O NH? L-Glutamine





Carbamoyl synihetase II

Carbamoyl phosphate

Figur* 8.13 L-Qut amine meiaboliim


Gin is the must abundant free amino acid in most cell types, with concentrations of several mmol 1. Muscle proteins contain 4 5% Gin (Kuhn et ai. 1999), of which about 0.01% is turned over per hour, A larger percentage can be mobilized in response to starvation, trauma, or infection.


Losses into feces arc negligible as long as gastrointestinal function is normal.

The kidneys filter about 3.3 g per day, most of which is reabsorbed under most conditions. The proximal renal tubules take up free Gin mainly through the sodium-ammo acid cotransport system B" {ASCT2, Avissar et ai. 2001; Bode, 2001 ), di- and tripeptides via pepTl and pepT2. Gin, if it is not metabolized in the renal epithelial cells, is then exported across the basolateral membrane via the sodium-dependent transporters ATA2 and ASC'Tl.

Most nitrogen from metabolized Gin is excreted into urine as urea, a much smaller amount as free ammonium ions.


Fasting, starvation, a lack of glucose, and high protein intake promote Gin deami nation. Glucagon is a major mediator of this effect. A cAMP-responsive clement increases expression ofglutaminasc (EC' in liver, kidneys and other tissues (Labow et al.. 2001).

Stress and infection increase Gin synthesis, acting to a large extent through glucocorticoid hormones. Several glucocorticoid response elements in the upstream transcription initiation site and the lirst intron of the giutamate ammonia ligase (EC6.3.1.2) gene promote its expression (Labow et ai. 20011, particularly in lung. Additional post-translattonal mechanisms appear to contribute to the tuning of enzyme activity, Gin concentration decreases synthesis by accelerating giutamate ammonia ligase degradation (Labow et a/., 2001),


Protein synthesis. Gin is a constituent of most proteins and peptides synthesized in the body. Glutamine-tRNA ligase (EC6,1.1.18) links Gin with its correspondent t-RNA in an ATP magnesium-dependent reaction.

Enerqy fuel: The u-ketoglutarate released upon transamination of Gin can be completely oxidized as an energy fuel yielding 3.089 kcal/g (May and Hill. 199(1). The necessary reactions are dependent on niacin, thiamin, ribollav in, pyridoxinc. pantothenate; Itpoalc. ubiquinone, iron, and magnesium. Gin is a major fuel for lymphocytes, macrophages, and other immune cells (Newsholme, 20011, w hich may explain a potentially beneficial role in times of heightened immune activity (trauma and infection).

Figure 8.14 The GABAgluramaie-glutamine cycle refurbishes neurotransmitter* in astroglia cells for renewed use by neurons

Neurotransmitter cycling. Gin is the critical precursor of L-glutamate and y-aminobutyric acid (GABA) in brain. After their release from neurons these neurotransmitters are laken up by astrocytes, converted into Gin. and transferred to neurons for renewed use (Behar and Rothman, 2001). 4-Ammobutyrate aminotransferase (GABA transaminase; EC2.6, 1.19) uses the amino group ofGABA for the synthesis of glutamate and succinate scm¡aldehyde dehydrogenase (EC1.2.1.24) shunts the carbon skeleton into the Krebs cycle. Continued metabolism through the Krebs cycle and addition of acetyl-CoA from glucose utilization then generates a-ketoglutarate for the earlier mentioned glutamate synthesis. An astroglia-specifie glutamate ammonia ligasc iglut-amine synthase: F.C6.3.1.2) am ¡dates glutamate. The sodium-dependent transporters N1 and ASCT2 export Gin and neurons can take it up v ia transporters ATA1 and ATA2 (Behar and Rothman. 2001). Phosphate-activated glutaminase (EC' in neuronal mitochondria then produces glutamate as needed.

Nucleotide synthesis; Gin provides two of the purine nucleotide nitrogens and one of the pvrimidme nucleotide nitrogens. The first step of purine synthesis is the transfer of the y-amino group from Gin to 5-phospho-a-D-ribosyl 1-pyrophosphate (PRPP). catalyzed by amidophosphori bosy Itran s ferase (EC2.4.2.14). Another amino group is transferred in the fourth step by phosphoribosylformylglycinamidine synthase (FGAM synthase; EC6.3.5.3). Pyrimidine nucleotide synthesis starts with the condensation reaction of Gin and bicarbonate to carbamoyl phosphate, catalyzed by glutamine-hydrolyzing carbamovl-phosphatc synthase (EC6.3 5.5).

Hexosamme synthesis: The synthesis of ammo sugars in glycoproteins (e.g. gapjunction proteins) and glyeans (glucosaminoglyeans in mucus, ehondroitins, kcratans, derma tans, hyaluronan, heparans and heparin in extracellular matrix) depends on an adequate supply of Gin. Glutamine: fhictose-6-phosphate transaminase isomerizing (GFAT; EC2.6.1.I6) transfers the y-amide from Gin to fiructose-6-phosphate to produce glucosamine-ii-phosphate. This key intermediate is the precursor of N-acetyl glucosamine. N-acetyl galactosamine, and other important hexosamines. pH regulation: An excess of free acids in blood (metabolic acidosisl increases the activity of glutaminase (EC3.5.1.2) and glutamate dehydrogenase NADlP) I (EC' I 4. 1.3) in renal distal and proximal tubular epithelia and causes the rapid release of ammonia into urine. The ammonia cations allow the increased secretion of anions and thereby facilitate an increase in blood pH. Since Gin transport across the basolateral tubular membrane reverses in response to low intracellular concentration, additional Gtn for ammonia production can be drawn from venous blood.


Avissar NE, Ryan CK. Ganapathy V. Sax Hi'. Na'-dependent neutral amino acid transporter ATBo is a rabhit epithelial cell brush border protein. Am J Physiol Cell Physio! 200l;28l:C963 71

Behar K1 . Rothinan 01 . hi vivo nuclear magnetic resonance studies of glutamate-y-a mi no butyric aeid-glutamme cycling in rodent and human cortex, the central role of glutamine../ Nuir 2001; 131:2498S-25i)4S Bode HP. Recent molecular advances in mammalian glutamine transport. J Nutr

2001:131:2475S-248SS Hammarqvist K Ejesson B, Wernerman J. Stress hormones initiate prolonged changes in the muscle amino acid pattern. Clin Physiol 2(K) 1:21:44 51) llatanaka I. Huang'W. Ling R. Prasad PD, Sugawara M. Lei bach l-IL Ganapathy V Evidence for the transport of neutral as well as cat ionic amino acids by ATA3. a novel and liver-spcc i tic subtype of amino acid transport system A. Biochim Biophys Acta 2001:1510:10-17

Indiveri C. Abbruzzo G, Strpani I. Palnucri E Identilication and purification of the reconsti-tutiveiy active glutamine carrier from rai kidney mitochondria. Biochem J 1998;333: 285-90

Kuhn KS. Schuhmann K. Stehle P. Durmaun D. Furst P. Determination of glutamine in muscle protein facilitates accurate assessment of proteolysis and de novo synthesis-derived endogenous glutamine production. Am J Clin Nutr 1999:70:484 li Labow HI. Souba WW, Abeouwer SF. Mechanisms governing the expression of the enzymes of glutamine metabolism glutaminase and glutamine synthetase.,/ Nutr 2001;131:24675 2474S May ME. Hill JO. Fncrgv content of diets ot variable amino acid composition, tin J Clin Nutr 1990;52:770 6

Muni/ R. Burguillo L, del Castillo JR. Effect of starvation on neutral amino acid transport in isolated small-intestinal cells from guinea pigs. Pj! Arch Eur J Physiol 1993; 423:59-66

Ncwsholme I' Why is 1 -glutamine metabolism important to cells of the immune system in health, postinjury, surgery or infection? J Murr 2001;2515S 2522S Sugawani M, Nakanishi T. Fei YJ, Huang W. Ganapathy ME, Leibach FH. Ganapathy V. Cloning of an amino acid transporter with functional characteristics and tissue expression pattern identical to that of system A, J Biol Client 2000:275:1647V 7 Varoqui H. /hu H, Yao D. Ming H, Erickson .ID Cloning and functional identification of a neuronal glutamine transporter. J Biol Chettt 20(10:275:4049 54


COOH Glycine I aminoacetic acid, aminoethanoic acid, glycocoll, Gyn-Hydralin. Glycosthene.

I one-letter code G: molecular weight 75) is a nonessential small neutral amino acid con-

taming 18.7% nitrogen.

Figure 8.15 Glycine


CoA coenzyme A

Gly glycine

GSH glutathione (reduced)

RDA recommended dietary allowance

Nutritional summary

Function: The nonessential amino acid glycine (Gly) is an inhibitory neurotransmitter. It is needed for the synthesis of peptides and proteins, creatine, glutathione, porphyrins and purines, and for the conjugation of bile acids and xenobiotics. Gly breakdown requires thiamin, riboflavin, niacin, vitamin B6, folate, vitamin BI2. pantothenate, lipoate. ubiquinone, iron, and magnesium.

Food sources: Adequate amounts are consumed when total protein intakes meet recommendations. Dietary supplements containing cry stalline Gly are commercially available. Requirements: With adequate total protein intake enough Gly is available directly and from conversion of serine and threonine.

Deficiency: Prolonged lack of Gly in protein deficiency causes growth failure, loss of muscle mass and organ damage.

Excessive intake: Very high intake of protein and or amino acids (more than three times the RDA or 2.4g'kg) is thought to increase the risk of renal glomerular sclerosis and accelerate osteoporosis. A small percentage is converted to oxalate which may increase the risk of kidney stones.

Endogenous sources

Daily endogenous production of glycine is around 125 mg'kg body weight with plentiful total protein intake, and slightly higher with marginally low protein intake (Gibson



ch3 L-Threonine



oh L-Serine

5,10 methylene THF THF

Serme hydroxy-metnyltransterase




L-Glutamme, L-Alanine. L-Phenylalanine. L-Tyrosme. L-Histidine. Kynurenme (Trp)

Figur« 8.16 Endogenous glycine iynthwii el at., 2002). Glycine is generated from serine (by glycine hydroxymethy¡transferase; EC2.1.2.1). threonine, choline or betaine, and transamination of glyoxylate.

The threonine cleavage complex generates a small amount of glycine in a two-stop sequence. L-Threonine dehydrogenase (EC 1.1.11)3) oxidizes threonine to 2-amino 3-ketobutyrate. The PLP-dependent 2-amino-3-ketobutyrate coenzyme A ligase (EC then links the acetyl moiety to CoA and releases glycine.

Choline and its metabolites can be metabolized to Gly \ia betaine aldehyde, betaine, dimethyl glycine, and sarcosine.

Pyndoxal 5-phosphate (PLP)-dependent transamination of glyoxylate by glycine aminotransferase (EC2.6.1.4), alanine-glyoxylate aminotransferase (EC2.6.1.-I4|, aromatic-ami no-acid—glyoxylate aminotransferase (EC2.6.1.60), or kynurenine-glyoxylate aminotransferase {EC2.6.1.63) provides additional small quantities of glycine.

Dietary sources

All food proteins contain Gly. Proteins with a relatively high percentage of Gly are in beef (55 mg/g protein) and pork (57 mg. g). Milk is at the other end of the spectrum, with only 21 mg g. The typical intake of American adults is 3X mg kg body weight or more (Gibson et til., 2002).

Digestion and absorption

Protein in food is denatured by gastric acid and the action of gastric, pancreatic, and enteric enzymes, many of them cleav ing peptide bonds between specific amino acids and Gly.

Intestinal lumen

Brush border membrane

Basolateral membrane

Figure 8.17 Intestinal absorption of glycine

Intestinal lumen

Brush border membrane

Basolateral membrane

Capillary lumen

Capillary endothelium

Figure 8.17 Intestinal absorption of glycine

Proteins as well as free Gly are nearly completely taken up from the smalt intestine; small residual amounts may also be absorbed from pans of the large intestine.

Gly as a component ot'di- or tripeptides is taken up via hydrogen ion;peptide cotrans-poner I (SLC15A1, PepTDand. to a lesser extent, 2 (SLC15A2, PepT2l.

The main conduit for Gly uptake from the intestinal lumen is the sodium-ammo acid cotransport system B" (Avissar el til, 2001). The sodium-independent rBAT (SLC3AI l glycoprotein-anchorcd transporter HAT I/b0-4 (SLC7A9) uses Scr in most situations as a counter molecule in exchange for the transport of other neutral amino acids and usually effects net Scr transport into the lumen.

The sodium- and chloride-dependent high-affinity transporter GLYTI at the haso-lateral membrane is a major route for concentration-dependent t ilv exchange with the pericapillary space (Christie ct at., 2001). The sodium-dependent transport systems A (ATA21. ASC (ASCTI) and N also accept Gly, The sodium-independent complex 4F2 LAI 2 (SLC3A2/SLC7A8) mediates transport m either direction in exchange for another neutral amino acid.

Transport and cellular uptake

Blood circulation: Plasma concentrations of Gly are typically around 24X praol I. Uptake from hlood into tissues occurs via transport systems A. ASC. N, and others (Barker et al., 1999). Expression patterns vary greatly between different cell types.

Blood brain barrier. LATl (SLC7A5) is expressed in brain capillary endothelial celts and is certain to contribute to Gly transport, hut its relative importance and the role of"other transporters is not completely understood.

Matemo-fetal transfer: While the sodium amino acid eotransport systems A, ASC'. and N arc available for uptake from maternal blood (Jansson, 2(H)!), there is no significant net extraction of Gly from maternal blood (Cetin, 2001). Substantial amounts of Gly arc. however, produced in the placenta from serine, and this is transported across the b&so-lateral membrane by the sodium -amino acid eotransport sy stem A (Anand et iiL, 19L)61. Transport system A and two additional sodium-dependent systems appear to contribute to Gly transfer into milk (Rchan ef a!.. 20(H)).


Conversion to serine. The gtycine-cleavage system (glycine hydroxymeihytransferase: EC2.1.2.1) converts Gly into serine by one-carbon transfer from 5.10-methylenete-trahydrofolate. Under some circumstances this reaction runs in the rev erse direction. The glycine cleavage system: Glycine is decarboxylated in mitochondria by a targe pyri-doxal 5-phosphate-dependent glycine dehydrogenase (EC1.4.4.21 complex composed of multiple subunits P.T. L. and II; the 11 subunit contains lipoamide. In a fashion similar to the three lipoate-dependent alpha-keto acid dehydrogenases, the lipoamide arm acts as an acceptor for a methylene group from glycine, transfers it to folate, and is reduced in the process.TheT subunit then transfers the hydrogen via FAD to NAD. Minor pathways. Alanine-glyoxylate aminotransferase (F.C2.6,1,44) in liver peroxisomes normally generates Gly by transferring the amino group from alanine to glv-oxylalc. Since this reaction is reversible, a small percentage (0.1%) of Gly is converted to glyoxyiatc and then to oxalate. Glyoxylate is also the product of the reaction of oxidative Gly deamination by the FAD-containing D-amino acid oxidase (EC1.4.3,3) in peroxisomes, and of several PLP-dependcnt transamination reactions (glycine aminotransferase; EC2.6,1.4: alanine-glyoxylate aminotransferase: EC2.6,1.44; aromatic-amino-acid glyoxylate aminotransferase; EC2.6.I.60: and kynurenine-glyoxylate aminotransferase; EC2.6.1.631,


On average, the proteins in the human body contain more than 5%. Gly is released during normal protein turnover.


Filtered free Gly is taken up into proximal renal tubules mainly by the sodium-amino acid eotransport system II" (Avissar el ut.. 2001). di- and tripeptides via PepTI and PepT2. Ala is then exported across the basolateral membrane via the sodium-dependent systems N (SN2). A (ATA2). and ASC (ASCT1). As a result of very efficient

5,10-methylene-letrahydrotolate +NH3

tetra hydro (otate

Dihydro-lipoamide SH SH

tetra hydro (otate


S-a mmome thy i dihydrolipoamide


OH L-Serine

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