Effect of intravenous amino acids on glutamine and protein kinetics in low-birth-weight preterm infants during the immediate neonatal period

Glutamine may be a conditionally essential amino acid in low-birth-weight (LBW) preterm neonates. Exogenously administered amino acids, by providing anaplerotic carbon into the tricarboxylic acid cycle, could result in greater cataplerotic efflux and glutamine de novo synthesis. The effect of dose and duration of amino acid infusion on glutamine and nitrogen (N) kinetics was examined in LBW infants in the period immediately after birth. Preterm neonates (<32 weeks gestation, birth weights 809-1,755 g) were randomized to initially receive either 480 or 960 micromol x kg(-1) x h(-1) of an intravenous amino acid solution for 19-24 hours, followed by a higher or lower amino acid load for either 5 h or 24 h. Glutamine de novo synthesis, leucine N, phenylalanine, and urea kinetics were determined using stable isotopic tracers. An increase in amino acid infusion from 480 to 960 micromol x kg(-1) x h(-1) for 5 h resulted in decreased glutamine de novo synthesis in every neonate (384.4 +/- 38.0 to 368.9 +/- 38.2 micromol x kg(-1) x h(-1), P < 0.01) and a lower whole body rate of proteolysis (P < 0.001) and urea synthesis (P < 0.001). However, when the increased amino acid infusion was extended for 24 h, glutamine de novo synthesis increased (369.7 +/- 92.6 to 483.4 +/- 97.5 micromol x kg(-1) x h(-1), P < 0.001), whole body rate of proteolysis did not change, and urea production increased. Decreasing the amino acid load resulted in a decrease in glutamine rate of appearance (R(a)) and leucine N R(a), but had no effect on phenylalanine R(a). Acutely stressed LBW infants responded to an increase in amino acid load by transiently suppressing whole body rate of glutamine synthesis, proteolysis, and oxidation of protein. The mechanisms of this transient effect on whole body protein/nitrogen metabolism remain unknown.     

Intestinal renal metabolism of L-citrulline and L-arginine following enteral or parenteral infusion of L-alanyl-L-[2,15N]glutamine or L-[2,15N]glutamine in mice

Previously, we observed increased plasma arginine (ARG) concentrations after glutamine (GLN)-enriched diets, in combination with clinical benefits. GLN delivers nitrogen for ARG synthesis, and the present study was designed to quantify the interorgan relationship of exogenous L-GLN or GLN dipeptide, by enteral or parenteral route, contributing to intestinal citrulline (CIT) and renal de novo ARG synthesis in mice. To study this, we used a multicatheterized mouse model with Swiss mice (n = 43) in the postabsorptive state. Stable isotopes were infused into the jugular vein or into the duodenum {per group either free L-[2,(15)N]GLN or dipeptide L-ALA-L-[2,(15)N]GLN, all with L-[ureido-(13)C-(2)H(2)]CIT and L-[guanidino-(15)N(2)-(2)H(2)]ARG} to establish renal and intestinal ARG and CIT metabolism. Blood flow was measured using (14)C-para-aminohippuric acid. Net intestinal CIT release, renal uptake of CIT, and net renal ARG efflux was found, as assessed by arteriovenous flux measurements. Quantitatively, more de novo L-[2,(15)N]CIT was produced when free L-[2,(15)N]GLN was given than when L-ALA-L-[2,(15)N]GLN was given, whereas renal de novo L-[2,(15)N]ARG was similar in all groups. In conclusion, the intestinal-renal axis is hereby proven in mice in that L-[2,(15)N]GLN or dipeptide were both converted into de novo renal L-[2,(15)N]ARG; however, not all was derived from intestinal L-[2,(15)N]CIT production. In this model, the feeding route and form of GLN did not influence de novo renal ARG production derived from GLN.     

Effect of glutamine on glutathione kinetics in vivo in dogs

To determine whether glutamine affects glutathione (GSH, gamma-glutamyl-cysteinyl-glycine) metabolism, seven healthy beagle dogs received 6-h infusions of [(15)N]glutamate and [(13)C]leucine after a 3-day fast. Isotope infusions were performed during oral feeding with an elemental regimen, supplemented with either l-glutamine or an isonitrogenous amino acid mixture, on two separate days and in randomized order. Timed blood samples were obtained, and a surgical duodenal biopsy was performed after 6 h of isotope infusion. GSH fractional synthesis rate (FSR) was assessed from [(15)N]glutamate incorporation into blood and gut GSH, and duodenal protein synthesis from [(13)C]leucine incorporation into gut protein. Glutamine supplementation failed to alter erythrocyte GSH concentration (2189+/-86 vs. 1994+/-102 micromol L(-1) for glutamine vs. control; ns) or FSR (64+/-17% vs. 74+/-20% day(-1); ns). In the duodenum, glutamine supplementation was associated with a 92% rise in reduced/oxidized GSH ratio (P=.024) and with a 44% decline in GSH FSR (96+/-15% day(-1) vs. 170+/-18% day(-1); P=.005), whereas total GSH concentration remained unchanged (808+/-154 vs. 740+/-127 micromol kg(-1); P=.779). We conclude that, in dogs receiving enteral nutrition after a 3-day fast: (1) glutamine availability does not affect blood GSH, and, (2) in contrast, in the duodenum, the preserved GSH pool, along with a decreased synthesis rate, suggests that glutamine may maintain GSH pool and intestinal redox status by acutely decreasing GSH utilization.     

Somatotropin-induced protein anabolism in hindquarters and portal-drained viscera of growing pigs

To differentiate the effect of somatotropin (ST) treatment on protein metabolism in the hindquarter (HQ) and portal-drained viscera (PDV), growing swine (n = 20) treated with ST (0 or 150 microg x kg(-1) x day(-1)) for 7 days were infused intravenously with NaH(13)CO(3) and [(2)H(5)]phenylalanine and enterally with [1-(13)C]phenylalanine while in the fed state. Arterial, portal venous, and vena cava whole blood samples, breath samples, and blood flow measurements were obtained for determination of tissue and whole body phenylalanine kinetics under steady-state conditions. In the fed state, ST treatment decreased whole body phenylalanine flux, oxidation, and protein degradation without altering protein synthesis, resulting in an improvement in whole body net protein balance. Blood flow to the HQ (+80%), but not to the PDV, was increased with ST treatment. In the HQ and PDV, ST increased phenylalanine uptake (+44 and +23%, respectively) and protein synthesis (+43 and +41%, respectively), with no effect on protein degradation. In ST-treated and control pigs, phenylalanine was oxidized in the PDV (34-43% of enteral and arterial sources) but not the HQ. In both treatment groups, dietary (40%) rather than arterial (10%) extraction of phenylalanine predominated in gut amino acid metabolism, whereas localized blood flow influenced HQ amino acid metabolism. The results indicate that ST increases protein anabolism in young, growing swine by increasing protein synthesis in the HQ and PDV, with no effect on protein degradation. Differing results between the whole body and the HQ and PDV suggest that the effect of ST treatment on protein metabolism is tissue specific.     

Effects of enteral glutamine on gut mucosal protein synthesis in healthy humans receiving glucocorticoids

In hypercatabolic patients, the beneficial effects of glutamine on gut mucosa could be partly due to a stimulation of protein synthesis. The fractional synthesis rate (FSR) of gut mucosal protein was measured in four groups of healthy volunteers treated with glucocorticoids for 2 days. Two groups were studied in the postabsorptive state while receiving glutamine or a nitrogen equivalent (control) and two groups in the fed state with or without glutamine, using a 5-h intravenous infusion of [(13)C]leucine, [(2)H(5)]phenylalanine, and cortisone. After nutrient and tracer infusion, duodenal biopsies were taken. In the postabsorptive state, FSR of gut mucosal protein were 87 and 76%/day in the control group and 130% (P = 0.058 vs. control) and 104% (P = 0.17 vs. control)/day in the glutamine group, with leucine and phenylalanine as tracers, respectively. During feeding, FSR did not increase and no significant difference was observed between glutamine and control groups. Overall, FSR of the four groups were two- to threefold higher than those obtained previously in healthy humans, suggesting that glucocorticoids may increase gut mucosal protein synthesis. However, in this situation, a moderate enteral glutamine supply failed to demonstrate a significant effect on gut mucosal protein synthesis in the postabsorptive state and during feeding.     

Protein turnover modifications induced by the protein feeding pattern still persist after the end of the diets

This study was undertaken to determine whether the protein feeding pattern could induce chronic adaptation of protein turnover. After a 15-day adaptive period, elderly (68 yr) and young (26 yr) women received, for 14 days, a diet providing 200 KJ x kg fat-free mass (FFM)(-1) x day(-1), where the daily protein intake (1.7 g protein x kg FFM(-1) x day(-1)) was either spread over 4 meals in the spread pattern or mainly (80%) consumed at noon in the pulse pattern. One day after the end of the dietary treatment, whole body leucine kinetics were measured by use of a continuous [(13)C]leucine infusion, both in the postabsorptive state and in the same fed state. The pulse pattern was able to induce, in young as in elderly women, a lower postabsorptive leucine oxidation and endogenous leucine flux than the spread pattern and improved the responsiveness of nonoxidative leucine disposal during 4-h oral feeding. Thus the pulse pattern was able to induce chronic regulation of protein metabolism in young as in elderly women.     

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.     

Modulation of glutamine uptake and phosphate-activated glutaminase activity in rat brain mitochondria by amino acids and their synthetic analogues

Uptake of L-[14C]Gln and phosphate-activated glutaminase (PAG) activity were measured in nonsynaptic mitochondria isolated from rat cerebral hemispheres, in the presence of protein and nonprotein amino acids and their synthetic structural analogues and derivatives. The uptake was inhibited by > 50% in the presence of a 10-fold excess of His, homocysteine (Hcy), Trp, Leu, Tyr, Ile, Thr, Ala, Phe, Met, Ser, by > 20% in the presence of a 10-fold excess of Val, Arg, Glu, and was not affected by a 10-fold excess of Orn, alpha-ketoglutarate, Tau and Pro. Uptake of L-[14C] Leu differed from Gln uptake by its resistance to Arg, Glu, and a relatively high sensitivity to the reference inhibitor of the plasma membrane transport of large neutral amino acids (L-system)--BCH (2-aminobicyclo[2.2.1]heptane-2-carboxylic acid), and a number of natural L-system substrates. A newly synthesized alanine analogue, 2'-cyano-(biphenyl) alanine, referred to as MRC01, was the only compound tested that inhibited Gln uptake more strongly than Leu uptake. The strongest Gln uptake inhibitors: MRC01, His, Hcy and Leu, inhibited PAG activity by > 50% when added at the inhibitor/Gln concentration ratio of 1:2. PAG activity was not affected by Tau, Lys or Pro, compounds which did affect Gln uptake. The results suggest that a number of natural amino acids function as common endogenous modulators of cerebral mitochondrial Gln uptake and its degradation. MRC01, because of its inhibitory potency towards both mitochondrial Gln uptake and PAG activity, may become a convenient tool in studying the role of Gln transport in its mitochondrial metabolism in intact CNS cell and tissues.     

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.     

Route of administration (enteral or parenteral) affects the contribution of L-glutamine to de novo L-arginine synthesis in mice: a stable-isotope study

A pathway from enteral L-glutamine as substrate for L-arginine synthesis is suggested by previous studies. L-Glutamine and L-glutamine dipeptides exhibit numerous beneficial effects in experimental and clinical studies. In trauma patients, enteral L-glutamine supply increased plasma L-arginine. The present study was designed to quantify the contribution of L-glutamine to the de novo L-citrulline and L-arginine synthesis in mice when L-glutamine is administered in a high dose of labeled L-glutamine or L-alanyl-L-glutamine by the enteral or parenteral route. For this purpose, male Swiss mice (n = 43) underwent a laparotomy, and catheters were inserted for sampling and infusion. A primed, constant, and continuous infusion of L-alanyl-L-[2-(15)N]glutamine (dipeptide groups) or L-[2-(15)N]glutamine (free L-glutamine groups), simultaneously with L-[ureido-(13)C,(2)H(2)]citrulline and L-[guanidino-(15)N(2),(2)H(2)]arginine, was given (steady-state model). Mice received the L-glutamine tracers intravenously (jugular vein) or enterally (duodenum). Enrichments of metabolites were measured by LC-MS. Arterial L-glutamine concentrations were the highest in the intravenous dipeptide group. L-Glutamine was converted to L-citrulline and L-arginine when L-[2-(15)N]glutamine and L-alanyl-L-[2-(15)N]glutamine were given by enteral or parenteral route. The contribution of L-glutamine to the de novo synthesis of L-citrulline and L-arginine was higher in the enteral groups when compared with the intravenous groups (P < 0.005). Therefore, the route of administration (enteral or parenteral) affects the contribution of L-glutamine, provided as free molecule or dipeptide, to the de novo synthesis of L-arginine in mice.     

Nutrition and metabolism of glutamate and glutamine in fish

Amino Acids - Glutamate (Glu) and glutamine (Gln) comprise a large proportion of total amino acids (AAs) in fish in the free and protein-bound forms. Both Glu and Gln are synthesized de novo from...     

Trypsin and splanchnic protein turnover during feeding and fasting in human subjects

Knowledge of the stimulatory effects of enteral and parenteral (intravenous) feeding on the synthesis and turnover of trypsin would help in the management of acute pancreatitis, because the disease is caused by the premature activation of trypsin. To investigate this, we labeled intravenous infusions with [1-(13)C]leucine and enterals with [(2)H]leucine and measured isotope enrichment of plasma, secreted trypsin, and duodenal mucosal proteins over 6 h by duodenal perfusion/aspiration and endoscopic biopsy. Thirty healthy volunteers were studied during fasting (n = 7), intravenous feeding (n = 6), or postpyloric enteral feeding [duodenal polymeric (n = 6), elemental duodenal (n = 6), and jejunal elemental (n = 5)]. All diets provided 1.5 g x kg(-1) x day(-1) protein and 40 kcal x kg(-1) x day(-1) energy. Results demonstrated that compared with fasting, enteral feeding increased the rate of appearance (71 +/- 4 vs. 91 +/- 5 min, P = 0.01) and secretion (546 +/- 80 vs. 219 +/- 37 U/h, P = 0.01) of newly labeled trypsin and expanded zymogen stores (1,660 +/- 237 vs. 749 +/- 133 units, P = 0.03). These differences persisted whether the feedings were polymeric or elemental, duodenal, or jejunal. In contrast, intravenous feeding had no effect on basal rates. Differential labeling of the plasma amino acid pool by enteral and intravenous isotope infusions suggested that 35% of absorbed amino acids were retained within the splanchnic bed during enteral feeding and that mucosal protein turnover increased from a fasting rate of 34 +/- 6 to 108 +/- 8%/day (P < 0.05) compared with no change after intravenous feeding. In conclusion, all common forms of enteral feeding stimulate the synthesis and secretion of pancreatic trypsin, and only parenteral nutrition avoids it.     

Glutamine uptake and expression of mRNA's of glutamine transporting proteins in mouse cerebellar and cerebral cortical astrocytes and neurons

The relative roles of the three sodium-dependent transport systems: A, ASC and N in the uptake of [3H]Gln, and the compatibility of the uptake characteristics with the expression of mRNAs coding for the Gln transporting molecules, were examined in primary cultures of astrocytes and neurons derived from mouse cerebellum, a glutaminergic system-enriched structure, and in cerebral cortex. Gln uptake activity (Vmax) was higher in cerebellar astrocytes or neurons than in their cerebral cortical counterparts. The N-methylamino-isobutyric acid (MeAiB)- and pH-sensitive, system A-mediated component of the uptake, and the uptake of [14C]MeAiB itself, was much more active in neurons than in astrocytes derived from either region. Also, the expression of mRNA for GlnT (SAT1), a system A isoform specific for Gln, was only expressed in neurons derived from both structures, while an alanine (Ala)-preferring system A transporter, SAT2, was expressed in neurons and astrocytes from either region. System ASC-mediated Gln uptake and expression of ASCT2 mRNA were in both structures more pronounced in astrocytes than in neurons, consistent with the postulated role of ASCT2 in the efflux of de novo synthesized Gln from astrocytes. System N-mediated (threonine+MeAiB-inhibitable) Gln uptake showed comparable activities in all four types of cells, which is compatible with the ubiquitous expression of NAT2 mRNA-a mouse brain-specific N-system isoform.     

Somatotropin increases protein balance by lowering body protein degradation in fed, growing pigs

Somatotropin (ST) administration enhances protein deposition in well-nourished, growing animals. To determine whether the anabolic effect is due to an increase in protein synthesis or a decrease in proteolysis, pair-fed, weight-matched ( approximately 20 kg) growing swine were treated with porcine ST (150 microg. kg(-1). day(-1), n = 6) or diluent (n = 6) for 7 days. Whole body leucine appearance (R(a)), nonoxidative leucine disposal (NOLD), urea production, and leucine oxidation, as well as tissue protein synthesis (K(s)), were determined in the fed steady state using primed continuous infusions of [(13)C]leucine, [(13)C]bicarbonate, and [(15)N(2)]urea. ST treatment increased the efficiency with which the diet was used for growth. ST treatment also increased plasma insulin-like growth factor I (+100%) and insulin (+125%) concentrations and decreased plasma urea nitrogen concentrations (-53%). ST-treated pigs had lower leucine R(a) (-33%), leucine oxidation (-63%), and urea production (-70%). However, ST treatment altered neither NOLD nor K(s) in the longissimus dorsi, semitendinosus, or gastrocnemius muscles, liver, or jejunum. The results suggest that in the fed state, ST treatment of growing swine increases protein deposition primarily through a suppression of protein degradation and amino acid catabolism rather than a stimulation of protein synthesis.     

Time course of nitric oxide production after endotoxin challenge in mice

Nitric oxide (NO) regulates numerous processes during endotoxemia and inflammation. However, the sequential changes in whole body (Wb) nitric oxide (NO) production during endotoxemia in vivo remain to be clarified. Male Swiss mice were injected intraperitoneally with saline (control group) or lipopolysaccharide (LPS group). After 0, 2, 4, 6, 9, 12, and 24 h, animals received a primed constant infusion of L-[guanidino-(15)N(2)-(2)H(2)]arginine, L-[ureido-(15)N]citrulline, L-[5-(15)N]glutamine, and L-[ring-(2)H(5)]phenylalanine in the jugular vein. Arterial blood was collected for plasma arginine (Arg), citrulline (Cit), glutamine (Gln), and phenylalanine (Phe) concentrations and tracer-to-tracee ratios. NO production was calculated as plasma Arg-to-Cit flux, Wb de novo Arg synthesis as plasma Cit-to-Arg flux, and Wb protein breakdown as plasma Phe flux. LPS reduced plasma Arg and Cit and increased Gln and Phe concentrations. Two peaks of NO production were observed at 4 and 12 h after LPS. Although LPS did not affect total Arg production, de novo Arg production decreased after 12 h. The second peak of NO production coincided with increased Wb Cit, Gln, and Phe production. In conclusion, the curve of NO production in both early and late phases of endotoxemia is not related to plasma Arg kinetics. However, because Wb Cit, Gln, and Phe fluxes increased concomitantly with the second peak of NO production, NO production is probably related to the catabolic phase of endotoxemia.     

Evidence for the glia-neuron protein transfer hypothesis from intracellular perfusion studies of squid giant axons

Incubation of intracellulary perfused squid giant axons in [3H]leucine demonstrated that newly synthesized proteins appeared in the perfusate after a 45-min lag period. The transfer of labeled proteins was shown to occur steadily over 8 h of incubation, in the presence of an intact axonal plasma membrane as evidenced by the ability of the perfused axon to conduct propagated action potentials over this time-period. Intracellularly perfused RNase did not affect this transfer, whereas extracellularly applied puromycin, which blocked de novo protein synthesis in the glial sheath, prevented the appearance of labeled proteins in the perfusate. The uptake of exogenous 14C-labeled bovine serum albumin (BSA) into the axon had entirely different kinetics than the endogenous glial labeled protein transfer process. The data provide support for the glia-neuron protein transfer hypothesis.     

Glutamine and leucine nitrogen kinetics and their relation to urea nitrogen in newborn infants

Glutamine kinetics and its relation to transamination of leucine and urea synthesis were quantified in 16 appropriate-for-gestational-age infants, four small-for-gestational-age infants, and seven infants of diabetic mothers. Kinetics were measured between 4 and 5 h after the last feed (fasting) and in response to formula feeding using [5-(15)N]glutamine, [1-(13)C,(15)N]leucine, [(2)H(5)]phenylalanine, and [(15)N(2)]urea tracers. Leucine nitrogen and glutamine kinetics during fasting were significantly higher than those reported in adults. De novo synthesis accounted for approximately 85% of glutamine turnover. In response to formula feeding, a significant increase (P = 0.04) in leucine nitrogen turnover was observed, whereas a significant decrease (P = 0.002) in glutamine and urea rate of appearance was seen. The rate of appearance of leucine nitrogen was positively correlated (r(2) = 0.59, P = 0.001) with glutamine turnover. Glutamine flux was negatively correlated (r(2) = 0.39, P = 0.02) with the rate of urea synthesis. These data suggest that, in the human newborn, glutamine turnover is related to a high anaplerotic flux into the tricarboxylic acid cycle as a consequence of a high rate of protein turnover. The negative relationship between glutamine turnover and the irreversible oxidation of protein (urea synthesis) suggests an important role of glutamine as a nitrogen source for other synthetic processes and accretion of body proteins.     

Acyl-CoAs are functionally channeled in liver: potential role of acyl-CoA synthetase

Acyl-CoA synthetase (ACS) catalyzes the activation of long-chain fatty acids to acyl-CoAs, which can be metabolized to form CO(2), triacylglycerol (TAG), phospholipids (PL), and cholesteryl esters (CE). To determine whether inhibiting ACS affects these pathways differently, we incubated rat hepatocytes with [(14)C]oleate and the ACS inhibitor triacsin C. Triacsin inhibited TAG synthesis 70% in hepatocytes from fed rats and 40% in starved rats, but it had little effect on oleate incorporation into CE, PL, or beta-oxidation end products. Triacsin blocked [(3)H]glycerol incorporation into TAG and PL 33 and 25% more than it blocked [(14)C]oleate incorporation, suggesting greater inhibition of de novo TAG synthesis than reacylation. Triacsin did not affect oxidation of prelabeled intracellular lipid. ACS1 protein was abundant in liver microsomes but virtually undetectable in mitochondria. Refeeding increased microsomal ACS1 protein 89% but did not affect specific activity. Triacsin inhibited ACS specific activity in microsomes more from fed than from starved rats. These data suggest that ACS isozymes may be functionally linked to specific metabolic pathways and that ACS1 is not associated with beta-oxidation in liver.     

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.