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PubMed1993

Simultaneous determination of ammonia nitrogen and L-glutamine in bioreactor media using flow injection.

A novel split stream flow injection (FI) system suitable for the simultaneous determination of L-glutamine and ammonia nitrogen (ammonia-N) in cell culture media is described. Potentiometric detection of ammonia-N in one portion of the manifold is achieved using a commercial ammonia gas-sensing electrode fitted with a wall-jet cap. L-Glutamine is quantified in the other part of the split sample by potentiometric detection of ammonium ions (by an ammonium-selective polymer membrane electrode), liberated from the hydrolysis of glutamine after the sample flows through a glass bead reactor containing immobilized glutaminase. Endogenous ammonia-N and potassium ions that would normally interfere with the glutamine measurement are removed upstream using a unique tubular cation-exchange unit. Using 50 microliters sample volumes and mixed solutions of ammonium chloride and L-glutamine in Iscove's Modified Dulbecco's Medium to calibrate the FI measuring system, values for ammonia-N and L-glutamine determined for 22 media samples obtained from a bioreactor growing retroviral producer cells correlate well with those measured with commercial, manual enzymic-spectrophotometric assay kits.

Ammonia↗
PubMed2001

Inhibition of Escherichia coli CTP synthase by glutamate gamma-semialdehyde and the role of the allosteric effector GTP in glutamine hydrolysis.

Cytidine 5'-triphosphate synthase catalyses the ATP-dependent formation of CTP from UTP with either ammonia or glutamine as the source of nitrogen. When glutamine is the substrate, GTP is required as an allosteric effector to promote catalysis. Escherichia coli CTP synthase, overexpressed as a hexahistidine-tagged form, was purified to high specific activity with the use of metal-ion-affinity chromatography. Unfused CTP synthase, generated by the enzymic removal of the hexahistidine tag, displayed an activity identical with that of the purified native enzyme and was used to study the effect of GTP on the inhibition of enzymic activity by glutamate gamma-semialdehyde. Glutamate gamma-semialdehyde is expected to inhibit CTP synthase by reacting reversibly with the active-site Cys-379 to form an analogue of a tetrahedral intermediate in glutamine hydrolysis. Indeed, glutamate gamma-semialdehyde is a potent linear mixed-type inhibitor of CTP synthase with respect to glutamine (K(is) 0.16+/-0.03 mM; K(ii) 0.4+/-0.1 mM) and a competitive inhibitor with respect to ammonia (K(i) 0.39+/-0.06 mM) in the presence of GTP at pH 8.0. The mutant enzyme (C379A), which is fully active with ammonia but has no glutamine-dependent activity, is not inhibited by glutamate gamma-semialdehyde. Although glutamate gamma-semialdehyde exists in solution primarily in its cyclic form, Delta(1)-pyrroline-5-carboxylate, the variation of inhibition with pH, and the weak inhibition by cyclic analogues of Delta(1)-pyrroline-5-carboxylate (L-proline, L-2-pyrrolidone and pyrrole-2-carboxylate) confirm that the rare open-chain aldehyde species causes the inhibition. When ammonia is employed as the substrate in the absence of GTP, the enzyme's affinity for glutamate gamma-semialdehyde is decreased approx. 10-fold, indicating that the allosteric effector, GTP, functions by stabilizing the protein conformation that binds the tetrahedral intermediate(s) formed during glutamine hydrolysis.

Allosteric Regulation↗
PubMed2005

Increasing the culture efficiency of hybridoma cells by the use of integrated metabolic control of glucose and glutamine at low levels.

The metabolism of HAb18 hybridoma cells was shifted to decrease metabolite accumulation and to improve culture efficiency by integrated metabolic control of glucose and glutamine at low levels. When glucose and glutamine levels were decreased to 0.5 and 0.3 mM respectively, lactate and ammonia production were reduced by 62.6 and 74% respectively, glucose-to-cell yield was increased from 0.23x10(9) to 0.66x10(9) cells.mmol-1, and glutamine-to-cell yield from 0.18x10(9) to 1.95x10(9) cells.mmol-1. Compared with high-level glucose and glutamine fed-batch cultures, low-level glucose and glutamine led to higher cell density (1.0x10(7) versus 0.3x10(7) cells.ml-1), longer culture span (14 as opposed to 8 days) and higher antibody yield (250 as against 150 mg.l-1). These results indicate that hybridoma culture efficiency would be increased by the integrated control of glucose and glutamine at 0.5 and 0.3 mM respectively. In contrast with previously reported glucose-and/or-glutamine-level-controlled fed-batch cultures, we demonstrated an efficient strategy of nutrient level selection and amino acid feeding. More importantly, our accurately and well-distributed Equable Feeding Control System opens a new avenue for reducing metabolites to low levels by controlling nutrients at low levels.

Algorithms↗
PubMed1971

The pathway of glutamine and glutamate oxidation in isolated mitochondria from mammalian cells.

1. Pyruvate strongly inhibited aspartate production by mitochondria isolated from Ehrlich ascites-tumour cells, and rat kidney and liver respiring in the presence of glutamine or glutamate; the production of (14)CO(2) from l-[U-(14)C]glutamine was not inhibited though that from l-[U-(14)C]glutamate was inhibited by more than 50%. 2. Inhibition of aspartate production during glutamine oxidation by intact Ehrlich ascites-tumour cells in the presence of glucose was not accompanied by inhibition of CO(2) production. 3. The addition of amino-oxyacetate, which almost completely suppressed aspartate production, did not inhibit the respiration of the mitochondria in the presence of glutamine, though the respiration in the presence of glutamate was inhibited. 4. Glutamate stimulated the respiration of kidney mitochondria in the presence of glutamine, but the production of aspartate was the same as that in the presence of glutamate alone. 5. The results suggest that the oxidation of glutamate produced by the activity of mitochondrial glutaminase can proceed almost completely through the glutamate dehydrogenase pathway if the transamination pathway is inhibited. This indicates that the oxidation of glutamate is not limited by a high [NADPH]/[NADP(+)] ratio. 6. It is suggested that under physiological conditions the transamination pathway is a less favourable route for the oxidation of glutamate (produced by hydrolysis of glutamine) in Ehrlich ascites-tumour cells, and perhaps also kidney, than the glutamate dehydrogenase pathway, as the production of acetyl-CoA strongly inhibits the first mechanism. The predominance of the transamination pathway in the oxidation of glutamate by isolated mitochondria can be explained by a restricted permeability of the inner mitochondrial membrane to glutamate and by a more favourable location of glutamate-oxaloacetate transaminase compared with that of glutamate dehydrogenase.

Acetates↗
PubMed1977

Glutamine as a precursor to N-terminal pyrrolid-2-one-5-carboxylic acid in mouse immunoglobulin lambda-type light chains. Amino acid-sequence variability at the N-terminal extra piece of lambda-type light-chain precursors.

The mRNA molecules coding for three mouse immunoglobulin lambda-type light (L) chains (MOPC-104E lambda(1), RPC-20 lambda(1), MOPC-315 lambda(2)) programme the cell-free synthesis of precursors larger than the mature proteins. Radioactive amino acid-sequence analyses of each of the three precursors labelled with [(3)H]alanine, [(3)H]serine, [(3)H]glutamine, [(3)H]glutamic acid and [(3)H]threonine showed that an extra piece, at least 18 residues long, is linked to the N-terminus of the mature L-chains. The N-terminal extra-peptide segment may be 19 residues long, since analyses of precursors labelled with [(35)S]methionine indicated an additional N-terminal methionine residue which was recovered in low yields. Presumably this is the initiator methionine, which is known to be short lived in eukaryotes. The mature forms of MOPC-104E, RPC-20 and MOPC-315 lambda L-chains are blocked at the N-termini by pyrrolid-2-one-5-carboxylic acid (pyroglutamic acid). Sequence analyses of precursors labelled with [(3)H]glutamine and [(3)H]glutamic acid showed incorporation only of glutamine in a position that matches with the position of pyrrolid-2-one-5-carboxylic acid in the mature forms of all three precursors, and incorporation of glutamic acid in other positions. The data showed the absence of glutamine-glutamic acid interconversion, since the radioactive peaks obtained from either (3)H-labelled amino acid were discrete, and free from cross-contamination. These results prove that glutamine is the precursor amino acid of pyrrolid-2-one-5-carboxylic acid at the N-termini of the mature MOPC-104E lambda(1), RPC-20 lambda(1) and MOPC-315 lambda(2) L-chains. Thus the formation of pyrrolid-2-one-5-carboxylic acid by cyclization of glutamine is a post-translational event which occurs after, or concomitant with, cleavage of the extra piece from the precursor to yield the mature L-chain. The variable (V) regions (110 amino acid residues) of mouse lambda L-chains are quite similar: when compared with that of MOPC-104E lambda(1) chain, the V-region of RPC-20 lambda(1) chain differs in one residue, and the V-region of MOPC-315 lambda(2) chain differs in 11 residues. The partial sequence data show that the N-terminal extra pieces of the two lambda(1) L-chain precursors have, so far, identical partial sequences; the extra piece of the lambda(2) L-chain precursor differs from these in at least three out of 19 positions.

Amino Acid Sequence↗
PubMed2004

Glutamine availability up-regulates expression of the amino acid transporter protein ASCT2 in HepG2 cells and stimulates the ASCT2 promoter.

Glutamine transport into the human hepatoma cell line HepG2 is catalysed primarily by an ASCT2-type transporter identical in sequence with that cloned previously from JAR cells. An antibody raised against the C-terminus of the ASCT2 protein was shown to recognize ASCT2 on Western blots. Using this antibody, it was found that variation in cell growth rate did not affect ASCT2 expression, but both growth rate and ASCT2 expression were significantly reduced by glutamine deprivation. Expression of a number of other proteins was shown to be unaffected under these conditions. The sequence of the 5'-flanking region of the ASCT2 gene was derived from the human genome database. A 907 bp fragment of this sequence was directionally ligated into a luciferase reporter vector and was shown to exhibit promoter activity when transfected into HepG2 cells. Promoter activity was greatly reduced when transfection was performed in glutamine-free medium and was restored when glutamine was added post-transfection. The absence of other essential amino acids did not affect promoter activity, and glutamine deprivation did not affect the MCT1 (monocarboxylate transporter 1) promoter. These results indicate that both ASCT2 promoter activity and ASCT2 protein expression in these cells are dependent on glutamine availability.

Amino Acid Sequence↗
PubMed1982

The stimulus-secretion coupling of glucose-induced insulin release. Environmental influences on L-glutamine oxidation in pancreatic islets.

L-Glutamine at a near-physiological concentration (1.0mM) was rapidly taken up and metabolized in rat pancreatic islets. The rate of glutamine deamidation much exceeded that of glutamate conversion into 2-oxoglutarate, the latter conversion being mediated mainly by transamination reactions. The production of 14CO2 from L-[U-14C]glutamine, which reflected the generation of ATP through the metabolism of exogenous glutamine, appeared to be regulated by the redox state of nicotinamide nucleotides and the ATP content of the islet cells. The influence of environmental factors on glutamine oxidation was examined in order to identify ATP-requiring processes. Glutamine oxidation was decreased in the absence of extracellular Ca2+, under conditions aiming at inhibition of the (Na+ + NA+)-dependent ATPase and, provided that glucose was present in the incubation medium, by cycloheximide. These findings were interpreted to suggest that the handling of Ca2+ by the islet cells, the active transport of univalent cations and the biosynthesis of proinsulin represent three major ATP-consuming processes in this fuel-sensor organ.

Adenosine Triphosphate↗
PubMed1983

Muscle glutamine production in diabetic ketoacidotic rats.

The mechanism of activation of glutamine production by the hindlimb during diabetic ketoacidosis (DKA) was investigated in rats. Muscle glutamine production was estimated to account for over 90% of the total glutamine produced by the hindlimb. DKA produced significant increases in the concentrations of NH4+ and IMP in hindlimb muscles, suggesting that AMP deaminase is activated by DKA. NH4Cl- and HCl-induced acidosis did not produce these changes, indicating either that acidosis itself is not the stimulus for increased AMP deaminase activity or that the more severe degree of acidosis accompanying DKA is necessary for activation. Muscle glutamine concentrations were depressed in DKA. Experiments with isolated epitrochlearis muscle showed that the transport and permeability properties of the muscle cells (as judged by uptake and release of alpha-aminoisobutyrate and glutamine) were not altered by DKA. However, glutamine uptake by muscle cells was significantly inhibited by L-leucine, the concentration of which, along with other branched-chain amino acids, is markedly elevated in DKA.

Adipose Tissue↗
PubMed1986

The transport of glutamine and alanine into rat colonocytes.

The transport of glutamine and alanine into isolated rat colonocytes was studied. The transport of both amino acids appears to be dependent on a Na+ gradient. The apparent Km values for the transport of glutamine and alanine were 2.56 +/- 0.84 and 5.35 +/- 1.20 mM respectively, but with similar Vmax. values. Glutamine and alanine transport were mutually competitive, and the transport of both amino acids was competitively inhibited by 2-methylaminoisobutyrate. In contrast, histidine inhibited the transport of both glutamine and alanine non-competitively. It is concluded that glutamine and alanine are transported into rat colonocytes by a common carrier system similar to System A of other cells. It is suggested that the metabolic function of this transport system in rat colonocytes might be the partial exchange of extracellular glutamine for intracellular alanine.

Alanine↗
PubMed1990

Glutamine synthesis from aspartate in guinea-pig renal cortex.

1. Glutamine was found to be the main carbon and nitrogen product of the metabolism of aspartate in isolated guinea-pig kidney-cortex tubules. Glutamate, ammonia and alanine were only minor products. 2. Carbon-balance calculations and the release of 14CO2 from [U-14C]aspartate indicate that oxidation of the aspartate carbon skeleton occurred. 3. A pathway involving aspartate aminotransferase, glutamate dehydrogenase, glutamine synthetase, phosphoenolpyruvate carboxykinase, pyruvate kinase, pyruvate dehydrogenase and enzymes of the tricarboxylic acid cycle is proposed for the conversion of aspartate into glutamine. 4. Evidence for this pathway was obtained by: (i) inhibiting aspartate removal by amino-oxyacetate, an inhibitor of transaminases, (ii) the use of methionine sulphoximine, an inhibitor of glutamine synthetase, which induced a large increase in ammonia release from aspartate, (iii) the use of quinolinate, an inhibitor of phosphoenolpyruvate carboxykinase, which inhibited glutamine synthesis from aspartate, (iv) the use of alpha-cyano-4-hydroxycinnamate, an inhibitor of the mitochondrial transport of pyruvate, which caused an accumulation of pyruvate from aspartate, and (v) the use of fluoroacetate, an inhibitor of aconitase, which inhibited glutamine synthesis with concomitant accumulation of citrate from aspartate.

Aminooxyacetic Acid↗
PubMed1991

Control and function of the transamination pathways of glutamine oxidation in tumour cells.

Parallel investigations of the transamination pathways of glutamine oxidation in Ehrlich ascites carcinoma (EAC) and AS 30D hepatoma revealed that hepatoma cells, unlike EAC, produce very little aspartate. This cannot be explained by differences in the activity of glutamine-metabolizing enzymes. Also, the mitochondria from the hepatoma respired at a similar rate to EAC mitochondria with glutamine as sole substrate producing substantial amounts of aspartate. Unlike their isolated mitochondria, intact hepatoma cells showed a very low rate of glutamine oxidation. Compared with EAC, the rate of L-[U-14C]glutamine consumption by AS 30D hepatoma cells was much lower, with insignificant production of 14C-labelled aspartate and CO2. This suggested that the glutamine-transporting system in the hepatoma cell plasma membrane had a very low activity. Isolated hepatoma mitochondria produced 3 times more pyruvate from malate than did EAC mitochondria, indicating a higher activity of NAD(P)-dependent malic enzyme. We postulate that an active malic enzyme may suppress the synthesis of aspartate in hepatoma cells, but further evidence is needed to confirm this assumption.

Alanine↗
PubMed1991

Physiological glucocorticoid levels regulate glutamine and insulin-mediated glucose metabolism in skeletal muscle of the rat. Studies with RU 486 (mifepristone).

This study examined the effects of antagonism of the peak level of glucocorticoids in vivo, which occurs as rats enter the feeding/activity (dark) period on glucose and glutamine metabolism in incubated isolated rat soleus muscle preparations. Thus the rats were treated with the potent glucocorticoid antagonist RU 486 2 h before and 1 and 2 h into the dark period. Both the content of glutamine in skeletal muscle in vivo and plasma glucose and glutamine concentrations were elevated midway through the dark period, compared with the beginning of the period. RU 486 prevented the increases in plasma glucose and glutamine and caused a significant decrease in both the rate of release of glutamine in soleus muscle in vitro and the content of glutamine in gastrocnemius muscle. The sensitivity of soleus muscle to insulin in vitro is markedly decreased when isolated midway through the dark period (i.e. at 03:00 h) [Leighton, Kowalchuk, Challiss & Newsholme (1988) Am. J. Physiol. 255, E41-E45]. We now show that the concentrations of insulin required to stimulate lactate formation and glycogen synthesis half-maximally were 95 and 250 muunits/ml respectively, and treatment of rats with RU 486 decreased these values to 55 and 90 muunits of insulin/ml respectively. Thus antagonism of the action of the normal circadian rise in the level of glucocorticoids in rats reverses insulin insensitivity in soleus muscles in vitro.

Alanine↗
PubMed1991

Hyperammonaemia does not impair brain function in the absence of net glutamine synthesis.

1. It has been established that chronic hyperammonaemia, whether caused by portacaval shunting or other means, leads to a variety of metabolic changes, including a depression in the cerebral metabolic rate of glucose (CMRGlc) increased permeability of the blood-brain barrier to neutral amino acids, and an increase in the brain content of aromatic amino acids. The preceding paper [Jessy, DeJoseph & Hawkins (1991) Biochem. J. 277, 693-696] showed that the depression in CMRGlc caused by hyperammonaemia correlated more closely with glutamine, a metabolite of ammonia, than with ammonia itself. This suggested that ammonia (NH3 and NH4+) was without effect. The present experiments address the question whether ammonia, in the absence of net glutamine synthesis, induces any of the metabolic symptoms of cerebral dysfunction associated with hyperammonaemia. 2. Small doses of methionine sulphoximine, an inhibitor of glutamine synthetase, were used to raise the plasma ammonia levels of normal rats without increasing the brain glutamine content. These hyperammonaemic rats, with plasma and brain ammonia levels equivalent to those known to depress brain function, behaved normally over 48 h. There was no depression of cerebral energy metabolism (i.e. the rate of glucose consumption). Contents of key intermediary metabolites and high-energy phosphates were normal. Neutral amino acid transport (tryptophan and leucine) and the brain contents of aromatic amino acids were unchanged. 3. The data suggest that ammonia is without effect at concentrations less than 1 mumol/ml if it is not converted into glutamine. The deleterious effect of chronic hyperammonaemia seems to begin with the synthesis of glutamine.

Amino Acids↗
PubMed1991

Substrate and pH effects on glutamine synthesis in rat liver. Consequences for acid-base regulation.

Switching in acidosis of hepatic nitrogen disposal from urea synthesis to NH4+ and net glutamine production was demonstrated in the isolated perfused livers of starved male Wistar rats. Lactate was preferred to glucose as the substrate for the carbon skeleton of glutamine synthesized over the pH range 6.9-7.5. This is necessary if the switch away from a proton-producing process (ureagenesis) in acidosis is to constitute an acid-base regulating system intrinsic to the liver. Glutamine balance shifted with pH from marked net uptake to small net output under acidotic conditions (pH 7.5-6.9), an effect due solely to a decrease in glutamine uptake. NH4+ uptake by the liver had a linear relationship with pH, being markedly decreased in acidosis because glutamine synthesis was insufficient to compensate for the decreased incorporation into urea. Animals rendered chronically acidotic showed a lower central venous plasma urea concentration and a raised NH4+ concentration, but their livers synthesized no more glutamine when perfused at an acidotic pH than did normal livers. We conclude that perivenous hepatocytes may not be efficient scavengers of NH4+ ions, which must be partly disposed of elsewhere by non-proton-generating pathways if inhibition of ureagenesis is to represent a hepatic acid-base regulating system.

Acid-Base Equilibrium↗
PubMed1993

Evidence that the glutamine-stimulated loss of nitrate reductase protein from the yeast Candida nitratophila is not the result of inducer exclusion.

Synthesis of nitrate reductase protein and increases in nitrate reductase activity occurred in cultures of the yeast Candida nitratophila when they were incubated in medium containing ammonium nitrate. Similar treatment with glutamine plus nitrate resulted in little increase in nitrate reductase activity, in cultures grown previously with reduced nitrogen compounds, and decreases in enzyme activity, in cultures adapted to nitrate. Labelling studies conducted in vivo revealed a rapid cessation of de novo nitrate reductase synthesis when glutamine was supplied to nitrate-adapted cultures in the presence of nitrate. Intracellular glutamine concentrations increased rapidly under these conditions and these cultures exhibited high glutamine: glutamate ratios. As nitrate was taken up in the presence of glutamine in these experiments, it is concluded that the glutamine-stimulated inhibition of nitrate reductase synthesis is a consequence of repression and rapid turnover of nitrate reductase mRNA and not inducer (nitrate) exclusion.

Candida↗
PubMed1988

Effect of glucocorticoid treatment on glucose and glutamine metabolism by the small intestine of the rat.

1. The effect of dexamethasone (30 micrograms day-1 100 g-1 body wt.) on the metabolism of glucose and glutamine was studied in the small intestine of rats after 9 days of treatment. 2. Dexamethasone treatment resulted in negative nitrogen balance (P less than 0.001), and produced increases in the concentrations of plasma glucose (22%, P less than 0.05), alanine (32%, P less than 0.001) and insulin (127%, P less than 0.001), but a decrease in the plasma concentration of glutamine (20%, P less than 0.05). 3. Portal-drained visceral blood flow increased by approximately 22% (P less than 0.001) in dexamethasone-treated rats, and was accompanied by a decrease in the arterio-venous concentration difference of glucose (43%, P less than 0.001) and an increase in that of lactate (22%, P less than 0.05), glutamine (35%, P less than 0.01), glutamate (33%, P less than 0.01) and alanine (21%, P less than 0.05). 4. Enterocytes isolated from dexamethasone-treated rats showed decreased and increased rates of glucose and glutamine utilization, respectively. 5. The maximal activities of hexokinase, 6-phosphofructokinase, citrate synthase and oxoglutarate dehydrogenase were decreased (30-64%, P less than 0.001) in intestinal mucosal scrapings of dexamethasone-treated rats, whereas the activity of glutaminase was increased (35%, P less than 0.001). 6. It is concluded that glucocorticoid administration decreases the rate of glucose utilization but increases that of glutamine (both in vivo and in vitro) by the epithelial cells of the small intestine. This may be caused by changes in the maximal activities of key enzymes in the pathways of glucose and glutamine metabolism in these cells.

Animals↗
PubMed1991

Glutamine and alanine metabolism in lungs of septic rats.

1. The metabolism of glutamine and alanine in the lung was studied in rats made septic by a caecal ligation and puncture technique. 2. The blood glucose concentration was not significantly different in septic rats, but blood pyruvate, lactate, glutamine and alanine concentrations were markedly increased as compared with sham-operated rats. Conversely, blood ketone body and plasma cholesterol concentrations were significantly decreased in septic rats. Both plasma insulin and plasma glucagon concentrations were markedly elevated in response to sepsis. Sepsis resulted in a negative nitrogen balance. 3. Sepsis increased the rates of production of glutamine (52.5%, P less than 0.001), alanine (38.9%, P less than 0.001) and glutamate (48.6%, P less than 0.001) by lung slices incubated in vitro. 4. Sepsis increased lung blood flow by 27.6% (P less than 0.05). Blood flow and arteriovenous concentration difference measurement across the lung of septic rats showed an increase in the net exchange rates of glutamine (142.5%, P less than 0.001), alanine (129.4%, P less than 0.001), glutamate (100.9%, P less than 0.001) and ammonia (138.0%, P less than 0.001) as compared with sham-operated control rats. 5. Sepsis produced significant decreases in the lung concentrations of glutamine (36.8%), glutamate (20.8%), 2-oxoglutarate (64.8%) and AMP (18.3%). The lung concentrations of alanine (95.9%), ammonia (67.7%) and pyruvate (89.7%) were increased. 6. The maximal activities of glutamine synthetase (20.4%, P less than 0.05), phosphate-dependent glutaminase (18.9%, P less than 0.05) and alanine aminotransferase (25.5%, P less than 0.05) were increased, but there was no marked change in that of glutamate dehydrogenase, in the lungs of septic rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Alanine↗
PubMed2001

The effect of glutamine on protein balance and amino acid flux across arm and leg tissues in healthy volunteers.

BACKGROUND: Glutamine is important in nitrogen transportation and the physiological control of acid-base regulation. In addition, it has been assumed that glutamine regulates protein balance in skeletal muscles based on findings in both experimental and clinical studies. However, little information on glutamine and its effect on protein dynamics in normal individuals is available. Therefore, the aim of this study was to evaluate whether glutamine improves protein balance and uptake of various indispensable amino acids across peripheral tissue in healthy individuals. MATERIAL AND METHODS: Standard primed constant infusions of L-[ring-2H5]phenylalanine and [ring 3,3-2H2]tyrosine (2 micromol kg(-1) h(-1)) were performed after overnight fast in five healthy male volunteers before and during infusions of a standard and a glutamine/tyrosine enriched amino acid solution. Flux measurements of amino acids (AA) including 3-methylhistidine, glucose, lactate and free fatty acids (FFA) were performed across arm and leg tissues. RESULTS: Infusion of the standard AA solution (0.2 g N kg(-1) day(-1)) increased the net uptake of individual amino acids, but provision of the enriched solution (0.4 g N kg(-1) day(-1)) with increased amounts of glutamine and tyrosine seemed to compete unfavourably with the net uptake of other key amino acids as methionine and phenylalanine, which are indispensable in muscles for protein synthesis. Increased flux of amino acids across peripheral tissues did not influence on flux of glucose, free fatty acid and lactate. CONCLUSIONS: Glutamine provision did neither stimulate protein synthesis nor attenuate breakdown of either globular or myofibrillar proteins in skeletal muscles of healthy volunteers.

Adult↗