A method for measuring both glutamine and glutamate levels and stable isotopic enrichments

Elevated Carbon Dioxide and Chronic Warming Together Decrease Nitrogen Uptake Rate, Net Translocation, and Assimilation in Tomato

The response of plant N relations to the combination of elevated CO₂ (eCO₂) and warming are poorly understood. To study this, tomato (Solanum lycopersicum) plants were grown at 400 or 700 ppm CO₂ and 33/28 or 38/33 °C (day/night), and their soil was labeled with ¹⁵NO₃⁻ or ¹⁵NH₄⁺. Plant dry mass, root N-uptake rate, root-to-shoot net N translocation, whole-plant N assimilation, and root resource availability (%C, %N, total nonstructural carbohydrates) were measured. Relative to eCO₂ or warming alone, eCO₂ + warming decreased growth, NO₃⁻ and NH₄⁺-uptake rates, root-to-shoot net N translocation, and whole-plant N assimilation. Decreased N assimilation with eCO₂ + warming was driven mostly by inhibition of NO₃⁻ assimilation, and was not associated with root resource limitations or damage to N-assimilatory proteins. Previously, we showed in tomato that eCO₂ + warming decreases the concentration of N-uptake and -assimilatory proteins in roots, and dramatically increases leaf angle, which decreases whole-plant light capture and, hence, photosynthesis and growth. Thus, decreases in N uptake and assimilation with eCO₂ + warming in tomato are likely due to reduced plant N demand.

Urea Cycle Related Amino Acids Measured in Dried Bloodspots Enable Long-Term In Vivo Monitoring and Therapeutic Adjustment

BACKGROUND

Dried bloodspots are easy to collect and to transport to assess various metabolites, such as amino acids. Dried bloodspots are routinely used for diagnosis and monitoring of some inherited metabolic diseases.

METHODS

Measurement of amino acids from dried blood spots by liquid chromatography-tandem mass spectrometry.

RESULTS

We describe a novel rapid method to measure underivatised urea cycle related amino acids. Application of this method enabled accurate monitoring of these amino acids to assess the efficacy of therapies in argininosuccinate lyase deficient mice and monitoring of these metabolites in patients with urea cycle defects.

CONCLUSION

Measuring urea cycle related amino acids in urea cycle defects from dried blood spots is a reliable tool in animal research and will be of benefit in the clinic, facilitating optimisation of protein-restricted diet and preventing amino acid deprivation.

Polyethylene terephthalate microplastics affect hydrogen production from alkaline anaerobic fermentation of waste activated sludge through altering viability and activity of anaerobic microorganisms

Alkaline (especially pH 10) anaerobic fermentation of waste activated sludge (WAS) has been reported to be an effective approach for hydrogen production through inhibiting the homoacetogenesis and methanogenesis. However, the potential effect of the widespread microplastics in sludge on the performance of hydrogen production has never been reported. To fill this knowledge gap, the dominant polyethylene terephthalate (PET) microplastics in WAS were selected as the model microplastics to evaluate their influences on hydrogen production during alkaline anaerobic fermentation of WAS as well as the key mechanisms involved. Experimental results demonstrated that hydrogen production from WAS decreased in the presence of PET microplastics (i.e., 10, 30 and 60 particles/g-TS) compared to the control, with the hydrogen yield at 60 particles/g-TS being only 70.7 ± 0.9% of the control. Although the hydrogen consumption (i.e., homoacetogenesis and methanogenesis) was restrained under alkaline (pH 10) condition, PET microplastics inhibited hydrolysis, acidogenesis and acetogenesis in alkaline WAS anaerobic fermentation, leading to the inhibitory effect on hydrogen production. This was further confirmed by the microbial analysis, which clearly showed PET microplastics caused the shift of the microbial community toward the direction against hydrolysis-acidification. Mechanism studies revealed that PET microplastics carried on their negative influence mainly through leaching the toxic di-n-butyl phthalate (DBP). The reactive oxygen species (ROS) and live/dead staining tests indicated that the increased ROS was induced by PET microplastics, causing more cells dead, which further resulted in the decreased production of hydrogen.

The dietary protein paradox and threonine 15 N-depletion: Pyridoxal-5'-phosphate enzyme activity as a mechanism for the δ15 N trophic level effect

RATIONALE

Nitrogen stable isotope ratios (δ¹⁵ N values) are used to reconstruct dietary patterns, but the biochemical mechanism(s) responsible for the diet to tissue trophic level effect and its variability are not fully understood. Here δ¹⁵ N amino acid (AA) values and physiological measurements (nitrogen intake, plasma albumin concentrations, liver-reduced glutathione concentrations and leucine oxidation rates) are used to investigate increased dietary protein consumption and oxidative stress (vitamin E deficiency) in rat total plasma protein.

METHODS

Using gas chromatography/combustion/isotope ratio mass spectrometry, the δ¹⁵ N values from N-pivaloyl-i-propyl esters of 15 AAs are reported for rats (n = 40) fed casein-based diets with: adequate protein (AP, 13.8%; n = 10), medium protein (MP, 25.7%; n = 10), high protein (HP, 51.3%; n = 10) or HP without vitamin E (HP-E; n = 10) for 18 weeks.

RESULTS

Between the HP and AP groups, the δ¹⁵ N_AA values of threonine (-4.0‰), serine (+1.4‰) and glycine (+1.2‰) display the largest differences and show significant correlations with: nitrogen intake, plasma albumin concentrations, liver-reduced glutathione concentrations and leucine oxidation rates. This indicates increased AA catabolism by the dietary induction of shared common metabolic pathways involving the enzymes threonine ammonia-lyase (EC 4.3.1.19), serine hydroxymethyltransferase (EC 2.1.2.1) and the glycine cleavage system (EC 2.1.2.10). The δ¹⁵ N_AA values of the HP-E and HP groups were not found to be significantly different.

CONCLUSIONS

The ¹⁵ N-depleted results of threonine are linked to increased activity of threonine ammonia-lyase, and show potential as a possible biomarker for protein intake and/or gluconeogenesis. We hypothesize that the inverse nitrogen equilibrium isotope effects of Schiff base formation, between AAs and pyridoxal-5'-phosphate cofactor enzymes, play a key role in the bioaccumulation and depletion of ¹⁵ N in the biomolecules of living organisms and contributes to the variability in the nitrogen trophic level effect. Copyright © 2017 John Wiley & Sons, Ltd.

Growth hormone enhances fat-free mass and glutamine availability in patients with short-bowel syndrome: an ancillary double-blind, randomized crossover study

BACKGROUND

Benefits of recombinant human growth hormone (rhGH) alone or combined with glutamine in patients with intestinal failure because of short-bowel syndrome remain controversial.

OBJECTIVE

We explored effects of rhGH on whole-body protein metabolism in patients with short-bowel syndrome with intestinal failure (SBS-IF) to gain insight into its mechanism of action.

DESIGN

Eight stable hyperphagic patients with severe SBS-IF received, in a double-blind, randomized crossover study, low-dose rhGH (0.05 mg · kg⁻¹ · d⁻¹) and a placebo for two 3-wk periods. Leucine and glutamine kinetics under fasting and fed conditions, fat-free mass (FFM), and serum insulin were determined on the final day of each treatment.

RESULTS

rhGH increased FFM and nonoxidative leucine disposal (NOLD; an index of protein synthesis) (P < 0.02), whereas FFM and NOLD were correlated in the fed state (r = 0.81, P = 0.015). With rhGH administration, leucine release from protein breakdown (an index of proteolysis) decreased in the fed compared with fasting states (P = 0.012), which was not observed with the placebo. However, the fast-to-fed difference in leucine release from protein breakdown was not significantly different between rhGH and placebo (P = 0.093). With rhGH, the intestinal absorption of leucine and glutamine increased (P = 0.036) and correlated with serum insulin (r = 0.91, P = 0.002). rhGH increased glutamine de novo synthesis (P < 0.02) and plasma concentrations (P < 0.03) in both fasting and fed states.

CONCLUSIONS

In SBS-IF patients, feeding fails to decrease proteolysis in contrast to what is physiologically observed in healthy subjects. rhGH enhances FFM through the stimulation of protein synthesis and might decrease proteolysis in response to feeding. Improvements in de novo synthesis and intestinal absorption increase glutamine availability over the physiologic range, suggesting that beneficial effects of rhGH in hyperphagic patients might be achieved without glutamine supplementation.

13C isotope-assisted methods for quantifying glutamine metabolism in cancer cells

Glutamine has recently emerged as a key substrate to support cancer cell proliferation, and the quantification of its metabolic flux is essential to understand the mechanisms by which this amino acid participates in the metabolic rewiring that sustains the survival and growth of neoplastic cells. Glutamine metabolism involves two major routes, glutaminolysis and reductive carboxylation, both of which begin with the deamination of glutamine to glutamate and the conversion of glutamate into α-ketoglutarate. In glutaminolysis, α-ketoglutarate is oxidized via the tricarboxylic acid cycle and decarboxylated to pyruvate. In reductive carboxylation, α-ketoglutarate is reductively converted into isocitrate, which is isomerized to citrate to supply acetyl-CoA for de novo lipogenesis. Here, we describe methods to quantify the metabolic flux of glutamine through these two routes, as well as the contribution of glutamine to lipid synthesis. Examples of how these methods can be applied to study metabolic pathways of oncological relevance are provided.

Stable isotope dilution mass spectrometric assay for PRPP using enzymatic procedures

5-Phosphoribosyl-1-pyrophosphate (PRPP) is an important regulator of de novo purine synthesis. A method for the measurement of PRPP in erythrocytes was designed, which is based on the determination of [(13)C(5)]glutamate derived from [(13)C(5)]glutamine following the utilization of PRPP by the action of amidophosphoribosyltransferase. The present study describes a gas chromatographic-mass spectrometric method for determination of [(13)C(5)]glutamate using [(13)C(2)]glutamate as an internal standard. The methods involved purification by anion-exchange chromatography using a BondElut SAX and derivatization with isobutyl chlorocarbonate in water-methanol-pyridine. Quantitation was performed by selected ion monitoring of the protonated molecular ions in the chemical ionization mode. The intra-day reproducibility in the amounts of [(13)C(5)]glutamate determined was in good agreement with the actual amounts added in erythrocytes. A linear relationship was found between the amount of PRPP added and the amount of [(13)C(5)]glutamate formed from [(13)C(5)]glutamine using amidophosphoribosyltransferase.

Oral citrulline does not affect whole body protein metabolism in healthy human volunteers: results of a prospective, randomized, double-blind, cross-over study

BACKGROUND & AIMS

Citrulline increases protein synthesis during refeeding in rodents with short bowel syndrome, aging and malnutrition, and improves nitrogen balance in fed healthy humans. The aim of the current study therefore was to determine whether citrulline had affected protein metabolism in healthy volunteers.

METHODS

In a randomized, double-blind, cross-over study, 12 healthy adults received a 5-h intravenous infusion of L-[1-(13)C]-leucine in the post-absorptive state, after a 7-day oral supplementation with 0.18 g/kg/day citrulline, or an iso-nitrogenous placebo. Treatment order was randomized, treatment periods were separated by 13-day wash out. Leucine appearance rate (Ra) was determined from plasma [1-(13)C]-keto-iso-caproate enrichment and leucine oxidation from expired (13)CO(2), and nitrogen balance was estimated from 6-h urinary urea excretion.

RESULTS

Compared with placebo, oral citrulline supplementation increased plasma citrulline, arginine and ornithine concentrations, but failed to affect albumin, transthyretin, free insulin and insulin-like growth factor (IGF)-1 plasma concentrations, urinary nitrate excretion, or nitrogen balance. Citrulline supplementation did not alter leucine Ra, leucine oxidation, nor whole-body protein synthesis.

CONCLUSION

In healthy, well nourished volunteers, oral citrulline increases plasma citrulline and arginine availability but does not affect whole-body protein kinetics in the post-absorptive state.

Glutamine is required for persistent epileptiform activity in the disinhibited neocortical brain slice

The neurotransmitter glutamate is recycled through an astrocytic-neuronal glutamate-glutamine cycle in which synaptic glutamate is taken up by astrocytes, metabolized to glutamine, and transferred to neurons for conversion back to glutamate and subsequent release. The extent to which neuronal glutamate release is dependent upon this pathway remains unclear. Here we provide electrophysiological and biochemical evidence that in acutely disinhibited rat neocortical slices, robust release of glutamate during sustained epileptiform activity requires that neurons be provided a continuous source of glutamine. We demonstrate that the uptake of glutamine into neurons for synthesis of glutamate destined for synaptic release is not strongly dependent on the system A transporters, but requires another unidentified glutamine transporter or transporters. Finally, we find that the attenuation of network activity through inhibition of neuronal glutamine transport is associated with reduced frequency and amplitude of spontaneous events detected at the single-cell level. These results indicate that availability of glutamine influences neuronal release of glutamate during periods of intense network activity.

Development and performance of a gas chromatography-time-of-flight mass spectrometry analysis for large-scale nontargeted metabolomic studies of human serum

A method for the preparation and GC-TOF-MS analysis of human serum samples has been developed and evaluated for application in long-term metabolomic studies. Serum samples were deproteinized using 3:1 methanol/serum, dried in a vacuum concentrator, and chemically derivatized in a two-stage process. Samples were analyzed by GC-TOF-MS with a 25 min analysis time. In addition, quality control (QC) samples were used to quantify process variability. Optimization of chemical derivatization was performed. Products were found to be stable for 30 h after derivatization. An assessment of within-day repeatability and within-week reproducibility demonstrates that excellent performance is observed with our developed method. Analyses were consistent over a 5 month period. Additional method testing, using spiked serum samples, showed the ability to define metabolite differences between samples from a population and samples spiked with metabolites standards. This methodology allows the continuous acquisition and application of data acquired over many months in long-term metabolomic studies, including the HUSERMET project (http://www.husermet.org/).

Corticosteroids increase glutamine utilization in human splanchnic bed

Glutamine is the most abundant amino acid in the body and is extensively taken up in gut and liver in healthy humans. To determine whether glucocorticosteroids alter splanchnic glutamine metabolism, the effect of prednisone was assessed in healthy volunteers using isotope tracer methods. Two groups of healthy adults received 5-h intravenous infusions of l-[1-(14)C]leucine and l-[(2)H(5)]glutamine, along with q. 20 min oral sips of tracer doses of l-[1-(13)C]glutamine in the fasting state, either 1) at baseline (control group; n = 6) or 2) after a 6-day course of 0.8 mg.kg(-1).day(-1) prednisone (prednisone group; n = 8). Leucine and glutamine appearance rates (Ra) were determined from plasma [1-(14)C]ketoisocaproate and [(2)H(5)]glutamine, respectively, and leucine and glutamine oxidation from breath (14)CO(2) and (13)CO(2), respectively. Splanchnic glutamine extraction was estimated by the fraction of orally administered [(13)C]glutamine that failed to appear into systemic blood. Prednisone treatment 1) did not affect leucine Ra or leucine oxidation; 2) increased plasma glutamine Ra, mostly owing to enhanced glutamine de novo synthesis (medians +/- interquartiles, 412 +/- 61 vs. 280 +/- 190 mumol.kg(-1).h(-1), P = 0.003); and 3) increased the fraction of orally administered glutamine undergoing extraction in the splanchnic territory (means +/- SE 64 +/- 6 vs. 42 +/- 12%, P < 0.05), without any change in the fraction of glutamine oxidized (means +/- SE, 75 +/- 4 vs. 77 +/- 4%, not significant). We conclude that high-dose glucocorticosteroids increase in splanchnic bed the glutamine requirements. The role of such changes in patients receiving chronic corticoid treatment for inflammatory diseases or suffering from severe illness remains to be determined.

Acute depletion of plasma glutamine increases leucine oxidation in prednisone-treated humans

BACKGROUND, AIMS & METHODS

To determine whether depletion in plasma glutamine worsens the catabolic response to corticosteroids, seven healthy volunteers received oral prednisone for 6 days on two separate occasions, at least 2 weeks apart, and in random order. On the sixth day of each treatment course, they received 5 h intravenous infusions of L-[1-(14)C]-leucine and L-[1-(13)C]-glutamine in the postabsorptive state (1) under baseline conditions (prednisone only day) and (2) after 24h of treatment with phenylbutyrate (prednisone+phenylbutyrate day), a glutamine chelating agent.

RESULTS

Phenylbutyrate treatment was associated with (1) an approximately 15% decline in plasma glutamine concentration (627+/-39 vs. 530+/-31 micromol l(-1); P<0.05), (2) no change in leucine appearance rate, an index of protein breakdown (124+/-9 vs. 128+/-9 micromol kg(-1) h(-1); NS) nor in non-oxidative leucine disposal, an index of whole body protein synthesis (94+/-9 vs. 91+/-7 micromol kg(-1) h(-1); NS), and (3) a approximately 25% rise in leucine oxidation (30+/-1 vs. 38+/-2 micromol kg(-1) h(-1), P<0.05), despite an approximately 25% decline (P<0.05) in leucine concentration.

CONCLUSIONS

In a model of mild, stress-induced protein catabolism, depletion of plasma glutamine per se may worsen branched chain amino acid and protein wasting.

Oral glutamine and amino acid supplementation inhibit whole-body protein degradation in children with Duchenne muscular dystrophy

BACKGROUND

Glutamine has been shown to acutely decrease whole-body protein degradation in Duchenne muscular dystrophy (DMD).

OBJECTIVE

To improve nutritional support in DMD, we tested whether oral supplementation with glutamine for 10 d decreased whole-body protein degradation significantly more than did an isonitrogenous amino acid control mixture.

DESIGN

Twenty-six boys with DMD were included in this randomized, double-blind parallel study; they received an oral supplement of either glutamine (0.5 g . kg(-1) . d(-1)) or an isonitrogenous, nonspecific amino acid mixture (0.8 g . kg(-1) . d(-1)) for 10 d. The subjects in each group were not clinically different at entry. Leucine and glutamine metabolisms were estimated in the postabsorptive state by using a primed continuous intravenous infusion of [1-(13)C]leucine and [2-(15)N]glutamine before and 10 d after supplementation.

RESULTS

A significant effect of time was observed on estimates of whole-body protein degradation. A significant (P < 0.05) decrease in the rate of leucine appearance (an index of whole-body protein degradation) was observed after both glutamine and isonitrogenous amino acid supplementation [x +/-SEM: 136 +/- 9 to 124 +/- 6 micromol . kg fat-free mass (FFM)(-1) . h(-1) for glutamine and 136 +/- 6 to 131 +/- 8 micromol . kg FFM(-1) . h(-1) for amino acids]. A significant (P < 0.05) decrease in endogenous glutamine due to protein breakdown was also observed (91 +/- 6 to 83 +/- 4 micromol . kg FFM(-1) . h(-1) for glutamine and 91 +/- 4 to 88 +/- 5 micromol . kg FFM(-1) . h(-1) for amino acids). The decrease in the estimates of whole-body protein degradation did not differ significantly between the 2 supplemental groups.

CONCLUSION

Oral glutamine or amino acid supplementation over 10 d equally inhibits whole-body protein degradation in DMD.

Decreased whole-body and splanchnic glutamate metabolism in healthy elderly men and patients with chronic obstructive pulmonary disease in the postabsorptive state and in response to feeding

Decreased plasma and muscle glutamate concentrations have been observed in patients with chronic obstructive pulmonary disease (COPD), suggesting disturbances in glutamate metabolism. The present study was conducted to further examine glutamate metabolism in 8 male COPD patients (68 +/- 4 y) by measurement of whole-body (WB) glutamate production and splanchnic glutamate extraction in the postabsorptive state as well as in response to feeding. Because COPD is particularly prevalent in the elderly and aging per se may also affect glutamate metabolism, 2 male control groups were included: 8 healthy elderly (63 +/- 3 y) and 8 young (22 +/- 1 y) subjects. On 2 test days, the stable isotope L-15N-glutamate was infused i.v. or enterally according to a primed constant and continuous infusion protocol. After 90 min of infusion, subjects ingested a carbohydrate-protein drink (28% milk protein, 72% maltodextrin) every 20 min for 2 h. Arterialized-venous blood samples were taken at the end of the postabsorptive and feeding periods. Postabsorptive WB glutamate production and splanchnic glutamate extraction were significantly lower in the elderly and COPD patients than in the young (P < 0.01). Feeding further decreased WB endogenous glutamate production in the elderly and COPD patients, with COPD patients tending (P = 0.07) to have a greater decrease. Splanchnic glutamate extraction increased during feeding in the elderly (P < 0.05) but did not change in COPD patients. In conclusion, aging reduces postabsorptive WB endogenous glutamate production and splanchnic glutamate extraction. COPD does not affect postabsorptive WB glutamate metabolism but may influence splanchnic glutamate metabolism during feeding.

Reducing plasma HIV RNA improves muscle amino acid metabolism

We reported (Yarasheski KE, Zachwieja JJ, Gischler J, Crowley J, Horgan MM, and Powderly WG. Am J Physiol Endocrinol Metab 275: E577-E583, 1998) that AIDS muscle wasting was associated with an inappropriately low rate of muscle protein synthesis and an elevated glutamine rate of appearance (Ra Gln). We hypothesized that high plasma HIV RNA caused dysregulation of muscle amino acid metabolism. We determined whether a reduction in HIV RNA (> or =1 log) increased muscle protein synthesis rate and reduced R(a) Gln and muscle proteasome activity in 10 men and 1 woman (22-57 yr, 60-108 kg, 17-33 kg muscle) with advanced HIV (CD4 = 0-311 cells/microl; HIV RNA = 10-375 x 10(3) copies/ml). We utilized stable isotope tracer methodologies ([13C]Leu and [15N]Gln) to measure the fractional rate of mixed muscle protein synthesis and plasma Ra Gln in these subjects before and 4 mo after initiating their first or a salvage antiretroviral therapy regimen. After treatment, median CD4 increased (98 vs. 139 cells/microl, P = 0.009) and median HIV RNA was reduced (155,828 vs. 100 copies/ml, P = 0.003). Mixed muscle protein synthesis rate increased (0.062 +/- 0.005 vs. 0.078 +/- 0.006%/h, P = 0.01), Ra Gln decreased (387 +/- 33 vs. 323 +/- 15 micromol.kg fat-free mass(-1).h(-1), P = 0.04), and muscle proteasome chymotrypsin-like catalytic activity was reduced 14% (P = 0.03). Muscle mass was only modestly increased (1 kg, P = not significant). We estimated that, for each 10,000 copies/ml reduction in HIV RNA, approximately 3 g of additional muscle protein are synthesized per day. These findings suggest that reducing HIV RNA increases muscle protein synthesis and reduces muscle proteolysis, but muscle protein synthesis relative to whole body protein synthesis rate is not restored to normal, so muscle mass is not substantially increased.

Gut mucosal and plasma concentrations of glutamine: a comparison between two enriched enteral feeding solutions in critically ill patients

Background

Addition of glutamine to enteral nutrition formulas is consistently associated with a significant decrease in septic morbidity in critically ill patients, possibly related to the attenuation of gut dysfunction. This pilot study was undertaken to compare the effects of enteral administration of two glutamine-enriched formulas containing either additional free glutamine or glutamine-rich proteins, with a standard solution on plasma and mucosal concentrations of glutamine in patients admitted in the Department of Intensive Care.

Methods

Following randomization, glutamine concentration was determined in endoscopically sampled duodenal biopsies and plasma, before and after a 7-day period of continuous administration of the designated solution.

Results

The mucosal concentration of glutamine increased in the duodenal biopsies sampled from patients randomized to the solution containing the glutamine-rich proteins (from 3.6 ± 2.2 to 6.7 ± 5.2 micro-mol/g protein), but not from the others. There were no differences between the 3 groups in the plasma concentrations of glutamine, which remained stable over time.

Conclusion

The source of supplemental glutamine can influence gut mucosal glutamine concentrations, suggesting differences in its availability or utilization.

Development of a rapid and convenient method to purify mucins and determine their in vivo synthesis rate in rats

The intestinal mucoprotein synthesis rate was measured in vivo for the first time. For this, a rapid, reproducible, and convenient method to purify mucoproteins from large numbers of intestinal samples at the same time was developed. The method takes advantage of both the high mucin resistance to protease activities due to their extensive glycosylations and the high mucin molecular size. Intestinal homogenates were partially digested with Flavourzyme. Nonprotected proteins partially degraded were easily separated from mucoproteins by small gel filtration chromatography using Sepharose CL-4B. Electrophoretically pure mucins were obtained. Their amino acid composition was typical of purified intestinal epithelial mucins. The mucoprotein synthesis rate was determined in vivo in rats using the flooding dose method with the stable isotope L-[1-13C]valine. Free L-[1-13C]valine enrichments in the intracellular pool were determined by GC-MS. L-[1-13C]valine enrichments into purified mucoproteins or intestinal mucosal proteins were measured by gas chromatography-combustion-isotope ratio mass spectrometry. In rats, we found that the gut mucosa protein synthesis rate (%/day) decreased regularly from duodenum (122%/day) to colon (43%/day). In contrast, mucoprotein fractional synthesis rates were in the same range along the digestive tract, between 112%/day (colon) and 138%/day (ileum).

Acute effects of intravenous glutamine supplementation on protein metabolism in very low birth weight infants: a stable isotope study

Although very low birth weight infants are subjected to severe stress and glutamine is now considered a conditionally essential amino acid that may attenuate stress-induced protein wasting in adults, current amino acid solutions designed for neonatal parenteral nutrition do not contain glutamine. To determine whether a short-term supplementation with i.v. glutamine would affect protein metabolism in very low birth weight infants, 13 preterm neonates (gestational age, 28-30 wk; birth weight, 820-1610 g) receiving parenteral nutrition supplying 1.5 g x kg(-1) x d(-1) amino acids and approximately 60 nonprotein kcal x kg(-1) x d(-1) were randomized to receive an i.v. supplement made of either 1) natural L-glutamine (0.5 g x kg(-1) x d(-1); glutamine group), or 2) an isonitrogenous glutamine-free amino acid mixture (control group), for 24 h starting on the third day of life. On the fourth day of life, they received a 2-h infusion of NaH(13)CO(3) to assess the recovery of (13)C in breath, immediately followed by a 3-h L-[1-(13)C]leucine infusion. Plasma ammonia did not differ between the groups. Glutamine supplementation was associated with 1) higher plasma glutamine (629 +/- 94 versus 503 +/- 83 microM, mean +/- SD; p < 0.05, one-tailed unpaired t test), 2) lower rates of leucine release from protein breakdown (-16%, p < 0.05) and leucine oxidation (-35%, p < 0.05), 3) a lower rate of nonoxidative leucine disposal, an index of protein synthesis (-20%, p < 0.05), and 4) no change in protein balance (nonoxidative leucine disposal - leucine release from protein breakdown, NS). We conclude that although parenteral glutamine failed to enhance rates of protein synthesis, glutamine may have an acute protein-sparing effect, as it suppressed leucine oxidation and protein breakdown, in parenterally fed very low birth weight infants.

Use of L-[(15)N] glutamic acid and homoglutathione to determine both glutathione synthesis and concentration by gas chromatography-mass spectrometry (GCMS)

A method for simultaneous measurement of both glutathione enrichment and concentration in a biological sample using gas chromatography mass spectrometry is described. The method is based on the preparation of N,S-ethoxycarbonylmethyl ester derivatives of glutathione, and the use of homoglutathione (glutamyl-cysteinyl--alanine) as an internal standard. A procedure for determination of glutamate concentration and enrichment is also reported. Both methods have within-day and day-to-day inter-assay coefficients of variation less than 5%, and recoveries of known added amounts of glutathione and glutamate are close to 100%. Taken together, these methods allowed determination of glutathione concentration and fractional synthesis rate in red blood cells using L-[(15)N] glutamic acid infusion. This approach was applied in vivo to investigate the effects of a 72 h fast, compared with a control overnight fast, on erythrocyte glutathione in a single dog. The 72 h fast was associated with a 39% decline in erythrocyte glutathione level, (2.9 +/- 0.4 versus 4.7 +/- 0.5 mmol l(-1), fasting versus control) with no change in glutathione fractional synthesis (67.4 versus 71.3% d(-1), fasting versus control).

Role of glucose in the regulation of glutamine metabolism in health and in type 1 insulin-dependent diabetes

To determine the effect of glucose availability on glutamine metabolism, glutamine kinetics were assessed under conditions of hyperglycemia resulting from 1) intravenous infusion of 7.5% dextrose in healthy adults and 2) insulin deficiency in young adults with insulin-dependent diabetes mellitus (IDDM). Eight healthy adults and five young adults with IDDM were studied in the postabsorptive state by use of a primed continuous infusion of D-[U-(14)C]glucose, L-[5,5,5-(2)H(3)]leucine, and L-[3, 4-(13)C]glutamine. Whether resulting from insulin deficiency or dextrose infusion, the rise in plasma glucose was associated with increased glucose turnover (23.5 +/- 0.7 vs. 12.9 +/- 0.3 micromol. kg(-1). min(-1), P < 0.01 and 20.9 +/- 2.5 vs. 12.8 +/- 0.4 micromol. kg(-1). min(-1), P = 0.03, in health and IDDM, respectively). In both cases, high blood glucose failed to alter glutamine appearance rate (R(a)) into plasma [298 +/- 9 vs. 312 +/- 14 micromol. kg(-1). h(-1), not significant (NS) and 309 +/- 23 vs 296 +/- 26 micromol. kg(-1). h(-1), NS, in health and IDDM, respectively] and the estimated fraction of glutamine R(a) arising from de novo synthesis (210 +/- 7 vs. 217 +/- 10 micromol. kg(-1). h(-1), NS and 210 +/- 16 vs. 207 +/- 21 micromol. kg(-1). h(-1), NS, in health and IDDM, respectively). When compared with the euglycemic day, the apparent contribution of glucose to glutamine carbon skeleton increased when high plasma glucose resulted from intravenous dextrose infusion in healthy volunteers (10 +/- 0.8 vs. 4.8 +/- 0.3%, P < 0.01) but failed to do so when hyperglycemia resulted from insulin deficiency in IDDM. We conclude that 1) the contribution of glucose to the estimated rate of glutamine de novo synthesis does not increase when elevation of plasma glucose results from insulin deficiency, and 2) the transfer of carbon from glucose to glutamine may depend on insulin availability.

Is glutamine a 'conditionally essential' amino acid in Duchenne muscular dystrophy?

To determine whether whole body protein kinetics are altered in Duchenne muscular dystrophy (DMD), six 9 +/- 1-year-old children with DMD and five weight and height matched controls, received intravenous infusion of L-[1-(13)C]leucine and L-[2-(15)N]glutamine in the post-absorptive state. Glutamine rate of appearance was approximatly 24% lower in DMD boys than in controls (321 +/- 22 vs 425 +/- 37 micromol kg(-1)h(-1), P< 0.05) resulting from a 32% decrease in glutamine de novo synthesis (230 +/- 21 vs 340 +/- 34 micromol kg(-1)h(-1), P< 0.05). Whereas there was no difference between groups in estimates of protein degradation and synthesis, leucine oxidation rate was 44% higher in DMD boys than in controls (23 +/- 2 vs 16 +/- 2 micromol kg(-1)h(-1), P< 0.05). The data suggest that the dramatic mucle mass loss observed in DMD boys is associated with a) significant protein wasting, since increased leucine oxidation reflects a more negative whole body leucine balance, and b) a significant decrease in glutamine availability in the postabsorptive state. Glutamine might therefore be a 'conditionally essential' amino-acid in DMD.

Measurement of homocysteine concentrations and stable isotope tracer enrichments in human plasma

Elevated levels of plasma homocysteine have been established as an independent risk factor for cardiovascular disease. Homocysteine is in low concentration in plasma (5-15 microM) and is bound to other thiols (e.g., cysteine in plasma proteins) via disulfide bonds. Existing methods for measuring homocysteine have difficulty in reducing and maintaining the reduction of homocysteine for measurement. We describe a GC/MS method that first reduces the disulfides in the physiological sample matrix and then immediately alkylates the free thiols with 4-vinylpyridine to prevent the reformation of the disulfide bonds. We use a deuterated internal standard, [3,3,3',3',4,4,4',4'-2H8]homocystine to account for losses associated with the isolation, derivatization, and measurement of the natural homocysteine. The amino acids are separated and derivatized to form the tert-butyldimethylsilyl derivatives. This method requires only 50 microL of plasma to measure homocysteine concentrations to 5 microM. Total homocysteine concentrations in plasma can be measured routinely from 0.5-mL samples with relative intra- and interday precisions of 1.3 and 4.0%, respectively. This method is sensitive enough to determine tracer enrichments of [1-13C]homocysteine with a detection limit of < 0.3 mol% excess and an average tracer precision of 0.6%.

Whole-body protein metabolism assessed by leucine and glutamine kinetics in adult patients with active celiac disease

To assess the effect of increased renewal of intestinal epithelial cells on leucine and glutamine (Gln) turnover, 4-hour intravenous infusions of L-[1-(13)C]leucine and L-[2-(15)N]Gln were administered to five adult patients with active celiac disease in the postabsorptive state. There was a 35% increase in leucine flux (micromoles per kilogram per hour) in patients (117 +/- 17) compared with healthy controls (96 +/- 11, P < .03). Gln flux was increased by 13% in patients (377 +/- 35) versus controls (335 +/- 16, P < .04). These results suggest that active celiac disease, characterized by villous atrophy and crypt cell hyperplasia, is associated with a dramatic increase in whole-body protein breakdown as assessed by 13C-leucine, which may contribute per se to the protein malnutrition status of the patients. The increase in Gln utilization as assessed by L-[2-(15)N]Gln was moderate, but may have been offset due to the villose atrophy and ensuing reduced intestinal epithelial cell mass. The results are consistent with the concept that increased renewal of intestinal epithelial cells represents a sizable fraction of whole-body protein turnover and that Gln is an important fuel for epithelial intestinal cells in vivo.

Increased plasma gln and Leu Ra and inappropriately low muscle protein synthesis rate in AIDS wasting

Muscle protein wasting occurs in human immunodeficiency virus (HIV)-infected individuals and is often the initial indication of acquired immunodeficiency syndrome (AIDS). Little is known about the alterations in muscle protein metabolism that occur with HIV infection. Nine subjects with AIDS wasting (CD4 < 200/mm3), chronic stable opportunistic infections (OI), and >/=10% weight loss, fourteen HIV-infected men and one woman (CD4 > 200/mm3) without wasting or OI (asymptomatic), and six HIV-seronegative lean men (control) received a constant intravenous infusion of [1-13C]leucine (Leu) and [2-15N]glutamine (Gln). Plasma Leu and Gln rate of appearance (Ra), whole body Leu turnover, disposal and oxidation rates, and [13C]Leu incorporation rate into mixed muscle protein were assessed. Total body muscle mass/fat-free mass was greater in controls (53%) than in AIDS wasting (43%; P = 0.04). Fasting whole body proteolysis and synthesis rates were increased above control in the HIV+ asymptomatic group and in the AIDS-wasting group (P = 0. 009). Whole body Leu oxidation rate was greater in the HIV+ asymptomatic group than in the control and AIDS-wasting groups (P < 0.05). Fasting mixed muscle protein synthesis rate was increased in the asymptomatic subjects (0.048%/h; P = 0.01) but was similar in AIDS-wasting and control subjects (0.035 vs. 0.037%/h). Plasma Gln Ra was increased in AIDS-wasting subjects but was similar in control and HIV+ asymptomatic subjects (P < 0.001). These findings suggest that AIDS wasting results from 1) a preferential reduction in muscle protein, 2) a failure to sustain an elevated rate of mixed muscle protein synthesis while whole body protein synthesis is increased, and 3) a significant increase in Gln release into the circulation, probably from muscle. Several interesting explanations for the increased Gln Ra in AIDS wasting exist.

Glucagon increases glutamine uptake without affecting glutamine release in humans

Glucagon causes transient hyperglycemia and persistent hypoaminoacidemia, but the mechanisms of this action are unclear. To address this question, the present study measured the effects of glucagon on glucose, leucine, phenylalanine, and glutamine kinetics. Seven healthy subjects each underwent three pancreatic clamp studies (octreotide 30 ng/kg/min, insulin 0.15 mU/kg/min, and glucagon 1.4 ng/kg/min) lasting 7 hours. During the last 3.5 hours of the studies, glucagon infusion was either unchanged (study 0) or increased to 4 and 7 ng/kg/min (studies 1 and 2). The higher glucagon infusion rates increased the glucagon concentration by 50% and 100%, respectively. [6,6-(2)H2]glucose, [2-(15)N]glutamine, 2H5-phenylalanine, and 2H3-leucine were infused to quantify the respective fluxes. Glucagon transiently increased glucose concentrations by stimulating glucose production, which peaked in 15 minutes to 3.82 +/- 0.36 and 4.21 +/- 0.33 mg/kg/min in studies 1 and 2 and then returned to the postabsorptive levels. Glucagon decreased the glutamine concentration (-10% +/- 2% and -22% +/- 2% in studies 1 and 2 v study 0, P < .05), because glutamine uptake became greater than glutamine release (balance from -1.9 +/- 0.9 in study 0 to -8.1 +/- 1.1 and -13.6 +/- 1.0 micromol/kg/h in studies 1 and 2, P < .01). Glucagon decreased the leucine concentration (-11% +/- 3% in study 2 v study 0, P < .02) and caused a small increment in proteolysis (+6% in study 2 v study 0, P < .01) that was related to the decrement in glutamine concentrations. Phenylalanine kinetics were not significantly affected. These results show that glucagon promotes the uptake of gluconeogenic substrates but does not increase their release, suggesting that glucagon-induced hyperglycemia is short-lived because glucagon fails to provide more fuel for gluconeogenesis. The small increase in proteolysis and the depletion of circulating glutamine prove that physiologic hyperglucagonemia can contribute to protein catabolism.

Phenylbutyrate-induced glutamine depletion in humans: effect on leucine metabolism

The present study was designed to determine whether sodium phenylbutyrate (phi B) acutely induces a decrease in plasma glutamine in healthy humans, and, if so, will decrease estimates of whole body protein synthesis. In a first group of three healthy subjects, graded doses (0, 0.18, and 0.36 g.kg-1.day-1) of phi B were administered for 24 h before study: postabsorptive plasma glutamine concentration declined in a dose-dependent manner, achieving an approximately 25% decline for a dose of 0.36 g phi B.kg-1.day-1. A second group of six healthy adults received 5-h infusions of L-[1-14C]leucine and L-[1-13C]glutamine in the postabsorptive state on two separate days: 1) under baseline conditions and 2) after 24 h of oral treatment with phi B (0.36 g.kg-1.day-1) in a randomized order. The 24-h phenylbutyrate treatment was associated with 1) an approximately 26% decline in plasma glutamine concentration from 514 +/- 24 to 380 +/- 15 microM (means +/- SE; P < 0.01 with paired t-test) with no change in glutamine appearance rate or de novo synthesis; 2) no change in leucine appearance rate (Ra), an index of protein breakdown (123 +/- 7 vs. 117 +/- 5 mumol.kg-1.h-1; not significant); 3) an approximately 22% rise in leucine oxidation (Ox) from 23 +/- 2 to 28 +/- 2 mumol.kg-1.h-1 (P < 0.01), resulting in an approximately 11% decline in nonoxidative leucine disposal (NOLD = Ra-Ox), an index of protein synthesis, from 100 +/- 6 to 89 +/- 5 mumol.kg-1.h-1 (P < 0.05). The data suggest that, in healthy adults, 1) large doses of oral phenylbutyrate can be used as a "glutamine trap" to create a model of glutamine depletion; 2) a moderate decline in plasma glutamine does not enhance rates of endogenous glutamine production; and 3) a short-term depletion of plasma glutamine decrease estimates of whole body protein synthesis.

Role of glucose and glutamine synthesis in the differential recovery of 13CO2 from infused [2-13C] versus [1-13C] acetate

Carbon exchange in the Krebs cycle may result in underestimation of substrate oxidation measured with 13C-labeled substrates, since carbon labeled in position 2 of acetyl-coenzyme A (CoA) could be incorporated into glucose (via gluconeogenesis) and glutamine. Five healthy volunteers were therefore infused with [1-13C] and [2-13C] acetate at a rate of 0.5 micromol x kg(-1) x min(-1) for 165 minutes on two different occasions in randomized order. Whole body acetate turnover did not differ between the two tracers: 7.9+/-0.3 and 7.5+/-0.6 micromol x kg(-1) x min(-1) (nonsignificant [NS]) for [1-13C] and [2-13C] acetate, respectively. Isotopic 13C enrichment was higher in expired CO2 (0.177+/-0.021 v 0.089+/-0.009 atom percent excess [APE], P < .01) and lower in glucose (0.074+/-0.017 v0.291+/-0.061 mole percent excess [MPE], P < .01) for [1-13C] acetate compared with [2-13C] acetate, respectively, at the end of the infusions. Glutamine isotopic enrichment was slightly but not significantly higher when infusing [1-13C] acetate versus [2-13C] acetate (0.348+/-0.038 v0.495+/-0.069 MPE, NS, respectively). At the end of the experiment, the recovery of 13CO2 from [1-13C] acetate was 44.8%+/-2.7%, and from [2-13C] acetate, 22.6%+/-1.3%. A significant correlation was observed between the differences in 13C enrichment of CO2 for the two tracers and glucose (deltaCO2=0.424 x deltaglucose + 0.001, R2=.9856, P=.0007) or glutamine (deltaCO2=0.621 x deltaglutamine + 0.004, R2=.9573, P=.0038) during the infusion. These results suggest that (1) although gluconeogenesis appears to be more responsible than glutamine for the differential recovery of [2-13C] versus [1-13C] acetate, other secondary pathways are probably also implicated; and (2) different recovery correction factors should be applied when measuring substrate oxidation with a stable isotope tracer depending on the expected position of 13C in acetyl-CoA.

Oral glutamine slows down whole body protein breakdown in Duchenne muscular dystrophy

We determined whether glutamine has a protein anabolic effect in six 8-13-y-old boys with Duchenne muscular dystrophy. Children received a 5-h i.v. infusion of L-[1-13C]leucine and L-[2-15N]glutamine in the postabsorptive state on two consecutive days while drinking: 1) flavored water on one day, and 2) the same drink mixed with L-glutamine (800 micromol x kg[-1] x h[-1]), the other day. Oral glutamine administration was associated with an 8% decrease in leucine release from protein breakdown, from 116 +/- 5 to 107 +/- 6 micromol x kg(-1) h(-1) (p < 0.01), and a 35% decrease in leucine oxidation rate from 23 +/- 2 to 15 +/- 2 micromol x kg(-1) x h(-1) (p < 0.01), resulting in no change in the nonoxidative leucine disposal, an index of protein synthesis. Whole body glutamine exchange in plasma doubled from 321 +/- 22 to 623 +/- 24 micromol x kg(-1) x h(-1), p < 0.01, but glutamine from protein degradation and glutamine de novo synthesis both decreased (91 +/- 4 versus 84 +/- 5 micromol x kg(-1) x h(-1), p < 0.01, and 230 +/- 21 versus 163 +/- 25 micromol x kg(-1) x (h-1), p = 0.02, respectively). These data suggest that acute oral glutamine administration might have a protein-sparing effect in children with Duchenne muscular dystrophy, decreasing estimates of whole body protein degradation and glutamine de novo synthesis, therefore sparing nitrogen precursors.

Glutamine metabolism in very low birth weight infants

To quantitate glutamine kinetics in premature infants and determine whether glutamine affects leucine metabolism. 11 very low birth weight (< 1250 g) neonates received 4-h i.v. infusions of L-[2H3]leucine and L-[13C5]glutamine, along with orogastric infusion of L-[I-13C]leucine and L-[I-13C]glutamine on the 10th d of life and in the fed state. Patients were receiving parenteral nutrition and were randomized to receive either hypocaloric, enteral preterm formula alone (controls; n = 5), or glutamine (0.2 g.kg-1.d-1 on the day of the study) supplemented formula (GL.n; n = 6). The rates of appearance (Ra) of leucine and glutamine, and their rates of splanchnic extraction were determined from isotopic enrichments in plasma at steady state. Leucine release from protein breakdown did not differ between groups (123 +/- 51 versus 162 +/- 94 mumol.kg-1h-1 in the controls and GLN group, respectively). Glutamine de novo synthesis accounted for > 80% of overall glutamine Ra, and was similar in both groups (626 +/- 177 versus 525 +/- 86 mumol.kg-1.h-1; NS); 46 +/- 16% and 53 +/- 31% of the enteral glutamine underwent first-pass splanchnic extraction in the controls and GLN group, respectively. These findings indicate that the pathways of glutamine de novo synthesis and glutamine utilization in the splanchnic bed are functional in very low birth weight humans by the 10th d of life. Glutamine supplementation provided at low doses on a hypocaloric regimen results in no apparent differences in flux of glutamine or leucine.

Effect of glutamine on leucine metabolism in humans

The aim of this study was to determine whether the putative protein anabolic effect of glutamine 1) is mediated by increased protein synthesis or decreased protein breakdown and 2) is specific to glutamine. Seven healthy adults were administered 5-h intravenous infusions of L-[1-14C]leucine in the postabsorptive state while receiving in a randomized order an enteral infusion of saline on one day or L-glutamine (800 mumol.kg-1.h-1, equivalent to 0.11 g N/kg) on the other day. Seven additional subjects were studied using the same protocol except they received isonitrogenous infusion of glycine. The rates of leucine appearance (RaLeu), an index of protein degradation, leucine oxidation (OxLeu), and nonoxidative leucine disposal (NOLD), an index of protein synthesis, were measured using the 14C specific activity of plasma alpha-ketoisocaproate and the excretion rate of 14CO2 in breath. During glutamine infusion, plasma glutamine concentration doubled (673 +/- 66 vs. 1,184 +/- 37 microM, P < 0.05), whereas RaLeu did not change (122 +/- 9 vs. 122 +/- 7 mumol. kg-1.h-1), OxLeu decreased (19 +/- 2 vs. 11 +/- 1 mumol.kg-1.h-1, P < 0.01), and NOLD increased (103 +/- 8 vs. 111 +/- 6 mumol. kg-1.h-1, P < 0.01). During glycine infusion, plasma glycine increased 14-fold (268 +/- 62 vs. 3,806 +/- 546 microM, P < 0.01), but, in contrast to glutamine, RaLeu (124 +/- 6 vs. 110 +/- 4 mumol. kg-1.h-1, P = 0.02), OxLeu (17 +/- 1 vs. 14 +/- 1 mumol.kg-1.h-1, P = 0.03), and NOLD (106 +/- 5 vs. 96 +/- 3 mumol.kg-1.h-1, P < 0.05) all decreased. We conclude that glutamine enteral infusion may exert its protein anabolic effect by increasing protein synthesis, whereas an isonitrogenous amount of glycine merely decreases protein turnover with only a small anabolic effect resulting from a greater decrease in proteolysis than protein synthesis.

Protein and energy metabolism in prepubertal children with sickle cell anemia

We hypothesized that, in children with homozygous sickle cell anemia (HbSS), the shortened life-span of erythrocytes places an increased demand on protein stores, accelerates whole body protein turnover, and consequently, energy expenditure, as well as the rate of utilization of glutamine, a major fuel for reticulocytes. Eight (11.2 +/- 0.4 y old) children with HbSS who were free of infection of vaso-occlusive disease, and seven (11.3 +/- 0.4 y old) healthy black children were therefore studied in the postabsorptive state. Each received a continuous 4-h infusion of L-[1-(13)C]leucine to determine the rate of leucine oxidation, leucine rate of appearance, and nonoxidative leucine disposal, indicators of whole body protein breakdown and synthesis, respectively. Infusion of L-[2-(15)N]glutamine was used to assess rates of glutamine utilization. Resting energy expenditure and cardiac output were measured using indirect calorimetry and echocardiography, respectively. Compared with control subjects, HbSS children had a 58 and 65% higher leucine rate of appearance and nonxidative leucine disposal, respectively (both p < 0.001), 47% higher rates of whole body glutamine utilization (p < 0.01), 19% higher resting energy expenditure (p < 0.05), and 66% higher cardiac output (p < 0.01). In conclusion, children with HbSS show evidence of hypermetabolism with regard to protein, energy, and glutamine utilization. Both increased Hb synthesis and increased cardiac workload may contribute to excess protein and energy utilization. Whatever the mechanism of hypermetabolism, the data suggest that children with HbSS may have greater protein and energy requirements than the general population.

Energy and protein metabolism in malnutrition due to nonneoplastic gastrointestinal diseases

Although a reduction in both energy expenditure and protein turnover has been demonstrated in starved volunteers, few metabolic data are available for patients in whom malnutrition is due to nonneoplastic gastrointestinal diseases. Chronically malnourished, unstressed adult patients with nonneoplastic gastrointestinal diseases (body mass index, 15.8 +/- 2.5 kg/m2, n = 13) and healthy control subjects (n = 10) were studied in the postabsorptive state using indirect calorimetry, as well as substrate fluxes of L[1-13C]leucine, L-[2-15N]glutamine (seven patients and six controls), and D[6,6-2H2]glucose (seven patients and eight controls). Resting energy expenditure (REE) expressed in kilocalories per 24 hours was significantly lower in patients than in controls; REE expressed per unit of fat-free mass (FFM) was not significantly different in both groups. Whole-body leucine turnover, oxidation, and nonoxidative disposal rates, based on either 13C-leucine or 13C-alpha-ketoisocaproic acid (KIC) enrichments, and glucose turnover rate were not significantly different between malnourished patients and controls. Moreover, glutamine turnover was increased by 28% in malnourished patients as compared with normal volunteers (429.8 +/- 86.8 v 334.9 +/- 15.9 mumol/kg/h, P = .02). These results suggest that hypometabolic adaptation, although previously documented in starved volunteers, is not operative during states of chronic malnutrition due to gastrointestinal disease. The increase in glutamine turnover rate might represent an adaptative mechanism to malnutrition for preservation of visceral mass or function.

Oxidation of glutamic acid by the splanchnic bed in humans

[1,2-13C2]glutamate and [ring-2H5]phenylalanine were infused for 7 h into postabsorptive healthy subjects on two occasions. The tracer infusion was by the intravenous route for 3.5 h and by the nasogastric route for 3.5 h. The order of tracer infusion routes was switched between the two occasions. From the plasma tracer enrichment measurements at plateau during the intravenous and enteral infusion periods, we determined that 33 +/- 3% of the enterally delivered phenylalanine and 96 +/- 1% of the glutamate were removed on the first pass by the splanchnic bed; 78 +/- 3% of the enterally delivered [13C]glutamate tracer was recovered as exhaled CO2 compared with 79 +/- 2% of the intravenously infused tracer. The fraction of the enterally delivered tracer that was sequestered specifically on the first pass by the splanchnic bed was 75 +/- 2%. These results verify the previously reported large uptake of [15N]glutamate by the splanchnic bed [Matthews et al. Am. J. Physiol. 264 (Endocrinol. Metab. 27): E848-E854, 1993] and demonstrate that the uptake of tracer is not due to an artifactual loss of the 15N tracer by reversible transamination but to glutamate uptake for oxidation.

Determination of amino- and amide-15N glutamine enrichment with tertiary butyldimethylsilyl derivatives

We have developed a simple and rapid method for the selective synthesis of tetra-tertiarybutyldimethylsilyl (TBDMS) glutamine, which allows the simultaneous quantitation of glutamine (2-15N) and (5-15N) isotopic enrichment by selected ion monitoring (SIM) gas chromatographic/mass spectrometric analysis. The tetra-TBDMS glutamine (4S-gln) has an electron impact mass fragment at m/z 258 that contains only the amino-N and larger fragments (e.g., at m/z 545) that have both nitrogens. Derivatization with acetonitrile (ACN) and N-methyl-N-(tert-butyldimethylsilyl)trifluoroacetamide yields primarily tri-TBDMS glutamine (3S-gln) and quantities of 4S-gln too small to allow accurate SIM and tracer/tracee ratio determinations. However, when N,N-dimethylformamide, a more polar aprotic solvent, was substituted for ACN and the sample was heated for 30 min at 125 degrees C, greater than 80% of derivatized glutamine appeared as 4S-gln. Derivatized plasma samples that had been mixed with amide- and/or amino-15N glutamine and analyzed by SIM demonstrated strong agreement (r > or = 0.998, p = 0.0001) between theoretical and observed enrichment values for the 4S-gln fragments at m/z 258 and 545. Deamidation of glutamine to glutamate is negligible during sample processing and analysis. This procedure will facilitate the investigation of the specific sources and fates of glutamine amide and amino nitrogen as well as stable isotope studies involving amino acid transamination, ammonia clearance, urea production and other areas of nitrogen metabolism.

Flow injection determination of glutamate in human serum and rat brain samples with immobilized glutamate oxidase and glutamate dehydrogenase reactors

Two methods are proposed for the determination of regional concentrations of glutamate in the rat brain as well as in human serum. Glutamate oxidase was immobilized on non-porous glass beads and glutamate dehydrogenase was immobilized on glass derivatives. These supports were employed for the construction of Single Bead String Reactors and Packed Bed Reactors, respectively, which in turn were linked to Flow Injection Analysis systems with either photometric or fluorometric detection. Analytical working curves are linear in the range 1-200 mumol/l for packed bed reactors and 10-500 mmol/l for single bead string reactors. The samples were pretreated depending on their origin and the applied measuring system. Optimal dilution factors were established for the two techniques. Optimal dilution ratios were established and the influence of several added substances was investigated. Recovery and method comparison studies including high performance liquid chromatography verified the accuracy of the proposed methods. Results from within-day and between-day measurements gave relative standard deviations of 4.7 and 5.9% for serum samples and 2.5 and 4.0% for brain samples, respectively.

Mechanism governing short-term fed-state adaptation to dietary protein restriction

The mechanism governing short-term adaptation to dietary protein restriction was investigated in nine normal adults by measuring their metabolic response to a standard mixed meal, first while they were adapted to a conventional, high-protein diet (day 1) and then again after they had eaten two low-protein meals (day 2). Urea appearance (measured as the sum of its urinary excretion and the change in body urea pool size), body retention of 15N-alanine included in each test meal, and whole-body protein turnover were calculated over the 9 hours following meal consumption on each day. Postprandial urea nitrogen appearance was 5.05 +/- 0.26 g/9 h on day 1 and decreased to 4.16 +/- 0.31 on day 2 (P < .05). Whole-body N flux (Q), protein synthesis (S), and protein breakdown (B) all decreased significantly on day 2 as assessed using either urea or ammonium end-product enrichments; however, recovery of 15N in the test meal as 15N-urea was similar on both days, approximately 22%. It is concluded that short-term metabolic adaptation occurs within two meals of reduced protein intake. The mechanism appears not to involve selectively an increased "first-pass" retention of dietary amino acids, but rather a general reduction in fed-state whole-body protein breakdown.

Calcium and protein kinetics in prepubertal boys. Positive effects of testosterone

We investigated the effects of 4-6-wk administration of testosterone on calcium and protein metabolism in six healthy prepubertal short boys (mean age +/- SE = 12.9 +/- 0.6 yr). At baseline, subjects received a 4-h infusion of L-[1-13C]leucine and L-[2-15N]glutamine, and were given 42Ca intravenously, and 44Ca PO. Testosterone enanthate (approximately 3 mg/kg) was given I.M. 2 wk apart (two doses n = 5, three doses n = 1), and the study was repeated 4-5 d after the last injection. After testosterone therapy, there were significant increases in serum testosterone and mean peak and total growth hormone concentrations. Net calcium absorption (Va) and retention (Vbal) also increased (Va 13.3 +/- 2.3 vs 21.5 +/- 2.3; mg.kg-1.d-1, Vbal 8.0 +/- 2.1 vs 16.6 +/- 2.5, mg.kg-1.d-1, P < .05 both), as well as Ca's net forward flow into bone and total exchangeable pool (16 and 20%, respectively). The rate of appearance of leucine (an indicator of proteolysis) increased by 17.6 +/- 5.9%, P = 0.036. Leucine oxidation decreased by 48.6 +/- 8.0%, P = 0.004; thus, nonoxidative leucine disappearance, which estimates protein synthesis, increased significantly by 34.4 +/- 7.7%, P = 0.009. Glutamine's rate of appearance also increased (+32%), mostly through enhanced glutamine de novo synthesis (+42%). In conclusion, short term testosterone administration significantly increases calcium's retention and net forward flow into bone in prepubertal humans, as well as whole body estimates of protein and calcium anabolism. These effects may represent a pure androgen effect, an amplification of growth hormone's action or some combination of these factors.

Determination of glutamine and alpha-ketoglutarate concentration and specific activity in plasma using high-performance liquid chromatography

A method is described for measuring glutamine (GLN) and alpha-ketoglutarate (KG) concentration and specific activity (SA) using high-performance liquid chromatography (HPLC). Plasma GLN and KG are separated on miniature ion-exchange columns. KG is derivatized with O-phenylene diamine, the derivative is extracted in ethyl acetate, dried, and dissolved in pH 7 phosphate buffer. The isolated GLN is enzymatically converted to KG and analysed as such. Derivatized samples are stable for weeks at -20 degrees C. Samples are injected onto a reversed-phase HPLC column. Absolute standards are injected to determine the nmol content of unknown samples. alpha-Ketoadipate and [3H]-glutamine are used as internal standards to quantitate KG and GLN concentrations, respectively. Collection of the entire peak of interest permits determination of the radioactivity in the GLN and KG peaks; this together with the determination of the nanomoles injected permits the calculation of the SA. Typical precision is 3.5 and 4.6% for GLN and KG concentrations and 5.3 and 3.3% for GLN and KG SA, respectively. Analysis time is ca. 7 min. Using this method, the turnover rate of GLN carbon was determined during a 5-h infusion of L-[U-14C]glutamine in a human subject.

Measurement of 15N enrichment in multiple amino acids and urea in a single analysis by gas chromatography/mass spectrometry

A precise and accurate procedure to measure the 15N isotopic enrichment of 18 common plasma amino acids and singly (15N1) and doubly (15N2) labeled urea in a single analysis by selected ion monitoring electron impact ionization gas chromatography/mass spectrometry analysis is presented. The choice of tert-butyldimethylsilyl derivatives allowed the enrichments in the amide and amino nitrogens of glutamine to be resolved. The ions monitored contained all the nitrogen atoms from the parent compounds except for arginine, which lost one guanidino nitrogen. Isotope ratios were determined with a coefficient of variation (within-assay precision) of 0.35% (range, 0.1-1.0%) on replicate measures averaged over all components; thus, the standard deviation associated with a nominal [m + 1]/[m + 0] isotope ratio of 0.2000 was 0.0007. The average error between measured and theoretical [m + 1]/[m + 0] isotope ratios was +0.0001 +/- 0.0086 for samples at natural abundance isotopic composition. The utility of the procedure is demonstrated by monitoring the incorporation of 15N into 18 plasma amino acids and urea during a 6 h oral administration of 15NH4Cl to a human volunteer. Highest levels of enrichment were achieved in arginine and urea, followed by glutamine. Approximately 80% of the label in glutamine was in the amino nitrogen. Excess 15N enrichment was observed in all plasma amino acids monitored with the exception of the essential amino acids phenylalanine, lysine and histidine. This method will facilitate the measurement of isotopic enrichment of multiple amino acids by a single analysis when it is necessary to monitor multiple stable-isotopically labeled amino acids in studies of amino acid and protein metabolic kinetics.

Incorporation of 15N from L-leucine into isoleucine by rat brain cerebral cortex slices

The fate of leucine nitrogen in the central nervous system was investigated by incubating rat cerebral cortex slices in the presence of 0.5 mM each of L-[15N]-leucine, L-isoleucine, and L-valine. Analysis of the slices and incubation media for free amino acids by gas chromatography-mass spectrometry revealed that the 15N from leucine is incorporated into isoleucine only. No 15N was detected in valine or any other amino acid. These results suggest that leucine, valine, and their corresponding aminotransferases may be compartmentalized in brain cerebral cortex.

A mass spectrometric method for measuring glycerol levels and enrichments in plasma using 13C and 2H stable isotopic tracers

The stable isotope tracer [1,1,2,3,3,-2H5]glycerol has been commonly used as a tracer to measure glycerol kinetics and lipolysis in vivo. The method for measuring samples using the trimethylsilyl derivative and electron impact gas chromatograph-mass spectrometry retains only three of the five deuteriums, resulting in the possibility of incorrectly identifying the whole glycerol tracer molecule. This reports preparation of glycerol as the heptafluorobutyrl derivative and measurement by negative ion chemical ionization gas chromatography-mass spectrometry to produce a derivative with an intense molecular ion that retains all five deuterium labels. Thus the heptafluorobutyrl derivative analyzed by negative ion mass spectrometry overcomes the problems associated with fragmentation and loss of the isotopic label. Glycerol concentration using a labeled internal standard can be determined in plasma with a precision of 3%. Nanomole amounts of glycerol can be analyzed for 13C or 2H enrichments with a precision of +/- 0.14 mol% excess isotope. This simple, sensitive method for measuring glycerol levels and stable isotopic enrichment in plasma uses a simple extraction procedure and requires a minimal volume of plasma (less than 300 microliters).

Glutamine nitrogen kinetics in insulin-dependent diabetic humans

To assess the effect of insulin deficiency on whole body glutamine kinetics, five young adults with type I (insulin-dependent) diabetes received 4-h primed continuous infusions of L-[1-13C]leucine and L-[2-15N]glutamine in the postabsorptive state after blood glucose had been clamped overnight at either a normoglycemic level (approximately 85 mg/dl) or a moderate hyperglycemic level (approximately 260 mg/dl) by means of an automated glucose control insulin infusion system. The hyperglycemic state was associated with a significant rise in leucine level [from 165 +/- 23 to 242 +/- 62 (SD) microM], appearance rate (from 125 +/- 11 to 142 +/- 17 mumol.kg-1.h-1), and oxidation (from 27 +/- 10 to 31 +/- 10 mumol.kg-1.h-1). In contrast, neither the plasma level nor the appearance rate of glutamine (333 +/- 51 vs. 318 +/- 58 mumol.kg-1.h-1) was affected. We conclude that insulin deficiency resulting in moderate hyperglycemia induces a 13% rise in whole body proteolysis and yet does not stimulate glutamine de novo synthesis, despite increased precursor availability.

Leucine and glutamine metabolism in septic rats

The rate of leucine C-2 incorporation into glutamine was compared in control and septic rats. Female Sprague-Dawley rats (n = 46, 210-260 g) were fed parenterally for 3 days and then randomized into two groups (control and septic). Sepsis was induced by the injection of 10(10) live Escherichia coli/kg on day 4 into the septic group. Rats in each group were given a continuous (8 h) infusion of one of three different isotopes. The isotopes were given 24 h after inoculation. Leucine oxidation and incorporation into protein were determined with [1-13C]leucine; glutamine flux and oxidation were determined with [5-13C]glutamine, and the fraction of leucine C-2 incorporated into glutamine was determined by giving [1,2-13C]leucine. Results were as follows: sepsis caused a significant increase in the rate of leucine C-2 incorporation into glutamine (66.0 +/- 3.7 as against 29.6 +/- 3.7 mumol/h per kg, P less than 0.01). This increase was due to both an increase in glutamine production (2331 +/- 76 as against 1959 +/- 94 mumol/h per kg, P less than 0.01) and an increase in the proportion of glutamine derived from leucine (2.83 +/- 0.27% as against 1.51 +/- 0.31%, P less than 0.01). The ratio of leucine C-2 incorporated into glutamine to leucine oxidized increased from 7.16 +/- 0.91% to 11.49 +/- 1.12% with sepsis (P less than 0.05).

Glutamine metabolism after small intestinal resection in humans

Glutamine and leucine kinetics were measured using stable isotopes in five enterectomized patients (residual small bowel, 80 +/- 25 cm [mean +/- SE]) who were in a near normal nutritional status at distance from surgery. While parameters of leucine metabolism were normal, rates of whole body glutamine utilization were reduced by 20% in the patients. The data suggest that the small intestine plays a prominent role in glutamine utilization in vivo in humans.