Amino Acids (2009) 37:1 17
`DOI 10.1007/s00726 009 0269 0
`
`R E V I E W A R T I C L E
`
`Amino acids: metabolism, functions, and nutrition
`
`Guoyao Wu
`
`Received: 8 February 2009 / Accepted: 1 March 2009 / Published online: 20 March 2009
`Ó Springer Verlag 2009
`
`Abstract Recent years have witnessed the discovery that
`amino acids (AA) are not only cell signaling molecules but
`are also regulators of gene expression and the protein
`phosphorylation cascade. Additionally, AA are key precur-
`sors for syntheses of hormones and low-molecular weight
`nitrogenous substances with each having enormous biolog-
`ical importance. Physiological concentrations of AA and
`their metabolites (e.g., nitric oxide, polyamines, glutathione,
`taurine, thyroid hormones, and serotonin) are required for
`the functions. However, elevated levels of AA and their
`products (e.g., ammonia, homocysteine, and asymmetric
`dimethylarginine) are pathogenic factors for neurological
`disorders, oxidative stress, and cardiovascular disease. Thus,
`an optimal balance among AA in the diet and circulation is
`crucial for whole body homeostasis. There is growing rec-
`ognition that besides their role as building blocks of proteins
`and polypeptides,
`some AA regulate key metabolic
`pathways that are necessary for maintenance, growth,
`reproduction, and immunity. They are called functional AA,
`which include arginine, cysteine, glutamine, leucine, pro-
`line, and tryptophan. Dietary supplementation with one or a
`mixture of these AA may be beneficial for (1) ameliorating
`health problems at various stages of the life cycle (e.g., fetal
`growth restriction, neonatal morbidity and mortality,
`weaning-associated intestinal dysfunction and wasting
`syndrome, obesity, diabetes, cardiovascular disease, the
`metabolic syndrome, and infertility); (2) optimizing effi-
`ciency of metabolic transformations to enhance muscle
`growth, milk production, egg and meat quality and athletic
`performance, while preventing excess fat deposition and
`
`G. Wu (&)
`Department of Animal Science, Faculty of Nutrition,
`Texas A&M University, College Station, TX 77843, USA
`e mail: g wu@tamu.edu
`
`reducing adiposity. Thus, AA have important functions in
`both nutrition and health.
`Keywords Amino acids Health Metabolism
`Nutrition
`
`Abbreviations
`AA
`Amino acids
`BCAA Branched-chain amino acids
`EAA
`Nutritionally essential amino acids
`eIF
`Eukaryotic translation initiation factor
`mTOR Mammalian target of rapamycin
`NEAA Nutritionally non-essential amino acids
`NO
`Nitric oxide
`PDV
`Portal-drained viscera
`
`Introduction
`
`Amino acids (AA) are defined as organic substances
`containing both amino and acid groups. Except for gly-
`cine, all AA have an asymmetric carbon and exhibit
`optical activity. The absolute configuration of AA (L- or
`D-isomers) is defined with reference to glyceraldehydes.
`Except for proline, all protein AA have a primary amino
`group and a carboxyl group linked to the a-carbon atom
`(hence a-AA). In b-AA (e.g., taurine and b-alanine), an
`amino group links to the b-carbon atom. Post-transla-
`tionally modified AA occur in some proteins (Galli 2007).
`Because of variations in their side chains, AA have
`remarkably different biochemical properties and functions
`(Brosnan 2001; Suenaga et al. 2008; Wu et al. 2007a).
`AA are generally stable in aqueous solution at physio-
`logical pH, except for (1) glutamine which is slowly
`
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`G. Wu
`
`cyclized to pyroglutamate (\1% per day at 1 mM at
`25°C) and (2) cysteine which undergoes rapid oxidation
`to cystine.
`Except for glycine, all AA can have L- and D-isoforms.
`Most D-AA, except for D-arginine, D-cystine, D-histidine,
`D-lysine, and D-threonine, can be converted into L-AA in
`animals via widespread D-AA oxidases and transaminases
`(Baker 2008; Fang et al. 2009). The efficiency of D-AA
`utilization, on a molar basis of the L-isomer, may be 20
`100%, depending on substrates and species (Baker 2008).
`Among more than 300 AA in nature, only 20 of them
`(a-AA) serve as building blocks of protein. However,
`non-protein a-AA (e.g., ornithine, citrulline, and homo-
`cysteine) and non-a AA (e.g., taurine and b-alanine) also
`play important roles in cell metabolism (Curis et al.
`2007; Hu et al. 2008b; Manna et al. 2009; Perta-Kajan
`et al. 2007). Because of its large mass (representing 40
`45% of body weight), skeletal muscle is the largest res-
`ervoir of both peptide-bound and free AA in the body
`(Davis and Fiorotto 2009). Over the past 20 years, much
`effort has been directed toward defining optimal require-
`ments of AA by livestock species [including pigs (Wu
`et al. 2007a) and ruminants (Firkins et al. 2006)], birds
`(Baker 2008), fish (Li et al. 2008), and humans (Elango
`et al. 2009) under various nutritional, developmental,
`environmental, and pathological conditions. Additionally,
`results of recent studies indicate that the small intestine is
`
`Table 1 Reactions initiating AA catabolism in animals
`
`a major site for extensive catabolism of AA in humans
`and animals, therefore modulating the entry of dietary AA
`into the portal circulation and the pattern of AA in plasma
`(Riedijk et al. 2007; Stoll et al. 1998; Wu 1998). Further,
`there is growing interest in regulatory functions of L- and
`D-AA in nutrition and physiology (Kim and Wu 2008;
`Tujioka et al. 2007; Wang et al. 2008b), as well as the
`underlying cellular and molecular mechanisms (Grillo and
`Colombatto 2007; Jobgen et al. 2006; Katane et al. 2008;
`Scolari and Acosta 2007; Wang et al. 2008c).
`Although each AA has its own unique catabolic
`pathway(s), the catabolism of many AA exhibit a number
`of common characteristics
`in organisms
`(Table 1).
`Important metabolites of AA include ammonia, CO2,
`long-chain and short-chain fatty acids, glucose, H2S,
`ketone bodies, nitric oxide (NO), urea, uric acid, poly-
`amines, and other nitrogenous substances with enormous
`biological importance (Blachier et al. 2007; Montanez
`et al. 2008; Morris 2007; Rider et al. 2007; Sugita et al.
`2007)
`(Table 2). Complete oxidation of AA carbons
`occurs only if their carbons are ultimately converted to
`acetyl-CoA, which is oxidized to CO2 and H2O via the
`Krebs cycle and mitochondrial electron transport system.
`On a molar basis, oxidation of AA is less efficient for
`ATP production,
`compared with fat
`and glucose
`(Table 3). Thus, the efficiency of energy transfer from
`L-AA to ATP ranges from 29% for methionine to 59%
`
`Reactions
`
`Transamination
`
`Deamidation
`
`Oxidative deamination
`
`Decarboxylation
`
`Hydroxylation
`
`Reduction
`
`Dehydrogenation
`
`Hydrolysis
`
`Dioxygenation
`
`One carbon unit transfer
`
`Condensation
`
`Oxidation
`
`Amidotransferation
`
`Deaminated oxidation
`
`Dehydration
`
`Cleavage
`
`?
`
`Examples
`Leucine ? a ketoglutarate $ a ketoisocaproate ? glutamate
`Glutamine ? H2O ? glutamate ? NH4
`Glutamate ? NAD? $ a ketoglutarate ? NH3 ? NADH ? H?
`Ornithine ? putrescine ? CO2
`Arginine ? O2 ? BH4 ? NADPH ? H? ? NO ? BH4 ? citrulline ? NADP?
`Lysine ? a ketoglutarate ? NADPH ? H? ? saccharopine ? NADP?
`Threonine ? NAD? ? 2 amino 3 ketobutyrate ? NADH ? H?
`Arginine ? H2O ? ornithine ? urea
`Cysteine ? O2 ? cysteinesulfinate
`Glycine ? MTHF $ serine ? THF
`Methionine ? Mg ATP ? S adenosylmethionine ? Mg PPi ? Pi
`Proline ? ‘O2 ? pyrroline 5 carboxylate ? H2O
`Glutamine ? F6P $ glucosamine 6 phosphate ? glutamate
`D Amino acid ? O2 ? H2O $ a ketoacid ? H2O2 ? NH3
`Serine ? aminoacrylate ? H2O
`Glycine ? NAD? ? THF $ MTHF ? CO2 ? NH3 ? NADH ? H?
`
`(1)
`
`(2)
`
`(3)
`
`(4)
`
`(5)
`
`(6)
`
`(7)
`
`(8)
`
`(9)
`
`(10)
`
`(11)
`
`(12)
`
`(13)
`
`(14)
`
`(15)
`
`(16)
`
`Enzymes that catalyze the indicated reactions are: (1) BCAA transaminase; (2) phosphate activated glutaminase; (3) glutamate dehydrogenase;
`(4) ornithine decarboxylase; (5) NO synthase; (6) lysine:a ketoglutarate reductase; (7) threonine dehydrogenase; (8) arginase; (9) cysteine
`dioxygenase; (10) hydroxymethyltransferase; (11) S adenosylmethionine synthase; (12) proline oxidase; (13) glutamine:fructose 6 phosphate
`transaminase; (14) D amino acid oxidase; (15) serine dehydratase; (16) glycine synthase (glycine cleavage system). F6P fructose 6 phosphate,
`MTHF N5 N10 methylene THF, THF tetrahydrofolate. BH4, tetrahydrobiopterin (required for hydroxylation of arginine, phenylalanine, tyrosine,
`and tryptophan)
`
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`

`Amino acids
`
`3
`
`Table 2 Major metabolites and functions of AA in nutrition and metabolism
`
`AA
`
`AA
`
`Alanine
`b Alanine
`
`Products
`
`Major functions
`
`Directly
`
`Directly
`
`Directly
`
`Dipeptides
`
`Protein synthesis; osmolytes; regulation of hormone secretion, gene expression and cell signaling
`
`Inhibition of pyruvate kinase and hepatic autophagy; gluconeogenesis; transamination; glucose alanine cycle
`
`A component of coenzyme A and pantothenic acid
`Carnosine (b alanyl L histidine), carcinine (b alanyl histamine), anserine (b alanyl 1 methyl L histidine), and
`balenine (b alanyl 3 methyl histidine) with antioxidative function
`
`Arginine
`
`Directly
`
`Activation of mTOR signaling; antioxidant; regulation of hormone secretion; allosteric activation of NAG synthase;
`ammonia detoxification; regulation of gene expression; immune function; activation of BH4 synthesis; N reservoir;
`methylation of proteins; deimination (formation of citrulline) of proteinsa
`
`NO
`
`Agmatine
`
`Ornithine
`
`Signaling molecule; regulator of nutrient metabolism, vascular tone, hemodynamics, angiogenesis, spermatogenesis,
`embryogenesis, fertility, immune function, hormone secretion, wound healing, neurotransmission, tumor growth,
`mitochondrial biogenesis, and function
`Inhibition of NOS, ODC, and monoamine oxidase; ligand for a2 adrenergic and imidazoline receptors
`Ammonia detoxification; syntheses of proline, glutamate, and polyamines; mitochondrial integrity; wound healing
`
`Methylarginines
`
`Competitive inhibition of NOS
`
`Asparagine
`
`Directly
`
`Cell metabolism and physiology; regulation of gene expression and immune function; ammonia detoxification;
`function of the nervous system
`
`Acrylamideb
`
`Oxidant; cytotoxicity; gene mutation; food quality
`
`Aspartate
`
`Directly
`
`Purine, pyrimidine, asparagine, and arginine synthesis; transamination; urea cycle; activation of NMDA receptors;
`synthesis of inositol and b alanine
`
`Citrulline
`
`Cysteine
`
`Glutamate
`
`Glutamine
`
`Directly
`
`Directly
`
`Taurine
`
`H2S
`
`Directly
`
`GABA
`
`Directly
`
`Glu and Asp
`
`Antioxidant; arginine synthesis; osmoregulation; ammonia detoxification; N reservoir
`
`Disulfide linkage in protein; transport of sulfur
`
`Antioxidant; regulation of cellular redox state; osmolyte
`
`A signaling molecule
`
`Glutamine, citrulline, and arginine synthesis; bridging the urea cycle with the Krebs cycle; transamination; ammonia
`assimilation; flavor enhancer; activation of NMDA receptors; NAG synthesis
`
`Excitatory neurotransmitter; inhibition of T cell response and inflammation
`
`Regulation of protein turnover through cellular mTOR signaling, gene expression, and immune function; a major fuel
`for rapidly proliferating cells; inhibition of apoptosis; syntheses of purine, pyrimidine, ornithine, citrulline, arginine,
`proline, and asparagines; N reservoir; synthesis of NAD(P)
`
`Excitatory neurotransmitters; components of the malate shuttle; cell Metabolism; ammonia detoxification; major fuels
`for enterocytes
`
`Glucosamine 6 P Synthesis of aminosugars and glycoproteins; inhibition of NO synthesis
`
`Glycine
`
`Histidine
`
`Ammonia
`
`Directly
`
`Heme
`
`Directly
`
`Renal regulation of acid base balance; synthesis of glutamate and CP
`
`Calcium influx through a glycine gated channel in the cell membrane; purine and serine synthesis; synthesis of
`porphyrins; inhibitory neurotransmitter in CNS; co agonist with glutamate for NMDA receptors
`
`Hemoproteins (e.g., hemoglobin, myoglobin, catalase, and cytochrome c); production of CO (a signaling molecule)
`
`Protein methylation; hemoglobin structure and function; antioxidative dipeptides; one carbon unit metabolism
`
`Histamine
`
`Allergic reaction; vasodilator; central acetylcholine secretion; regulation of gut function
`
`Urocanic acid
`
`Modulation of the immune response in skin
`
`Isoleucine
`
`Leucine
`
`Directly
`
`Directly
`
`Synthesis of glutamine and alanine; balance among BCAA
`
`Regulation of protein turnover through cellular mTOR signaling and gene expression; activator of glutamate
`dehydrogenase; BCAA balance; flavor enhancer
`
`Gln and Ala
`
`Interorgan metabolism of nitrogen and carbon
`
`Lysine
`
`HMB
`
`Directly
`
`Regulation of immune responses
`
`Regulation of NO synthesis; antiviral activity (treatment of Herpes simplex); Protein methylation (e.g., trimethyllysine
`in calmodulin), acetylation, ubiquitination, and O linked glycosylation
`
`OH lysine
`
`Structure and function of collagen
`
`Methionine
`
`Homocysteine
`
`Oxidant; independent risk factor for CVD; inhibition of NO synthesis
`
`Betaine
`
`Choline
`
`Cysteine
`
`DCSAM
`
`Taurine
`
`Methylation of homocysteine to methionine; one carbon unit metabolism
`
`Synthesis of betaine, acetylcholine, phosphatidylcholine, and sarcosine
`
`Cellular metabolism and nutrition
`
`Methylation of proteins and DNA; polyamine synthesis; gene expression
`
`Antioxidant; osmoregulation; organ development; vascular, muscular, cardiac, and retinal functions; anti inflammation
`
`Phospholipids
`
`Synthesis of lecithin and phosphatidylcholine cell signaling
`
`Phenylalanine
`
`Directly
`
`Activation of BH4 (a cofactor for NOS) synthesis; synthesis of tyrosine; neurological development and function
`
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`
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`

`

`4
`
`Table 2 continued
`
`G. Wu
`
`AA
`
`Products
`
`Major functions
`
`Proline
`
`Directly
`
`Collagen structure and function; neurological function; osmoprotectant
`
`H2O2
`
`P5C
`
`Killing pathogens; intestinal integrity; a signaling molecule; immunity
`
`Cellular redox state; DNA synthesis; lymphocyte proliferation; ornithine, citrulline, arginine and polyamine synthesis;
`gene expression; stress response
`
`OH proline
`
`Structure and function of collagen
`
`Sarcosine
`
`Directly
`
`Serine
`
`Directly
`
`Glycine
`D Serinec
`
`Directly
`
`Theanine
`
`An intermediate in the synthesis of glycine from choline; possible treatment of certain mental disorders; a source of
`formaldehyde and H2O2; inhibition of glycine transport
`
`One carbon unit metabolism; syntheses of cysteine, purine, pyrimidine, ceramide and phosphatidylserine; synthesis of
`tryptophan in bacteria; gluconeogenesis (particularly in ruminants); protein phosphorylation
`
`Antioxidant; one carbon unit metabolism; neurotransmitter
`
`Activation of NMDA receptors in brain
`
`An amino acid (glutamine analog) in tea leaves; antioxidant; increasing levels of GABA, dopamine, and serotonin in
`brain; neuroprotective effect
`
`Threonine
`
`Directly
`
`Synthesis of the mucin protein that is required for maintaining intestinal integrity and function; immune function;
`protein phosphorylation and O linked glycosylation; glycine synthesis
`
`Tryptophan
`
`Serotonin
`
`Neurotransmitter; inhibiting production of inflammatory cytokines and superoxide
`
`NAS
`
`Inhibitor of BH4 synthesis; antioxidant; inhibition of the production of inflammatory cytokines and superoxide
`
`Melatonin
`
`Antioxidant; inhibition of the production of inflammatory cytokines and superoxide
`
`ANS
`
`Niacin
`
`Directly
`
`Tyrosine
`
`Inhibiting production of proinflammatory T helper 1 cytokines; preventing autoimmune neuroinflammation;
`enhancing immune function
`
`A component of NAD and NADP, coenzymes for many oxidoreductases
`
`Protein phosphorylation, nitrosation, and sulfation
`
`Dopamine
`
`Neurotransmitter; regulation of immune response
`
`EPN and NEPN Neurotransmitters; cell metabolism
`
`Valine
`
`Melanin
`
`Directly
`
`Arg and Met
`
`Polyamines
`
`Arg, Met, and
`Gly
`
`Cys, Glu, and
`Gly
`
`Creatine
`
`Glutathione
`
`Antioxidant; inhibition of the production of inflammatory cytokines and superoxide
`
`Synthesis of glutamine and alanine; balance among BCAA
`
`Gene expression; DNA and protein synthesis; ion channel function; apoptosis; signal transduction; antioxidants; cell
`function; cell proliferation and differentiation
`
`Antioxidant; antiviral; antitumor; energy metabolism in muscle and brain; neurological and muscular development and
`function
`
`Free radical scavenger; antioxidant; cell metabolism (e.g., formation of leukotrienes, mercapturate,
`glutathionylspermidine, glutathione NO adduct and glutathionylproteins); signal transduction; gene expression;
`apoptosis; cellular redox; immune response
`
`Gln, Asp, Gly,
`and Ser
`
`Nucleic acids
`
`Coding for genetic information; gene expression; cell cycle and function; protein and uric acid synthesis; lymphocyte
`proliferation
`
`Uric acid
`
`Antioxidant; the major end product of amino acid oxidation in avian species
`
`Lys, Met, and Ser Carnitine
`
`Transport of long chain fatty acids into mitochondria for oxidation; storage of energy as acetylcarnitine; antioxidant
`
`ANS anthranilic acid, BCAA branched chain AA, BH4 tetrahydrobiopterin, CNS central nervous system, CP carbamoylphosphate, CVD cardiovascular
`disease, DCSAM decarboxylated S adenosylmethionine, EPN epinephrine, GABA c aminobutyrate, HMB b hydroxy b methylbutyrate, NAG N acetylglu
`tamate, NAS N acetylserotonin, NEPN norepinephrine, NOS NO synthase, ODC ornithine decarboxylase, P5C pyrroline 5 carboxylate, Tau-Cl taurine
`chloramine
`a Including myelin basic protein, filaggrin, and histone proteins
`b Formed when asparagine reacts with reducing sugars or reactive carbonyls at high temperature
`c Synthesized from L serine by serine racemase
`
`for isoleucine. However, glutamine is a preferred major
`fuel for rapidly dividing cells,
`including enterocytes,
`lymphocytes, macrophages, and tumors (Curthoys and
`Watford 1995; Rhoads et al. 1997). The major objective
`of this article is to provide insights into new develop-
`ments
`in AA nutrition research, as well as
`their
`implications for both nutrition and health.
`
`Definitions of essential, non-essential, and functional
`AA
`
`On the basis of needs from the diet for nitrogen balance or
`growth, AA were traditionally classified as nutritionally
`essential (indispensable) or non-essential (dispensable) for
`humans and animals (Table 4). Essential AA (EAA) are
`
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`

`

`Amino acids
`
`Table 3 Energetic efficiency of oxidation of amino acids, protein, and other substrates in animals
`
`Nutrients
`
`Combustion energya
`
`Net atp production
`
`kJ per
`
`mol AA
`
`1,577
`
`3,739
`
`1,928
`
`1,601
`
`2,249
`
`2,244
`
`2,570
`
`973
`
`3,213
`
`3,581
`
`3,582
`
`3,683
`
`g AA
`
`17.7
`
`21.5
`
`14.6
`
`12.0
`
`18.6
`
`15.3
`
`17.6
`
`13.0
`
`20.7
`
`27.3
`
`27.3
`
`25.2
`
`mol per
`
`mol AA
`
`16
`
`29
`
`14
`
`16
`
`13
`
`25
`
`23
`
`13
`
`21
`
`41
`
`40
`
`35
`
`Alanine
`
`Arginine
`
`Asparagine
`
`Aspartate
`
`Cysteine
`
`Glutamate
`
`Glutamine
`Glycinec
`
`Histidine
`
`Isoleucine
`
`Leucine
`
`Lysine
`
`5
`
`Efficiency of energy
`transfer to ATPb (%)
`
`g AA
`
`0.180
`
`0.167
`
`0.106
`
`0.120
`
`0.107
`
`0.170
`
`0.157
`
`0.173
`
`0.135
`
`0.313
`
`0.305
`
`0.239
`
`52.4
`
`40.0
`
`37.5
`
`51.6
`
`29.8
`
`57.5
`
`46.2
`
`68.9
`
`33.7
`
`59.1
`
`57.6
`
`49.0
`
`28.6
`
`Methionine
`
`Ornithine
`
`Phenylalanine
`
`Proline
`
`Serine
`
`Threonine
`
`Tryptophan
`
`Tyrosine
`
`Valine
`Proteind
`
`Glucose
`Starche
`
`Palmitate
`Fatf
`
`3,245
`
`3,030
`
`4,647
`
`2,730
`
`1,444
`
`2,053
`
`5,628
`
`4,429
`
`2,922
`
`2,486
`
`2,803
`
`2,779
`
`9,791
`
`31,676
`
`23.0
`
`22.9
`
`28.1
`
`23.7
`
`13.7
`
`17.2
`
`27.6
`
`24.4
`
`25.0
`
`22.6
`
`15.6
`
`17.2
`
`38.2
`
`39.3
`
`18
`
`29
`
`39
`
`30
`
`13
`
`21
`
`38
`
`42
`
`30
`
`24
`
`38
`
`38
`
`129
`
`409
`
`0.121
`
`0.219
`
`0.236
`
`0.261
`
`0.124
`
`0.176
`
`0.186
`
`0.232
`
`0.256
`
`0.218
`
`0.211
`
`0.235
`
`0.504
`
`0.507
`
`49.4
`
`43.3
`
`56.7
`
`46.5
`
`52.8
`
`34.8
`
`48.9
`
`53.0
`
`49.8
`
`70.0
`
`70.6
`
`68.0
`
`66.6
`
`a Adapted from Cox (1970)
`b Calculated on the basis of 51.6 kJ/mol for one high energy bond in ATP (moles of net ATP production/mol substrate 9 51.6 kJ/
`mol 7 combustion energy of kJ/mol substrate 9 100)
`c When 1 mol glycine is catabolized by the glycine cleavage system, 1 mol ATP is produced. When 1 mol glycine is converted to serine and
`then oxidized, 13 mol ATP are produced
`d Assuming that the average molecular weight of an AA residue in protein is 110
`e The average molecular weight of glucose residue in starch is 162
`f Tripalmitoylglycerol is used as an example
`
`defined as either those AA whose carbon skeletons cannot
`be synthesized or those that are inadequately synthesized
`de novo by the body relative to needs and which must be
`provided from the diet
`to meet optimal requirements.
`Conditionally essential AA are those that normally can be
`synthesized in adequate amounts by the organism, but
`which must be provided from the diet to meet optimal
`needs under conditions where rates of utilization are
`greater than rates of synthesis. However, functional needs
`(e.g., reproduction and disease prevention) should also be a
`
`criterion for classification of essential or conditionally
`essential AA. Non-essential AA (NEAA) are those AA
`which can be synthesized de novo in adequate amounts
`by the body to meet optimal requirements. It should be
`recognized that all of the 20 protein AA and their metab-
`olites are required for normal cell physiology and function
`(El Idrissi 2008; Lupi et al. 2008; Novelli and Tasker 2008;
`Phang et al. 2008). Abnormal metabolism of an AA
`disturbs whole body homeostasis,
`impairs growth and
`development, and may even cause death (Orlando et al.
`
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`

`

`6
`
`G. Wu
`
`Table 4 EAA and NEAA in mammals, fish and poultry
`
`Mammals and fish
`
`EAA
`
`NEAA
`
`Argininea
`Histidine
`
`Isoleucine
`
`Leucine
`
`Lysine
`
`Methionine
`
`Phenylalanine
`
`Threonine
`
`Tryptophan
`
`Valine
`
`Alanine
`
`Asparagine
`
`Aspartate
`Cysteine2
`Glutamate
`Glutamineb
`Glycine
`Prolinec
`Serine
`Taurined
`
`Tyrosine
`
`Poultry
`
`EAA
`
`Arginine
`
`Glycine
`
`Histidine
`
`Isoleucine
`
`Leucine
`
`Lysine
`
`Methionine
`
`NEAA
`
`Alanine
`
`Asparagine
`
`Aspartate
`Cysteineb
`Glutamate
`Glutamineb
`Serine
`
`Phenylalanine
`
`Taurine
`
`Tyrosine
`
`Proline
`
`Threonine
`
`Tryptophan
`
`Valine
`
`a Arginine is an EAA for young mammals. Although it may not be
`required in the diet to maintain nitrogen balance in the adults of most
`species (including humans, pigs, and rats), dietary deficiency of
`arginine can result in metabolic, neurological or reproductive dys
`function. Thus, on the basis of
`functional needs, arginine is
`considered an EAA for vascular homeostasis, spermatogenesis, and
`fetal growth
`b Conditionally essential AA in neonates and under stress conditions
`c EAA for young pigs and some fish
`d EAA for carnivores (e.g., cats), neonates, and some fish
`
`2008; Willis et al. 2008; Wu et al. 2004c). Growing evi-
`dence shows that besides their role as building blocks of
`proteins and polypeptides, some AA are important regulators
`of key metabolic pathways that are necessary for mainte-
`nance, growth, reproduction, and immunity in organisms,
`therefore maximizing efficiency of
`food utilization,
`enhancing protein accretion,
`reducing adiposity, and
`improving health (Suenaga et al. 2008; Wu et al. 2007a, b,
`c). They are called functional AA, which include arginine,
`cysteine, glutamine, leucine, proline, and tryptophan.
`
`Dynamic changes of AA in physiological fluids
`
`Concentrations of AA in plasma are maintained relatively
`constant
`in the post-absorptive state of healthy adults.
`However, circulating levels of most AA undergo marked
`changes during the neonatal period, under catabolic condi-
`tions and in disease (Field et al. 2002; Flynn et al. 2000;
`Manso Filho et al. 2009). Additionally, results of recent
`studies indicate dynamic changes of free AA in milk (Haynes
`et al. 2009), skeletal muscle of lactating mammals (Clowes
`et al. 2005), and fetal fluids during pregnancy (Gao et al.
`2009a; Kwon et al. 2003a). For example, concentrations of
`free glutamine in sow’s milk increase from 0.1 to 4 mM
`between days 1 and 21 of lactation (Wu and Knabe 1994) and
`
`1 3
`
`those in ovine allantoic fluid increase from 0.1 to 25 mM
`between days 30 and 60 of gestation (Kwon et al. 2003a). In
`contrast, intramuscular glutamine levels decrease by[50%
`in lactating sows (Clowes et al. 2005) and mares (Manso
`Filho et al. 2009), compared with their nonlactating coun-
`terparts; therefore, restoring intramuscular glutamine may
`provide a novel strategy to enhance milk production by
`mammals. Strikingly, arginine, ornithine, and citrulline are
`unusually abundant in porcine allantoic fluid (e.g., 4 6 mM
`arginine on day 40) and ovine allantoic fluid (e.g., 10 mM
`citrulline on day 60) during early to mid-gestation, compared
`with their plasma levels (e.g., 0.1 0.2 mM arginine and
`citrulline) (Wu et al. 1996b; Kwon et al. 2003a). These three
`AA plus glutamine represent approximately 70% of total
`a-AA nitrogen in the fetal fluids. The great increase (up to
`80-fold) in their concentrations in allantoic fluid occurs
`during the most rapid period of placental growth. More
`recently, Gao et al. (2009a) reported that total recoverable
`amounts of glutamine, leucine, and isoleucine in ovine
`uterine flushings increased by 20-, 3-, and 14-fold, respec-
`tively, between days 10 and 15 of pregnancy, whereas those
`of arginine, histidine, ornithine, and lysine increased 8-, 22-,
`5-, and 28-fold, respectively, between days 10 and 16. Such
`dynamic changes of AA in physiological fluids support the
`view that these nutrients play a crucial role in growth and
`development of the fetus and neonate.
`
`Interorgan metabolism of AA and extensive catabolism
`of AA in the gut
`
`Several NEAA (including glutamine, glutamate, and aspar-
`tate) are extensively oxidized by absorptive epithelial cells
`(enterocytes) of the mammalian small intestine, such that
`nearly all of them in a conventional diet do not enter the
`portal vein (Stoll et al. 1998; Wu1998 ). Nitrogenous prod-
`ucts include ornithine, citrulline, arginine, and alanine. The
`small intestine utilizes glutamine from both the arterial cir-
`culation and intestinal lumen, but takes up glutamate and
`aspartate only from the intestinal lumen. The circulating
`glutamine is synthesized from branched-chain AA (BCAA)
`and a-ketoglutarate (derived primarily from glucose) in
`skeletal muscle, adipose tissue, heart, and placenta (Curth-
`oys and Watford 1995; Self et al. 2004). Enterocytes also
`actively degrade proline via the proline oxidase pathway to
`produce ornithine, citrulline, and arginine (Wu 1997). In
`adult mammals, the citrulline released from the small intes-
`tine is converted into arginine primarily in kidneys and, to a
`lesser extent, in other cell types (including endothelial cells,
`leukocytes, and smooth muscle cells) (Fig. 1). However, in
`neonates, most of the gut-derived citrulline is utilized locally
`for arginine synthesis (Wu and Morris 1998). Of particular
`note, enterocytes of post-weaning mammals have a high
`
`Page 6 of 17
`
`

`

`Amino acids
`
`—: Skeletal Muscle .—
`
`BCAA
`
`Glutamine
`
`
`
`AA
`
`BCKA Alanine
`
`
`
`G H Glucose
`
`
`
`Cells of the
`
`
`immune system
`GSH¢— Gln ->ATP<- Glucose
`
`flag NADPH +02
`+H" 7) As /<
`
`Cit
`p
`
`
`0;
`Glucose + Arginine
`NO
`
`NADP‘
`
`Fig. 1 Interorgan metabolism of branched chain amino acids. gluta
`mine and arginine and its role in immune fimction. Skeletal muscle
`lakes up BCAA from the arterial blood. synthesizes both alanine and
`glutamine from BCAA and at ketoglutarate. and releases these two
`amino acids into the circulation. The small intestine utilizes glutamine
`to synthesize citrulline, which is converted into arginine in kidneys.
`cells of the immune system. and other cell types. The liver is the
`primary organ for the synthesis of glutathione from glutamate.
`glycine. and cysteine and of glucose from alanine for use by
`extrahepatic cells (including immunocytes) and tissues. Arg arginine.
`Asp aspartate, Cit citrulline, BCKA branched chain at ketoacids. Glue
`glucose. GSH glutathione. Reprinted from British Journal of Nutrition
`Li et al. (2007) with permission from The Nutrition Society
`
`l996c) via both
`ability to catabolize arginine (Wu et al.
`cytosolic type I and mitochondrial type II arginase (Davis
`and Wu 1998), which contributes to the extensive intestinal
`
`nitrogen recycling (Fuller and Redes 1998). As a mechanism
`for sparing proline and arginine carbons, their oxidation to
`C02 is limited in mucosal cells of the gut due to a low activity
`of pyrroline-S-carboxylate dehydrogenase (Wu 1997).
`An exciting new aspect of AA nutrition is the finding that
`30 50% of EAA in the diet may be catabolized by the small
`intestine in first-pass metabolism (Stoll et al. 1998; Wu 1998).
`For example, in milk protein-fed piglets, 40% of leucine, 30%
`of isoleucine, and 40% of valine in the diet were extracted by
`the portal-drained viscera (PDV) in first-pass, with d0% of
`the extracted BCAA being utilized for intestinal mucosal
`protein synthesis (Stoll et a]. 1998). Similarly, large amounts
`of BCAA were catabolized by the sheep gastrointestinal tract
`(El-Kadi et a1. 2006). This is consistent with a high activity of
`BCAA transaminase in intestinal mucosa] cells (Chen et al.
`
`2007, 2009). Accordingly, BCAA are actively transarninated
`in enterocytes to yield branched-chain a—ketoacids at rates
`
`Page 7 of 17
`
`comparable to those in skeletal muscle of young rats and
`chickens (Wu and Thompson I987). The concept of intestinal
`AA metabolism has important implications for understanding
`efficiency of AA utilization and defining protein/AA
`requirements by humans and animals. The ammonia gener—
`ated from intestinal AA catabolism either enters the portal
`vein or is utilized locally for urea synthesis (Wu 1995). The
`presence of a functional urea cycle in enterocytes serves as the
`first line of defense against ammonia toxicity in mammals.
`Methionine, phenylalanine, lysine, threonine, and histi-
`dine were traditionally considered not to be catabolized by
`the intestinal mucosa (Wu 1998). However, Stoll et al.
`(1998) demonstrated that 50% of lysine and methionine,
`45% of phenylalanine, and 60% of threonine in the diet
`were extracted in first-pass metabolism by the PDV of milk
`protein-fed pigs, with 30% of the extracted AA being
`catabolized by the small intestine. In addition, van Gou—
`doever et al.
`(2000) found that
`intestinal oxidation of
`enteral
`lysine contributed one-third of total body lysine
`oxidation in growing pigs fed a high-protein diet. Subse-
`quently, Riedijk et
`al.
`(2007) discovered extensive
`transmethylation and transsulfuration of methionine in the
`piglet gastrointestinal
`tract. Collectively,
`these in vivo
`findings suggest extensive oxidation of EAA in the gut.
`Using the viable technique of enterocyte incubation,
`Chen et al. (2007) reported that there was no production of
`C02 or tricarboxylic-acid-cycle intermediates from carbon-
`1 or all carbons of lysine, histidine, threonine, and tryp—
`tophan in enterocytes of post—weaning pigs. Likewise,
`oxidation of methionine and phenylalanine in enterocytes
`was quantitatively negligible (Chen et al. 2007). Consistent
`with the metabolic data, there were no detectable activities
`
`of saccharopine dehydrogenase, threonine dehydrogenase,
`threonine hydratase, histidine decarboxylase, or phenylal-
`anine hydroxylase in pig enterocytes (Chen et al. 2009).
`These results provide direct evidence for the lack of
`quantitatively significant catabolism of histidine,
`lysine,
`methionine, phenylalanine,
`threonine, and tryptophan in
`intestinal mucosal cells. The reported extensive catabolism
`of these EAA by the pig small intestine may result from the
`action of lumenal microbes (Fuller and Redes 1998). This
`may help explain why dietary supplementation with anti—
`biotics or prebiotics improves efficiency of utilization of
`dietary AA for protein deposition and growth performance
`in pigs (Deng et al. 2007; Kong et a1. 2008).
`
`Regulatory roles of AA
`
`Gene expression
`
`Regulation of gene expression by AA can occur in any step
`of the highly specific processes that involve the transfer of
`
`E Springer
`
`

`

`Fig. 2 Possible mechanisms
`responsible for AA regulation of
`gene expression in cells. AA
`may regulate gene expression in
`animal cells at the levels of
`transcription, translation, and
`post translational protein
`modifications. Post translational
`protein modifications include
`acetylation. ADP ribosylation,
`biotinylation. y carboxylation,
`disulfide linkage. flavin
`attachment. glutamylation,
`glycation. glyeosylation,
`glycylation. heme attachment,
`hydroxylation, methylation,
`myristoylation. nitrosylation.
`oxidation, phosphorylation.
`palmitoylation. proteolytic
`cleavage, racemization,
`selenoylation. sulfation, and
`ubiquitination
`
`Synthesisip I\\\I/’\\\I/,\\\I/I\\\I/’\\\I/’\\\/ A—A>1)egradation
`DNA
`
`G. Wu
`
`
`
`
`Epigenetics
`AA
`
`DNA methylation
`
`Methylation and
`aeetylation of
`historic proteins
`
`Non-histone
`DNA -binding
`chromosomal p roteins
`
`1
`
`—D Chromatin Structure
`
`Transcription factor assembly
`Promoter actiw'ty
`. ..
`A
`.
`.
`.
`Transcription :> RNA polymerase lnltlatlon
`Polyad enylation
`mRNA splicing, export and stability
`
`mRNA
`
`Ribosome number and activity
`AA ®-mTORl
`Translation :) ®-86K1
`®-4EBP1
`®-eEF2
`
`Post-translational AA
`modifications
`
`Addition of functional groups
`Addition of polypeptides
`Change in AA chemical nature
`Change in protein structure
`
`‘—
`
`Modified Protein
`
`(RNA
`information encoded in a gene into its product
`and/or protein)
`(Fig. 2). These biochemical events are
`transcription, transla