters of Amino Acids and Related Substances in Animal Cells Gene

Protein

Substrate(s)

SLC25A13

AGC2 (Citrin)

L-Asp and L-Glu (mitochondria)

SLC25A15

ORC1 (ORNT1)

SLC25A18 SLC25A22 SLC25A29 SLC32A1 SLC36A4 SLC38A1

SLC38A5

GC2 GC1 ORNT3 VIAAT PAT4 SNAT1 (SAT1; ATA1; GlnT) SNAT2 (SAT2; ATA2) SNAT3 (SN1) SNAT4 (SAT3; ATA3) SNAT5 (SN2)

SLC66A4 NA

Cystinosin Unknown

Basic L-AAs (Arg, Lys, His, Orn), and Cit (mitochondria); an exchanger of Orn/Cit L-Glu L-Glu Basic AAs (Arg, Lys, Orn, His, HA) Gly, GABA L-Pro, L-Trp L-AAs (Gln, Ala, Asn, Cys, His, Met, Ser), Gly MeAIB, D-Ser Gly, L-AAs (Pro, Ala, Ser, Cys, Gln, Asn, His, Met), MeAIB, D-Ser L-Gln, L-Asn, L-Cit, L-His, L-Ala L-AAs (Ala, Ser, Cys, Gln, Asn, Met), Gly, MeAIB; no D-Ser L-Gln, L-Asn, L-Cit, L-His, L-Ser, L-Ala, Gly L-Cystine L-Phe and L-Met

SLC6A4

SERT

SLC6A8 SLC22A5

CRT OCTN2

SLC38A2 SLC38A3 SLC38A4

SLC15A1

PEPT1

SLC15A2

PEPT2

SLC22A1

OCT1

SLC22A2

OCT2

SLC22A3

OCT3

System (Comment) Asp/Glu carrier; an exchanger of the mitochondrial Asp for the cytosolic Glu; broadly distributed in tissues, but mainly in the liver, kidney, heart, and intestine Orn/Cit carrier; an exchanger of the cytosolic Orn for the mitochondrial Cit Glu carrier Glu carrier Basic AA carrier in the mitochondria VGGT Amino acid sensor A (both AM and BM) A (both AM and BM) N (Na+- and H+-coupled transporter) A; both AM and BM of the intestine; other tissues (liver and skeletal muscle) N (Na+- and H+-coupled transporter) LCT Phe; the AM of small intestine

(3) Na+-Dependent Transport of Substances Related to AAs Serotonin, carnitine, and organic 5-HTT; also requires Clcations Creatine Creatine; widespread in tissues; also requires ClL-Carnitine, acetyl-L-carnitine, High-affinity; AM; widespread in tissues; organic cations (e.g., intestinal Na+-independent for the transport of OC absorption of dietary substances) (4) Na+-Independent Transport of Substances Related to AAs the AM of intestine and renal tubules Dipeptides and tripeptides (H+-dependent) Dipeptides and tripeptides the AM of renal tubules; extraintestinal tissues (H+-dependent) Organic cations (e.g., agmatine, Mainly in the BM of liver; also in other serotonin, histamine, dopamine, tissues norepinephrine, PA) Organic cations (e.g., agmatine, Mainly in the BM of renal tubules; also in serotonin, histamine, dopamine, placenta, lung, brain, and intestine norepinephrine, PA) Organic cations (e.g., agmatine, Mainly in the BM of renal tubules; also serotonin, histamine, dopamine, broadly in tissues norepinephrine, PA) (Continued)

92

Amino Acids

TABLE 2.8 (Continued) Transporters of Amino Acids and Related Substances in Animal Cells Gene SLC22A4

Protein OCTN1

Substrate(s) Ergothioneine (an antioxidant from fungi and mycobacteria; not synthesized by animal cells)

System (Comment) the AM of intestine; neurons; requires H+; does not transport carnitine

Source: Banjarnahor, S. et al. 2020. J. Clin. Med. 9:3975; Bröer, S. and M. Palacín. 2011. Biochem. J. 436:193–211; Hatanaka, T. et al. 2002. Biochem. Biophys. Res. Commun. 291:291–295; Hyde, R. et al. 2003. Biochem. J. 373:1– 18; Kandasamy, P. et al. 2018. Trends Biochem. Sci. 43:752–789; Kantipudi, S. et al. 2020. Int. J. Mol. Sci. 21:7573; and Pochini, L. et al. 2014. Front. Chem. Cell. Biochem. 2:61. Note: AAs, amino acids; ADMA, asymmetric dimethylarginine; AM, apical membrane of the small intestine; BCAAs, branched-chain amino acids (leucine, isoleucine, and valine); BM, basolateral membrane of the small intestine; Cit, citrulline; EAAT1, excitatory amino acid transporter 1; 4F2hc, the heavy chain of the 4F2 cell surface antigen; GABA, γ-aminobutyric acid; HA, homoarginine; HAT, heteromeric AA transporters [e.g., the b0,+AT1/rBAT consists of b0,+AT1 (the light-chain with transport activity) and rBAT (heavy-chain mediating trafficking to the plasma membrane) linked by the disulfide bond]; HC-HAAT, the heavy chain of heteromeric AAT; 5-HTT, 5-­hydroxytryptamine transporter; LAT1, L-type/large neutral amino acid transporter 1; LAT2, L-type/large neutral amino acid transporter 2; LCT, lysosomal cystine transporter; MeAIB, 2-methylaminoisobutyric acid; NA, not assigned; NBB, neutral brush border; NIENB, Na+-independent exchanger of neutral/basic amino acids; OC, organic cations; OCT, organic cation transporter (polyspecific); Orn, ornithine; PAT, proton-coupled amino acid transporter; PA, p­ olyamines (putrescne, spermidine, and spermine); rBAT, related to b0,+ amino acid transporters; SDMA, s­ ymmetric dimethylarginine; TauT, taurine transporter; UB, ubiquitous; VGGT, vesicular Gly/GABA transporter; VGT, ­vesicular Glu transporter. a LAT1 locates in the cytosol in the absence of 4F2hc and, upon combination with 4F2hc, translocates together to the plasma membrane. This transporter occurs in many tissues, e.g., the small intestine, kidneys, brain, mammary gland, ovary, testis, placenta, spleen, colon, blood–brain barrier, fetal liver, lymphocytes, and tumors. This transporter does not transport basic or acidic amino acids. b This transporter transports basic and neutral amino acids in an Na+-independent and Na+-dependent manner, respectively. c LAT2 locates in the cytosol in the absence of 4F2hc and, upon combination with 4F2hc, translocate together to the plasma membrane. This transporter localizes mainly in the basolateral membrane of the small intestine and kidney tubules, but also occurs in many other tissues, including the lung, heart, spleen, liver, brain, placenta, prostate, ovary, fetal liver, testis, and skeletal muscle. d This transporter does not transport some D-AAs (e.g., Arg, Asn, Ile, Lys, and Val).

and (5) decrease in AA transport due to an inhibition of one of the enzymes in the γ-glutamyl cycle. On a theoretical basis, this γ-glutamyl cycle does not seem to be efficient for AA transport for the following reasons. First, the turnover of GSH has a high requirement for ATP. Note that only 1 mol ATP is required for the uptake of 1 mol of AA by a Na+-dependent transporter. Second, γ-glutamyl transpeptidase has a poor affinity toward proline, and thus, the γ-glutamyl cycle plays only a minor role in proline transport by animal cells. It has been shown that the γ-glutamyl cycle is not a major mechanism for AA transport in mammalian cells, such as mammary epithelial, placental, and intestinal cells (Vina et al. 1989), but may contribute to the transport of some AAs by the endothelial cells of the blood–brain barrier (Hawkins and Vina 2016). These authors also suggest that the oxoproline formed is a potent activator of a number of AA transporters, including ASC and the EAATs. 2.3.5.3  Transport of Small Peptides by the Small Intestine As noted previously, Newey and Smyth (1959) suggested that di- and tripeptides were transported by the small intestine across the apical membrane of the enterocyte into the cell. This concept had been largely established by 1975 based on both in vitro and vivo studies involving humans and other animals (Navab and Asatoor 1970; Silk et al. 1975a, b). About two decades later, the peptide transporter 1 (PepT1) gene (SLC15A1) was first cloned from rabbit tissues in 1994, providing ­definitive proof for the presence of peptide transport capacity by the small intestine

93

Protein Digestion and Absorption R CH COO

-

+

NH3 Amino acid (outside the cell) γ−Glutamyl

Cell membrane

Transpeptidase CH 2 SH

R C O NH CHCOO (CH 2 ) 2 HC

+ NH 3 -

C O NH CH CO NH CH 2 COO

CH2SH + H3N CH CO NH CH2 COO

-

(CH 2 ) 2 HC

Cysteinyl-glycine

+ NH 3 -

COO

γ-Glutamyl-cysteinyl-glycine (glutathione) ADP + Pi Glutathione synthetase

γ−Glutamyl-amino acid

ATP

Peptidase

CH2SH

COO

+

H3N CH2 COO

γ−Glutamyl cyclotransferase

COO

NH3

+

+

H COO

5-Oxoproline

CH2SH

H3N CHCOO

Amino acid (inside the cell)

N H2

(CH 2) 2

-

ATP

-

(CH 2) 2

+

HC NH3 COO

ADP + Pi

γ-Glutamylcysteine

γ-Glutamyl-cysteine synthetase COO

ATP

-

-

ADP + Pi

Cysteine

5-Oxoprolinase

+

HC NH3

-

+

O

CO-NHCHCOO

-

Glycine

R CH COO

-

-

Glutamic acid

FIGURE 2.7  A proposed role for the γ-glutamyl cycle in the transport of amino acids (AAs) by animal cells. γ-Glutamyl transpeptidase is required for AA transport across the cell membrane. In general, 3 molecules of ATP would be required for the uptake of 1 molecule of AA. Experimental evidence shows that the γ-glutamyl cycle plays a minor role in AA transport by animal cells. (Adapted from Meister, A. 1973. Science 180:33–39.)

(Fei et al. 1994), followed immediately by the cloning of the human intestinal PepT1 (Liang et al. 1995). Subsequently, J.C. Matthews et al. (1996) demonstrated the expression of PepT1 in ruminant forestomach tissues by overexpressing size-fractionated mRNA isolated from the forestomach tissues of sheep that had classic PepT1 activity in Xenopus laevis oocytes. Five years later, PepT1 mRNA was isolated from sheep small intestine and cloned by Pan et al. (2001). To date, PepT1 has been cloned in many other animal species, including the Atlantic cod, cattle, chicken, dog, human, monkey, mouse, pig, rat, sheep, turkey, and zebrafish (Gilbert et al. 2008; Spanier 2014; Wang et al. 2017). In the small intestine, this peptide transporter is expressed primarily in the villus tip. PepT1 is a high-capacity, low-affinity peptide transporter with 12 transmembrane domains (Adibi 1997). It is encoded by the SLC15A1 gene and has broad specificity for dipeptides and tripeptides. Dipeptides or tripeptides in the lumen of the small intestine can be directly transported into the enterocytes (i.e., the absorptive epithelial cells) through their apical membrane by H+ gradient-driven (Na+-independent) PepT1 (Daniel 2004). Neither free AAs nor peptides containing four or more AA residues are accepted as substrates for PepT1. The discovery of PepT1 in the small intestine helps to explain the findings that (1) the perfused human jejunum exhibited a kinetic absorptive advantage of di- and tripeptides over a mixture of the equivalent free AAs (Silk et al. 1975a); and (2) human patients with cystinuria (Silk et al. 1975b) and Hartnup disease (Navab and Asatoor 1970), which are characterized by genetic defects in the intestinal transport of certain free AAs, could obtain

94

Amino Acids

adequate AAs that were enterally provided as dipeptides. Clearly, hydrolysis of p­ eptides on the mucosal brush border is not required for their uptake by the small intestine. Compelling evidence indicates that the small intestine of humans (Adibi 1997) and other animals [including fish (Dabrowski et al. 2008)] transports small peptides (2–3 AA residues) at a faster rate than free AAs. When jejunal enterocytes (~5 mg cell protein) from 7-day-old pigs were incubated at 37°C in oxygenated (95% O2/5% CO2) Krebs bicarbonate buffer (pH 7.4) containing 5 mM alanyl-glutamine, about 60% of the dipeptide disappeared from the medium in 5 min (Haynes et al. 2009). The transport of di- and tripeptides or free AAs from the lumen into the enterocytes is associated with an influx of both Na+ and water. Once inside enterocytes, the small peptides are hydrolyzed rapidly by intracellular peptidases to form free AAs, which are utilized in multiple pathways (Wu 1998). A small proportion of these peptides (e.g., di- and tripeptides containing proline or hydroxyproline) may exit the enterocytes via their basolateral membrane into the bloodstream (Osawa et al. 2018). Available evidence supports the view that a peptide transporter other than PepT1 is expressed in the basolateral membrane of the enterocytes for the movement of small peptides from inside the cell into the portal circulation. However, the identity of basolateral peptide transporters remains elusive. Besides the small intestine, other tissues also contain transporters for small peptides so that diand tripeptides in the plasma can be rapidly utilized in the body. Specifically, PepT1 is also present in the proximal kidney tubule, and PepT2 (encoded by the SLC15A2 gene) is widely expressed in extraintestinal tissues (including kidney tubules). In the kidneys, PepT2 has much higher affinity for small peptides than PePT1 and is also the predominant peptide transporter. The wide distribution of PepT2 in tissues explains why most of the intravenously administered di- and tripeptides disappear within a few minutes from the blood in humans and other animals (Furst et al. 1990). The transport of small peptides offers distinct advantages over transport of free AAs. First, some free AAs are not highly stable (e.g., glutamine and cysteine) or have low solubility (e.g., tyrosine, tryptophan, and cysteine) in aqueous solutions. These shortcomings can be overcome by the delivery of small peptides (e.g., Ala-Gln, Gly-Gln, and Glu-Cys-Gly) so as to increase the availability of the constituent AAs to the body. Second, for equal molar concentrations of AAs, the use of dipeptides and tripeptides can reduce the osmolarity of free AAs by 50% and 67%, respectively. Third, dipeptides and tripeptides are absorbed faster and more efficiently by the intestine than free AAs. This can reduce catabolism of peptides by microorganisms in the lumen of the gastrointestinal tract and improve the balance of AA supply to the portal circulation. Thus, compared with intact proteins or a mixture of free AAs, addition of small peptides or hydrolyzed proteins to diets can offer a greater nutritional value to enhance protein retention, growth and development, and recovery from malnutrition and illness in humans and other terrestrial animals (Adibi 1997; Boza et al. 2000; Daenzer et al.; 2001; Dangin et al. 2001; Trocki et al. 1986). Likewise, free AA-based diets are generally inferior to protein- or peptide-based diets for the growth of fish (Dabrowski et al. 2010). However, because of their high costs, the use of small peptides as the sole source of dietary AAs is not sustainable practically in human or animal nutrition. Under some conditions, such as those encountered during the immediate post-enterectomy period, feeding a mixture of free AAs may be advantageous to intact protein for the nutritional support of individuals (Iglesias et al. 1994). In pigs, poultry, and fish, dietary supplementation with certain free AAs to achieve a balanced provision of AAs can improve their growth performance, feed efficiency, and health (Wu 2018).

2.3.6 Net Balance of AAs across the Small Intestine in Monogastric Animals In humans and other animals, absorbed AAs have different metabolic fates in enterocytes (Wang et al. 2009; Wu 1998). Note that some AAs, such as chemically modified arginine and lysine that may be produced during the processing or cooking of food, may be absorbed as chemical complexes that are not utilized or metabolized by animals (Moughan 2003). It had been a long-standing belief that all dietary AAs entered the portal vein intact. However, this concept has recently been

95

Protein Digestion and Absorption

TABLE 2.9 Percentages of Dietary Amino Acids (AAs) Entering the Portal Venous Blood in Young Swine (6–10 kg), Gestating Swine, and Adult Humans AANA Cysteine Histidine Isoleucine Leucine Lysine Methionine Phenylalanine Threonine Tryptophan Tyrosine Valine

Young Swinea

Gestating Swineb

Lactating Swinec

Adult Humansd

AASA

Young Swinea

Gestating Swineb

Lactating Swinec

Adult Humansd

69 71 66 64 55 69 63 50 75 71 65

71 69 56 57 65 68 67 62 70 69 56

72 71 58 59 66 71 69 62 71 72 57

----70–80 76–80 70–82 --73–81 82 ----70–80

Alanine Arginine Asparagine Aspartate Glutamate Glutamine Glycine Proline Serine Taurine β-Alanine

86 60 74 5 3 33 69 59 66 100 100

75 52 75 4 3 28 71 52 71 100 100

72 54 75 4 3 28 71 53 74 100 100

--62 ----4 33 ------100 100

Source: Chapman, K.P. et al. 2013 J. Nutr. 143:290–294; Stoll, B. and D.G. Burrin. 2006. J. Anim. Sci. 84 (Suppl.):E60– E72; Wu, G. 1998. J. Nutr. 128:1249–1252; Wu, G. 2020. Amino Acids 52:329–360; Wu et al. 2010. In: Dynamics in Animal Nutrition (Doppenberg, J. and P. van der Aar, eds). Wageningen Academic Publishers, The Netherlands, pp. 69–98. Note: AANA, amino acids that are not synthesized de novo in animal cells; AASA, amino acids that are synthesizable de novo in animal cells; ---, Data are not available. a Young swine were fed a milk protein-based diet. The values include both the digestibility of dietary protein and the ­catabolism of AAs in the portal drained viscera. b Gestating gilts were fed a corn- and soybean meal-based diet containing 12.2% crude protein. The values include both the digestibility of dietary protein and the bioavailability of orally administered free AAs. c Lactating sows were fed a corn- and soybean meal-based diet containing 18% crude protein. The values include both the digestibility of dietary protein and the bioavailability of orally administered free AAs. d The values refer to the percentages of orally administered free AAs entering the portal blood circulation.

challenged by findings from studies with young pigs that AAs in the enteral diet are degraded extensively by the small intestine in first pass (Table 2.9), with 150 >150

300 2547b >150 >150 >150 >150 >150

Comment Postabsorptive state Without arginine treatment After treatment with argininea Postabsorptive state At the end of a 2 h 70% of VO2 Max exercise Postabsorptive state Postabsorptive state Postabsorptive state Postabsorptive state Postabsorptive state Postabsorptive state Postabsorptive state Monitored for up to 8 h Postabsorptive state Postabsorptive state

Source: Brunton, J.A. et al. 1999. Am. J. Physiol. 277:E223–E231; Chen et al. 2020. Sci. Rep. 10:6065; Gilbreath, K.R. et al. 2020a. J. Anim. Sci. 98:skz370; Gilbreath, K.R. et al. 2020b. J. Anim. Sci. 98:skaa164; Graham, T.E. and D.A. MacLean. 1992. Can. J. Physiol. Pharmacol. 70:132–141; Harris, R.C. et al. 1999. Equine Vet. J. 30(Suppl.):546–551; Heird, W.C. et al. J. Pediatr. 81:162–165; Meyer, R.A. et al. 1980. J. Appl. Physiol. 49:1037– 1041; Morris, J.G. 1985. J. Nutr. 115:524–531; Morris, J.G. and Q.R. Rogers. 1978. J. Nutr. 108:1944-1953; Raina, R. et al. 2020. Nat. Rev. Nephrol. 16:471-482; Sakusic, A. et al. 2018. Crit. Care Med. 46:e897–e903; Wu et al. 2014. J. Nutr. Biochem. 15:442–451. a Intravenous administration of 1 mmol arginine–glutamate/kg BW/day. b Fed a diet containing no arginine. Note: IG, intragstric; TPN, total parenteral nutrition.

337

Synthesis of Urea and Uric Acid

have very high concentrations of blood ammonia (e.g., 107–371 µM), and the values are reduced to 25–60 µM after the intravenous administration of 0.5–1 mmol arginine–glutamate (a  mixture of arginine and glutamate)/kg BW/day (Johnson et al. 1972; Heird et al. 1972). Poorly managed infants with a urea cycle defect have plasma ammonia concentrations of >300 µM and often as high as 500–1,500 µM (Wijburg and Nassogne 2012). Major factors responsible for hyperammonemia in humans include arginine deficiency, liver failure, inherited metabolic diseases (a defect in a urea cycle enzyme or fatty acid oxidation), an excessive AA load, and intensive exercise. Like humans, companion (Che et al. 2021; Oberbauer and Larsen 2021), zoo (Herring et al. 2021), and farm (Gilbreath et al. 2021; Zhang et al. 2021) animals are also at high risks for hyperammonemia when fed either high-protein or arginine-deficient diets. Of particular note, overnight-fasted cats rapidly develop hyperammonemia within 45 min after consuming an arginine-free purified diet with proper amounts of other AAs (Table 6.3), with the concentration of ammonia in plasma increasing from 0.16 mM immediately before feeding to 1.4 mM at 2 h after the meal. Likewise, hyperammonemia occurs rapidly in neonatal pigs within 8 h after receiving an arginine-free diet via intragastric or intravenous feeding (Brunton et al. 1999). These findings indicate that dietary arginine is essential for the detoxification of ammonia produced from AA oxidation and for the health of animals.

6.1.4  Toxicity of Ammonia to Humans and Other Animals As noted above, hyperammonemia results in multiple organ dysfunctions and death in humans and other animals, particularly preterm neonates (Adeva et al. 2011; Lee et al. 2016; Ozanne et al. 2012). For example, elevating the concentrations of ammonia in plasma from 30 to 80 μM can generally result in vomiting, nausea, and seizures in humans (particularly infants), contributing to high rates of morbidity and mortality (Walker 2012). Similar syndromes occur in other mammals (e.g., cats, dogs, and pigs) with hyperammonemia [e.g., the plasma concentration of ammonia being >150 μM in piglets (Table 6.2)]. Interestingly, hyperammonemia (the median concentration of ammonia in serum being 43.5 µM, ranging from 35 to 58 µM) unrelated to hepatic dysfunction appears to be a biomarker for a poor prognosis and deaths in adults hospitalized in the intensive care unit (Sakusic et al. 2018).

TABLE 6.3 Time Course in Concentrations of Ammonia (NH3 + NH4+) in the Plasma of Overnight Fasted Cats Fed Diets with Arginine, without Arginine, or with Ornithine but without Argininea Time after the consumption of diet (min) Diet

0

45

90

120

150

210

300

Basal (n = 8 cats except for n = 7 at time 300 min) Basal – arginine (n = 8 cats)

Experiment “D” 174 169 156 344

– –

135 1,396

– –

– –

129 335

Basal – arginine (n = 1 cat) Basal – arginine + 1.52% ornithine (n = 5 cats)

Experiment “E” 91 311 111 154

1,085 141

– –

1,337 129

969 179

135 93

Source: Morris, J.G. and Q.R. Rogers. 1978. J. Nutr. 108:1944–1953. Values, expressed as µM. The basal diet contains the following (% of diet): L-Arg·HCl, 2.0; L-His-HCl·H2O, 1.2; L-Ile, 1.8; L-Leu, 2.4; L-Lys·HCl, 2.8; L-Met, 1.1; L-Cys-Cys, 0.80; L-Phe, 1.5; L-Tyr, 1.4; L-Thr, 1.4; L-Trp, 0.40; L-Val, 1.8; L-Asn, 2.0; L-Ser, 1.0; L-Pro, 2.0; Gly, 2.0; L-Glu, 6.0; L-Ala, 1.0; sodium acetate, 2.5. Crude protein content in the basal diet is 27.2% and represents 23% of total calories.

a

338

Amino Acids

Birds are also negatively affected by elevated levels of ammonia in the blood, as indicated by their reduced growth, impaired behavior, and abnormal metabolic profiles (Baker 2009). In both mammals and birds, hyperammonemia causes severe neurologic impairment, cerebral edema, coma, and death, indicating the potential for extremely toxic effects of ammonia on the central nervous system. Ammonia exists as both NH3 and NH4+ ion in physiological fluids (Chapter 4). Elevated levels of atmospheric ammonia have an irritating effect on the respiratory system, eyes, and skin of humans and other animals (particularly poultry and pigs, housed indoors), while compromising their immune response, health, growth, and development. These problems also occur in fish and crustaceans living in water with high concentrations of ammonia and are exacerbated in a hot environment. Note that the ammonia in the housing facilities of farm animals comes from both endogenous (e.g., AA catabolism) and exogenous (e.g., the microbial fermentation of manure and the outside air) production.

6.1.5  B  iochemical Mechanisms Responsible for Ammonia Toxicity to the Nervous System It had long been thought that high levels of ammonia drain α-ketoglutarate (α-KG) from the Krebs cycle, thereby reducing the production of ATP by cells and reducing intracellular ATP concentrations (Krebs 1964). This, in turn, causes dysfunction of cells in the central nervous system. However, such an effect of ammonia on ATP depletion can potentially also occur in other cell types (e.g., enterocytes and lymphocytes), and yet they do not exhibit an accelerated rate of apoptosis when extracellular concentrations increase from 0.05 to 0.5 mM in culture medium (Haynes et al. 2009; Wu 1995; Wu et al. 1994). Thus, in the presence of adequate buffering mechanisms, ammonia itself is not toxic to cells. Interestingly, hyperammonemia in humans and other animals is often associated with elevated levels of glutamine in plasma (up to ≥2 mM) due to the enhanced synthesis of glutamine by multiple tissues (Chapter 3). Thus, the plasma concentrations of both ammonia (>40 µM in humans) and glutamine (>900 µM in humans) in an overnight fasting state are often used as biomarkers of disease control in patients with urea cycle disorders (Lee et al. 2015). Patients with inborn urea cycle disorders frequently have >2 mM glutamine in their plasma, compared with the values of 0.5–0.6 mM in the normal, postabsorptive adults (Wijburg and Nassogne 2012). However, the culture medium of cells and tissues usually contains 2–4 mM glutamine to maintain their metabolism, growth, and survival, and these levels of glutamine are not cytotoxic at all (Meininger et al. 1988). Warren and Schenker (1964) reported that the administration of methionine sulfoximine (an inhibitor of glutamine synthetase) improved the survival of rats acutely exposed to toxic concentrations of ammonia. Subsequent works showed that inhibition of brain glutamine accumulation by methionine sulfoximine also prevented cerebral edema in hyperammonemic rats (Takahashi et al. 1991). Interestingly, the beneficial effect of methionine sulfoximine occurs despite a concomitant increase in brain ammonia levels. Emerging evidence from animal studies shows that hyperammonemia per se does not result in coma or death when glutamine synthesis is inhibited in vivo (Albrecht and Norenberg 2006; Fries et al. 2014). However, high extracellular concentrations of glutamine (e.g., ≥1 mM) inhibit nitric oxide (NO) synthesis by vascular endothelial cells in vitro and in vivo (Arnal et al. 1995; Lee et al. 1996; Okada et al. 2000), whereas an increase in endothelial NO synthesis can ameliorate the adverse effects of hyperammonemia on rats (Kawaguchi et al. 2005). These results indicate that prolonged elevation of glutamine in plasma is potentially harmful to organisms in an NO-dependent manner. A possible mechanism is that high concentrations of glutamine (e.g., >2 mM) inhibit NO synthesis via NO synthase (NOS) in endothelial cells through its catabolism to glucosamine-6-phosphate (Wu et al. 2001). This hexosamine, an analog of glucose6-phosphate, competitively inhibits the generation of NADPH (an essential cofactor for NOS) via the pentose cycle, thereby reducing blood flow and the supply of oxygen and nutrients to the brain, as well as ATP production by neuronal cells (Figure 6.1).

339

Synthesis of Urea and Uric Acid Ammonia

Glucose

3

Fructose-6-P 1

2

Glutamate

Glutamine

GFAT

Glucosamine-6-P (

Glucose-6-P Brain damage

)

G6PD (PC)

L-Arginine NADPH

Nitric oxide synthase

Reduced provision Nitric oxide of O2 and nutrients

FIGURE 6.1  Inhibition of endothelial nitric oxide (NO) synthesis by glutamine as a possible mechanism to mediate hyperammonemia-induced toxicity to the brain in humans and other animals. The concentrations of glutamine in plasma are increased in patients with high levels of ammonia due to the synthesis of glutamine. Glutamine is metabolized to glucosamine-6-phosphate, which competitively inhibits the activity of glucose-6-phosphate dehydrogenase, a key enzyme of the pentose cycle to generate NADPH. The latter is an essential cofactor of NO synthase for converting arginine into NO (a major vasodilator). Thus, in individuals with hyperammonemia, the supply of oxygen and nutrients (including glucose) from the blood to the brain is impaired possibly due, in part, to a glutamine-induced deficiency of NO, leading to tissue damage in the central nervous system. Enzymes that catalyze the indicated reactions are: (1) hexokinase; (2) phosphohexose isomerase; and (3) glutamine synthetase. GFAT, glutamine:fructose-6-phosphate transaminase; G6PD, glucose-6-phosphate dehydrogenase; P, phosphate; ↑, increase; ↓, decrease.

6.1.6  Treatment of Hyperammonemia Effective strategies for the treatment of hyperammonemia depend on its underlying causes (Cooper 2012; Enns et al. 2007). For example, in neonates and adults, ammonia toxicity induced by arginine deficiency can be successfully prevented by oral or intravenous administration of arginine, citrulline, or ornithine (Heird et al. 1972; Meijer et al. 1990; Morris 2002). Oral administration of proline is also effective to ameliorate neonatal death brought about by diet-induced hypoargininemia (Brunton et al. 1999). In patients who have defects in enzymes of the hepatic urea cycle and elevated levels of ammonia in the circulation, the intravenous administration of sodium benzoate is often used to prevent death (Chapter 5). This method can also apply to birds and other animal species. In the case of N-acetylglutamate (NAG) deficiency due to low mitochondrial NAG synthase-I activity, the oral or intravenous administration of N-carbamoylglutamate (a metabolically stable analog of NAG) can specifically reduce high concentrations of ammonia in the blood of both humans and other mammals (e.g., pigs) by the allosteric activation of CPS-I (Wu et al. 2004). In ruminants, vinegar (consisting mainly of acetic acid and water) is often used to treat ammonia toxicity because H+ from this acid rapidly combines with free NH3 to form NH4+, which is then quantitatively excreted with an anion (e.g., Cl−) in urine (Bates and Payne 2017). Additionally, acetic acid can help to normalize blood pH, while providing the energy required for the liver to convert ammonia into urea (Wu 2018).

6.2  UREA PRODUCTION IN MAMMALS 6.2.1  Historical Perspectives The Dutch physician-chemist H. Boerhaave discovered urea in urine in 1727. Indeed, urea was the first animal metabolite to be isolated in crystalline form. In 1773, H. Rouelle (a French chemist) prepared urea from dog’s urine and, in 1816, the British physician-chemist W. Prout reported the presence of urea in the blood plasma. With the improved method, urea composition was first

340

Amino Acids

determined accurately in 1817 by Prout. By the late 19th century, it was known that urea is the major nitrogenous product of protein and AA catabolism in mammals. Research on urea metabolism was greatly facilitated when F. Wöhler (a German chemist) first synthesized urea from silver isocyanate and ammonium chloride in 1828. In search of the origin of urinary urea, A. Clementi reported in 1913 that the mammalian liver can convert ammonia and AAs into urea. The pathway for urea biosynthesis had been elusive until H.A. Krebs and K. Henseleit proposed the urea (ornithine) cycle in 1932. Extensive research from the 1950s to the 1980s greatly expanded our knowledge of the metabolic control of the urea cycle by substrates, cofactors, protein turnover, allosteric regulators, and hormones (Meijer et al. 1990).

6.2.2  The Hepatic Urea Cycle in Mammals 6.2.2.1  Discovery of the Urea Cycle In the early 1930s, H.A. Krebs observed that either ornithine or arginine stimulated the conversion of ammonia to urea in rat liver slices incubated in the presence of physiological concentrations of major cations and anions (including sodium and bicarbonate). Interestingly, there was no change in the amount of ornithine in the medium or tissue. In his 1981 book entitled “Reminiscences and Reflections”, Krebs wrote that “The interpretation of this finding was not at once obvious. It took a full month to find the correct interpretation. At first, we were skeptical about the correctness of the observations. Was the ornithine perhaps contaminated with arginine? The answer was no. Then it occurred to us that the effect of ornithine might be related to the presence of arginase in the liver, the enzyme which converts arginine into ornithine and urea, known since the work of Kossel and Dakin in 1904”. Based on this consideration, Krebs and Henseleit proposed the following sequential reactions:

Ornithine + NH 3 + CO 2 → Citrulline → Arginine → Urea + Ornithine

After H.A. Krebs published his paper on the urea cycle, he was congratulated on this important discovery by some biochemists, but he also received severe criticisms from other scientists who could not reproduce his findings from studies with perfused rat livers. In his reminiscence of the discovery of the ornithine cycle, Krebs (1981) wrote the following comments. “Luck, it is true, is necessary, but the more experiments are carried out, the greater is the probability of meeting with luck. The story also shows that adverse criticisms are liable to be raised on the grounds that either the observations are not confirmed or that some other observations do not fit in with the interpretation of the findings. Almost every major development in science meets with criticisms of this kind”. The basic concept of the originally proposed cycle has stood the test of time over the past 90 years. The kinetics, activators, and substrate concentrations of the urea cycle enzymes are summarized in Tables 6.4, 6.5, and 6.6, respectively. Based on decreases in NAGS and N-acetylglutamate deacetylase activities in the rat liver over a 3-day period of fasting (Gomez et al. 1983), the half-lives of these two hepatic proteins are estimated to be 2.3 and 2.7 min, respectively. The mammalian liver releases urea to the bloodstream via a urea transporter (UT), UT-A (Sands and Blount 2014). 6.2.2.2  Characteristics of the Urea Cycle One of the most remarkable features of the urea cycle is its compartmentation (Figure 6.2). Namely, urea synthesis involves both the cytoplasm and mitochondria in the liver. Note that NH3 (rather than NH4+) and HCO3− (rather than CO2) are substrates for CPS-I. However, NH3 is in chemical equilibrium with NH4+, whereas HCO3− is produced from CO2 and H2O by the mitochondrial carbonic anhydrase. Both NH3 and HCO3− are formed from the catabolism of AAs (including glutamate, glutamine, and glycine) in the mitochondria. Glutamate dehydrogenase is the major intramitochondrial source of ammonia for urea production in mammals (Meijer et al. 1990). Once ammonia is generated within the mitochondria or the blood ammonia enters this organelle, it reacts with HCO3− to generate carbamoyl phosphate by CPS-I, with the hydrolysis of ATP to ADP and Pi

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TABLE 6.4 Kinetics of Urea Cycle Enzymes in the Liver and Enterocytes Rat livera Enzyme CPS-I

OCT ASS

ASL Arginase-I NAGS

Pig liverb

Reactant or activator

Vmax

Km (mM)

Vmax

Ammonia HCO3− Mg-ATP Mg2+ NAG (free) NAG (total) Ornithine CP Citrulline Aspartate Mg-ATP AS Arginine Glutamate Acetyl-CoA Arginine

21 21 21 21 21 21 799 799 7.4 7.4 7.4 13.3 5,143 0.22 0.22 0.22

1–2 (0.6) 4–5 (2) 0.5–3 (1.2) 0.17–2 (0.15) 0.04 (>0.2) 0.02 0.15 0.04–0.13 (>0.03) 3.5 (>0.06) 3 0.7 0.01 (0.05)

18.6 18.6 18.6 18.6 18.6 18.6 921 921 8.45 8.45 8.45 17.7 3,072 0.28 0.28 0.28

Pig enterocytesb

Km (mM) 1.2 5.77 1.69 ND 0.13 0.96 1.58 0.46 0.068 0.031 0.24 0.12 3.38 3.56 0.84 0.061

Vmax

Km (mM)

6.68 6.68 6.68 6.68 6.68 6.68 706 706 1.89 1.89 1.89 3.50 7.13 0.064 0.064 0.064

1.34 58.6 15.2 ND 0.82 ND 5.13 17.1 0.15 0.054 2.86 0.63 7.46 4.25 1.02 0.11

Source: Meijer, A.J., W.H. Lamers, and R.A. Chamuleau. 1990. Physiol. Rev. 70:701–748; and Davis, P.K. and G. Wu. 1998. Comp. Biochem. Physiol. B 119:527–537. a Values in parentheses are K values for enzymes in situ, either in permeabilized mitochondria (CPS-I), intact mitochondria m (OCT), or intact hepatocytes (ASS, ASL, arginase, and NAGS). Vmax is expressed as µmol/min/g of dry weight (adult rats). b V max is expressed as µmol/min/g of tissue protein (60-day-old growing pigs). Note: ND, Not determined. Ammonia is the sum of NH4+ and NH3

(PO43−). The carbamoyl phosphate combines with ornithine to yield citrulline by OCT. The source of ornithine for OCT is the diet, blood, or cytosolic arginase. In either case, ornithine is transported by ORNT1 (mitochondrial ornithine transporter 1) from the cytoplasm to the mitochondrial matrix in the mammalian hepatocyte (Monné et al. 2015). ORNT1 is an antiporter whereby mitochondrial citrulline is exchanged for cytosolic ornithine across the inner mitochondrial membrane to exit into the cytoplasm. In the cytosol of the hepatocytes, citrulline is rapidly metabolized to arginine and then to urea plus ornithine. Argininosuccinate synthase [ASS (a cytosolic enzyme)] converts citrulline and aspartate into argininosuccinate, and a major source of this aspartate is the mitochondrial aspartate through the action of citrin (a transporter in the mitochondrial membrane). Citrin exchanges the cytosolic glutamate for the mitochondrial aspartate (Monné et al. 2015). The mitochondrial aspartate can be derived from ammonia and oxaloacetate via glutamate dehydrogenase and glutamate–oxaloacetate transaminase (Figure 6.3). Glutamate is required for the synthesis of NAG (an allosteric activator of CPS-I) in the mitochondria, whereas aspartate is essential for the conversion of citrulline into argininosuccinate by ASS in the cytosol. The fumarate (derived from the mitochondrial oxaloacetate) is generated from argininosuccinate by argininosuccinate lyase (ASL) and converted into L-malate by cytosolic fumarase (Tuboi et al. 1986), with the cytosolic L-malate entering the mitochondria for the conversion into oxaloacetate. In support of this view, there is evidence for the transport of the urea cycle–derived L-malate from the cytosol into the mitochondria (Pesi et al. 2018). The formation of L-malate from fumarate in the cytosol as an intermediate allows regeneration of oxaloacetate in the mitochondria for the incorporation of ammonia into aspartate for another turn of the urea cycle.

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TABLE 6.5 Subunits, Cofactors, and Allosteric Activator of Urea Cycle Enzymes in the Mammalian Livera Enzyme

Half-life (days; means)

Structure

Subunit molecular mass (kDa)

2.3b 7.7 7.5 7.9 7.8 5.0

Homotrimer Homodimer Homotrimer Homotetramer Homotetramer Homotrimer

57 155 36 46.25 51.7 35

NAGS CPS-I OCT ASS ASL Arginase-I

Total molecular mass (kDa) 171 310 108 185 206.8 105

Cofactor or allosteric factor Argininec N-Acetylglutamated None Mg2+ None Mn2+

Source: Das, T.K. and J.C. Waterlow. 1974. Br. J. Nutr. 32:353–373; Jackson, M.J. et al. 1986. Annu. Rev. Genet. 20:431– 464; Morris, S.M. Jr. 2002. Annu. Rev. Nutr. 22:87–105; Schmike, R.T. 1973. Adv. Enzymol. 37:135–187; Wallace, R. et al. 1986. FEBS Lett. 208:427–430. a The half-life of N-acetylglutamate in the mitochondria of rat liver is 45 min. For comparison, the half-lives of glutamate dehydrogenase, alanine transaminase, aspartate transaminase, cytosolic fumarase, and mitochondrial fumarase in the rat liver are 10.1, 8.8, 8.2, 4.8, and 9.7 days, respectively. b Estimated on the basis of a decline in N-acetylglutamate synthase activity in the rat liver over a 3-day period of fasting (Gomez et al. 1983). c Allosteric activator of N-acetylglutamate synthase. d Allosteric activator of carbamoyl phosphate synthetase-I. Notes: ASL, argininosuccinate lyase; ASS, argininosuccinate synthase; NAGS, N-acetylglutamate synthase; CPS-I, carbamoyl phosphate synthetase-I; OCT, ornithine carbamoyltransferase.

TABLE 6.6 Concentrations of Urea Cycle Enzyme Substrates or Activators in the Pig Liver and Enterocytesa Pig liver Substrate

Cytoplasm

Ammonia (NH  + NH3) ATP Aspartate Citrulline Arginine Ornithine Argininosuccinate Carbamoyl phosphate N-Acetylglutamate + 4

0.35 5.97 2.26 0.10 0.13 0.65 0.078 1.63 ND

Pig enterocytes

Mitochondria 0.52 12.6 2.50 0.12 0.14 0.82 ND 0.50 1.08

Cytoplasm 0.45 4.28 5.56 0.36 0.84 0.30 0.042 2.26 ND

Mitochondria 1.18 8.56 2.41 0.52 0.75 0.61 ND 0.59 0.64

Source: Davis, P.K. and G. Wu. 1998. Comp. Biochem. Physiol. B 119:527–537. a Values are expressed as mM. The liver and enterocytes were obtained from 60-day-old fed pigs. Note: ND, not detectable.

The detailed steps of the urea cycle and its associated reactions in the liver are:

NH 3 + HCO3− → Carbamoyl phosphate ( mitochondria ) (6.1)



Ornithine + Carbamoyl phosphate → Citrulline ( mitochondria ) (6.2)

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Synthesis of Urea and Uric Acid -

NH2

HCO 3 + NH 3

C O

2 Mg-ATP NAG

NH2 Urea

CPS-I

NH2 H 2O

+

H2C NH3

2 Mg-ADP + Pi

H2C NH Arginase

CH2 CH2

+

H2N C O P O O Carbamoyl phosphate

HC

H C NH3

-

O

CH2 CH2

+

H C NH3 O

+ NH2

C

COO L-Ornithine

ASL OCT

Pi

+ NH2

COO C NH CH

Mitochondrion NH2 C O

Cytoplasm

-

OOC CH Fumarate

-

COO L-Arginine

-

COO

AMP + PPi

Mg-ATP ASS

+ NH3 -

L-Citrulline

CH2

COO

-

+

CH2 H C COO

CH2

CH2

H2C NH C H2

CH2 NH

-

COO

H C NH3 COO

Argininosuccinate

+

H C NH3 CH2

-

COO L-Aspartate

FIGURE 6.2  The urea cycle in mammals. The synthesis of urea from ammonia and bicarbonate involves both the mitochondrion and the cytoplasm. Citrulline exits the mitochondrion into the cytoplasm where it is converted into arginine, which is rapidly hydrolyzed by arginase into urea plus ornithine. Ornithine is then reused for another turnover of the cycle. ASL, argininosuccinate lyase; ASS, argininosuccinate synthase; CPS-I, carbamoyl phosphate synthetase -I; NAG, N-acetylglutamate.



NH 3 + α-Ketoglutarate → Glutamate ( mitochondria ) (6.3)



Glutamate + Oxaloacetate → α-Ketoglutarate + Aspartate ( mitochondria ) (6.4)

The net reaction of (6.3) and (6.4): NH 3 + Oxaloacetate → Aspartate

Citrulline + Aspartate → Argininosuccinate ( cytosol ) (6.5)



Argininosuccinate → Arginine + Fumarate ( cytosol ) (6.6)



Arginine + H 2 O → Ornithine + Urea ( cytosol ) (6.7)

344

Amino Acids

NH4+ + α-Ketoglutarate

Mitochondrion Cytosol

NAD+

NADH + H+ GDH

Glutamate

GOT Glutamate

Aspartate

Oxaloacetate

MDH

NADH + H+

L-Malate NAD+

Citrin

Aspartate

ASS

H2O

Fumarase

Fumarate

Citrulline Glutamate

L-Malate

Argininosuccinate

ASL

Arginine

FIGURE 6.3  The source of aspartate required for and the fate of fumarate produced in the hepatic urea cycle. Ammonia (either produced from the mitochondrial oxidation of AAs or derived from the blood) and oxaloacetate are the ultimate sources of aspartate for ASS in the cytosol. The mitochondrial aspartate is transported by citrin into the cytosol in exchange for the cytosolic glutamate. ASL generates fumarate in the cytosol, which is converted into L-malate by the cytosolic fumarase. After the L-malate enters the mitochondria via the malate shuttle, this intermediate is oxidized to oxaloacetate for the regeneration of oxaloacetate to allow another turn of the urea cycle. ASL, argininosuccinate lyase; ASS, argininosuccinate synthase; GDH, glutamate dehydrogenase; GOT, glutamate–oxaloacetate transaminase; MDH, NAD+-linked malate dehydrogenase.



Fumarate + H 2 O → L-Malate L-Malate → Oxaloacetate

( cytosol ) (6.8) ( mitochondria ) (6.9)

Thus, the two nitrogen atoms of the urea molecule are ultimately derived from ammonia. The net reaction of the urea cycle in terms of nitrogen and carbon balances is:

2NH 3 + HCO3− → Urea

( H 2 N–CO–NH 2 )

Another important feature of the urea cycle is metabolic channeling, which can be defined as the restricted flow of substrates and products in a series of enzyme-catalyzed reactions (Srere 1987). Studies of the urea cycle in the 1980s involving the use of labeled substrates or intermediates led to the development of this concept in cell metabolism. Interestingly, the hepatic pool of arginine in the cytosol involved in the urea cycle seems to be metabolically sequestered and is not in equilibrium with, or rapidly exchangeable with, arginine in plasma or other hepatic arginine pools (Yang et al. 2000). Metabolic channeling facilitates the immediate transfer of intermediates between enzymes and helps maintain relatively high concentrations of substrates in catalytic sites. This ensures the rapid and efficient formation of end products via a series of biochemical reactions. Available evidence shows that metabolic channeling in the hepatic urea cycle occurs in both the cytoplasm and mitochondria (Cheung et al. 1989). 6.2.2.3  Coupling of the Urea Cycle with Gluconeogenesis in the Liver Under physiological conditions, the mammalian liver does not completely oxidize the carbon skeletons of AAs to CO2 due to the constraints of oxygen consumption and ATP turnover (Jungas et al. 1992). Some of the carbons of AAs are metabolized to glucose (for glucogenic AAs), ketone bodies (for ketogenic AAs), both glucose and ketone bodies (for glucogenic plus ketogenic AAs), and fatty acids and cholesterol (for all AAs), depending on the physiological needs (Watford 2020). In the fed state, some of the glucose and fatty acids are converted into glycogen and triacylglycerols,

345

Synthesis of Urea and Uric Acid

respectively, in the liver and other tissues, whereas some of the AA carbons can also be used for cholesterol synthesis. When the catabolism of AAs generates ammonia, oxygen- and hydrogencontaining carbon skeletons, NADH, and ATP in the mammalian liver, most of the carbon skeletons (except for those from ketogenic AAs) in the form of C3 units, NADH, and ATP are used for the synthesis of glucose to account for carbon, nitrogen, oxygen, and hydrogen balances in the biochemical reactions (Watford 2020). The regeneration of NAD+ and ADP is essential for AA catabolism to continue in the liver. Thus, the urea cycle for the removal of AA-derived ammonia is always coupled with gluconeogenesis in mammalian hepatocytes under both fed and fasting conditions (Figure 6.4). This is analogous to glutamine degradation in the kidneys, where the resulting ammonia and α-KG are used for NH4+ and glucose production, respectively (Watford 2020).

6.2.3 Synthesis of Urea from Ammonia in the Extrahepatic Cells of Mammals In studying intestinal AA metabolism, G. Wu found in 1995 that enterocytes in postweaning mammals (e.g., pigs and rats) contain all the enzymes required for the synthesis of urea from either ammonia and CO2 or glutamine. These cells synthesize urea from either the extracellular ammonia or the ammonia generated from the catabolism of AAs (e.g., glutamine) within the mitochondria in a concentration-dependent manner (Table 6.7). This physiologically relevant pathway was established by measuring the formation of urea in enterocytes from these nitrogenous substrates at concentrations present in the lumen of the small intestines of pigs and rats. Results from studies with postweaning pigs indicate that, at the same concentrations of substrates, the rate of ureagenesis from extracellular ammonia or glutamine in enterocytes is approximately 5% of that in hepatocytes (Wu 1995). Thus, enterocytes are capable of producing significant amounts of urea from extracellular- and intramitochondrially-derived ammonia. Urea is also formed from arginine via arginase in the enterocytes of weaned mammals. Urea synthesis by enterocytes in postweaning mammals is the first line of defense against the potential toxicity of ammonia that is: (1) produced by extensive intestinal degradation of dietary and blood-derived glutamine (a major fuel for enterocytes) and (2) derived from diets and luminal microorganisms. Additionally, F. Blachier and B.J. Bequette reported, respectively, that colonocytes of the rat’s large intestine (Mouille et al. 1999) and the Glycogen

Fed conditions

Amino acids

Glucose

Urea

NAD+ Both fed and fasting conditions

Cholesterol

ADP

ATP

ATP

NADH + H+

NH3

ions

ondit

Fed c

ADP

Both fed and fasting conditions

HCO3-

Carbon skeletons

Fed conditions Fastin

g cond

itions

Fatty acids and TAGs Ketone bodies

FIGURE 6.4  Coupling of the urea cycle with gluconeogenesis in the mammalian liver. When the catabolism of AAs is the source of ammonia in the mitochondria, their α-ketoacids (for glucogenic AAs) undergo partial oxidation to CO2 and are converted into glucose in the liver under both fed and fasting conditions, and also serve as substrates for fatty acid/cholesterol synthesis and ketogenesis in the liver under fed and fasting conditions, respectively. In the fed state, glucose can be converted into glycogen in periportal hepatocytes. Carbon, nitrogen, NAD+, and ATP must be balanced in all reactions. TAGs, triacylglycerols.

346

Amino Acids

TABLE 6.7 Synthesis of Urea from Ammonia and Glutamine in the Enterocytes of Pigsa Urea Synthesis (nmol/mg protein/30 min) Age of pigs (days) 0–21 29 58

No substrate

1 mM glutamine

5 mM glutamine

ND ND ND

ND 6.27 ± 0.74b 7.91 ± 0.82b

ND 15.2 ± 1.28c 16.5 ± 1.43c

2 mM NH4Cl + 2 mM 0.5 mM NH4Cl + 2 mM ornithine + 2 mM aspartate ornithine + 2 mM aspartate ND 13.4 ± 1.56c 14.6 ± 1.28c

ND 21.6 ± 2.07d 23.4 ± 3.26d

Source: Wu, G. 1995. Biochem. J. 312:717–723. a Values are means ± SEM, n = 8. Pigs were weaned at 21 days of age to a corn- and soybean meal–based diet. Jejunal enterocytes were incubated at 37°C for 0 to 30 min in Krebs–Henseleit buffer (pH 7.4), containing 5 mM D-glucose and the substrate(s) as indicated. b–d Within a row, means not sharing the same superscript letter differs (P 9 g/day) are occasionally associated with nausea, gastrointestinal discomfort, and diarrhea for some people, which may result from a rapid and excess production of NO by the gastrointestinal tract and from impaired intestinal absorption of other dietary basic AAs (lysine and histidine) (McNeal et  al. 2016). Also, supplementing 4% or more arginine to diets for animals can cause skin lesions, reduce food intake, and inhibit growth

Functions and Supplementation of Amino Acids

603

(Wu et al. 2009). A solution to this potential problem may be the alternative use of L-citrulline, a precursor for arginine synthesis (Faure et al. 2012; Wu and Meininger 2000). As a neutral AA, L-citrulline does not compete with basic AAs for transport by cells, its conversion to arginine consumes one mole of ammonia in the form of aspartate, and its administration does not require equimolar HCl. Thus, enteral or parenteral L-citrulline may be particularly useful for patients with elevated ammonia concentrations, impaired L-arginine transport, enhanced intestinal L-arginine catabolism, abnormal muscle function, or a high activity of constitutively expressed arginase. Finally, because excessive production of NO is destructive to cells, it would not be advisable to administer L-arginine to animals or patients with severe infections, active inflammatory or autoimmune disorders, active malignancy (e.g., late stages of tumorigenesis), or pathological angiogenesis. 11.6.2.3  Glutamine as an Example for the Safety of AA Supplementation Glutamine itself is not toxic to cells, as large amounts of glutamine (e.g., 4 mM or approximately 8–15 times physiological concentration in the plasma of mammals) are usually included in the culture medium for all human and animal cell lines (Curi et al. 2005). However, the central nervous system is highly sensitive to ammonia generated from glutamine degradation in the body (Chapter 6). Based on a comprehensive review of published studies, Shao and Hathcock (2008) have indicated that healthy adult humans can tolerate the oral administration of up to 14 g glutamine/day without any adverse effects. Adult athletes showed no adverse response to dietary supplementation with 28 g glutamine/day for 14 days (Gleeson 2008). Adult patients undergoing bone marrow transplantation reported improvements in mood and well-being after receiving a TPN solution containing >20 g glutamine/day (Young et al. 1993). Likewise, the addition of glutamine (0.35 g/kg BW/day; as 0.5 g alanyl-glutamine/kg BW/day) to a TPN solution did not have any negative effect on critically ill patients (Dechelotte et  al. 2006). Similarly, postsurgery patients had no adverse response to the administration of glutamine (0.5 g/kg BW; as alanyl-glutamine) via TPN feeding (Jiang et al. 1993). However, concerns over the safety of glutamine supplementation have arisen from two clinical studies with TPN-fed or enterally fed patients in the intensive care unit. First, an intravenous provision of glutamine (0.35 g/kg BW/day; as 0.5 g alanyl-glutamine) could increase mortality in critically ill patients with multiorgan failure and maintained on mechanical ventilation (Heyland et al. 2013). Second, 28-day enteral feeding of high-protein diets supplemented with immune-modulating nutrients (including glutamine) to critical patients managed with mechanical ventilation increased mortality (van Zanten et al. 2014). It is unknown whether (1) the alanine released from the intravenously administered alanyl-glutamine or other factors may contribute to the adverse effect of the nutritional intervention, (2) glutamine may interact with a high-protein enteral diet to cause hyperammonemia or other metabolic/physiological abnormalities (e.g., reduced blood flow to tissues) in the intensive care unit patients, and (3) the nonglutamine immune-modulating nutrients (e.g., vitamins and ω-3 polyunsaturated fatty acids) supplemented to the enteral diets may contribute to increased mortality in those patients. Nonetheless, the American Society for Parenteral and Enteral Nutrition (McClave et al. 2016), Society for Clinical Care Medicine (McClave et al. 2016), and the European Society for Parenteral and Enteral Nutrition (Singer et  al. 2019) now recommend that glutamine supplementation may be used for patients with burn injury or intestinal dysfunction and possibly for some elective surgery patients, but not for patients with multiorgan failures. Clearly, the use of glutamine in clinical nutrition requires the careful design of diets and the careful monitoring of vital physiological variables. Extensive feeding studies have shown the safety of appropriate doses of supplemental glutamine in swine and rodents (Watford 2008; Wong et al. 2011; Wu et al. 2011a; Zhu et al. 2018). For example, short- and long-term supplementation with glutamine at an appropriate dose (e.g., up to 1% in diet on an as-fed basis; 90% dry matter content in the diet) is safe for neonatal, gestating, and lactating swine. Based on feed intake, the supplemental doses of glutamine per kg BW for neonatal, gestating, and lactating swine are 1.0, 0.20, and 0.32 g/kg BW/day, respectively, in addition to the glutamine in the basal diets (1.2, 0.24, and 0.55 g glutamine/kg BW/day, respectively),

604

Amino Acids

without any adverse effects (Wu et al. 2011). In all of these experiments, dietary supplementation with up to 1% glutamine (on an as-fed basis) for 3 weeks did not result in feed-intake reduction, sickness, or death in any pigs. In addition, no side effects of glutamine supplementation of up to 1% in the diet (on an as-fed basis) were observed in postweaning pigs within at least 3 months after termination of a 2-week, 1-month, or 3-month period of supplementation. However, a high supplemental dose of glutamine (e.g., 2% glutamine in a corn- and soybean meal-based diet for weanling piglets) may have an undesirable effect (e.g., reduced feed intake) due to an AA imbalance plus increased amounts of ammonia in the plasma and this must be avoided in dietary supplementation and clinical therapy. Furthermore, growth, metabolic, and histological analyses indicated that dietary supplementation with 1.25% glutamine was safe for both male and female rats for at least 13 weeks (Tsubuku et al. 2004b). This level of supplementation was equivalent to 0.83 and 0.96 g glutamine/kg BW/day in male and female rats, respectively, based on their food intake per kg BW (Tsubuku et al. 2004b). Broiler chickens (from birth to market weight) well tolerated 1% supplemental glutamine in diets between 7 and 42 days or between 2 and 42 days of age without any adverse effects on feed intake, growth, or health (Abdulkarimi et al. 2019; Ayazi 2014). Similarly, 21- to 42-day-old turkey poults did not exhibit any adverse response to dietary supplementation with 0.7% glutamine (Salmanzadeh and Shahryar 2013). Moreover, dietary supplementation with 0.8 or 1% glutamine for 40–42 days had no negative effects on laying hens (Dong et al. 2010) or laying guinea fowl (Gholipour et al. 2017). Based on reduced feed intake, reduced weight gain, and abnormal intestinal morphology, broilers did not tolerate dietary supplementation with ≥2% glutamine (Bartell and Batal 2007; Khempaka et al. 2011; Soltan 2009). However, in view of intestinal health, growth performance, and feed efficiency, ducklings well tolerated dietary supplementation with 2% glutamine (Zhang et al. 2017a). Thus, there is a species difference in the sensitivity of poultry to dietary glutamine intake. Like terrestrial animals, the small intestine of fish also extensively oxidizes glutamine to CO2 (Wu et al. 2020a). Results of extensive research have shown that (1) dietary supplementation with glutamine (up to 3%) does not have adverse or undesirable effects on fish; (2) diets containing a total of up to 5% glutamine (dry matter basis) are not toxic to hybrid striped bass and largemouth bass; (3) dietary glutamine supplementation has beneficial effects on the growth, health, and feed efficiency of fish without adverse effects on health (Li et al. 2020b). For example, channel catfish could well tolerate dietary supplementation with 3% glutamine for 70 days (Pohlenz et al. 2012a, b), and turbots grew well when their typical diets were supplemented with 2% glutamine for 84 days (Zhang et al. 2017b). Also, grass carp did not exhibit any adverse response to dietary supplementation with at least 2% glutamine for 80 days (Yan and Zhou 2006). Similar results were obtained for red drum (Cheng et al. 2011), hybrid striped bass (Cheng et al. 2012), half-smooth tongue sole (Cynoglossus semilaevis Günther) postlarvae (Liu et  al. 2015), and gilthead seabream (Coutinho et  al. 2016). Considering glutamine content (1.5%–2%) in the basal diets, various species of fish can tolerate 4%–5% of glutamine in enteral diets (Li et al. 2020b). 11.6.2.4  Glutamate as an Example for the Safety of AA Supplementation Different cells use glutamate for different purposes. For example, the enterocytes of the small intestine actively take up and oxidize a large amount of glutamate from its lumen (e.g., 1–10 mM) as a major energy substrate (Chapter 4), glutamate activates the taste cells of the tongue and the gastrointestinal tract via signal transduction (San Gabriel and Uneyama 2013), and glutamate is the major excitatory neurotransmitter in the central nervous system with its extracellular concentrations being