Measurement of the transfer of the nitrogen moiety of intestinal lumen glutamic acid in man after oral ingestion of l-[15N]glutamic acid

Origin and Roles of Alanine and Glutamine in Gluconeogenesis in the Liver, Kidneys, and Small Intestine under Physiological and Pathological Conditions

Alanine and glutamine are the principal glucogenic amino acids. Most originate from muscles, where branched-chain amino acids (valine, leucine, and isoleucine) are nitrogen donors and, under exceptional circumstances, a source of carbons for glutamate synthesis. Glutamate is a nitrogen source for alanine synthesis from pyruvate and a substrate for glutamine synthesis by glutamine synthetase. The following differences between alanine and glutamine, which can play a role in their use in gluconeogenesis, are shown: (i) glutamine appearance in circulation is higher than that of alanine; (ii) the conversion to oxaloacetate, the starting substance for glucose synthesis, is an ATP-consuming reaction for alanine, which is energetically beneficial for glutamine; (iii) most alanine carbons, but not glutamine carbons, originate from glucose; and (iv) glutamine acts a substrate for gluconeogenesis in the liver, kidneys, and intestine, whereas alanine does so only in the liver. Alanine plays a significant role during early starvation, exposure to high-fat and high-protein diets, and diabetes. Glutamine plays a dominant role in gluconeogenesis in prolonged starvation, acidosis, liver cirrhosis, and severe illnesses like sepsis and acts as a substrate for alanine synthesis in the small intestine. Interactions among muscles and the liver, kidneys, and intestine ensuring optimal alanine and glutamine supply for gluconeogenesis are suggested.

Metabolomics reveals the mechanism of polyethylene microplastic toxicity to Daphnia magna

Microplastic exposure leads to various toxic effects in Daphnia magna; however, the effects of microplastics on the metabolic processes in D. magna and the corresponding molecular toxicity mechanisms remain unclear. In the present study, the effects of acute exposure to polyethylene microplastics with different particle sizes (20 μm [MPs-20] and 30 μm [MPs-30]) on metabolites in D. magna and the mechanisms of toxicity were investigated by combining metabolomics and traditional toxicology techniques. Exposure to both MPs-20 and MPs-30 resulted in significant accumulation of microplastics in the gut of D. magna and significantly reduced D. magna survival and heart rate. Metabolomics analysis revealed that MPs-20 and MPs-30 induced significant changes in up to 88 and 91 differential metabolites, respectively, and collectively induced significant changes in 75 metabolites in D. magna. Among lipid metabolites, MPs-20 specifically downregulated phosphatidylcholine and upregulated phosphatidylethanolamine, which mainly affected phospholipid metabolism, whereas MPs-30 specifically downregulated amino acid metabolites l-glutamine, l-glutamate and malic acid, which mainly interfered with energy metabolism. The results of this study provide novel insights into the mechanism of effects of microplastics on metabolic processes in D. magna.

Effects of histidine supplementation on amino acid metabolism in rats

Histidine (HIS) is investigated for therapy of various disorders and as a nutritional supplement to enhance muscle performance. We examined effects of HIS on amino acid and protein metabolism. Rats consumed HIS in a drinking water at a dose of 0.5 g/l (low HIS), 2 g/l (high HIS) or 0 g/l (control) for 4 weeks. At the end of the study, the animals were euthanized and blood plasma, liver, soleus (SOL), tibialis (TIB), and extensor digitorum longus (EDL) muscles analysed. HIS supplementation increased food intake, body weight and weights and protein contents of the liver and kidneys, but not muscles. In blood plasma there were increases in glucose, urea, and several amino acids, particularly alanine, proline, aspartate, and glutamate and in high HIS group, ammonia was increased. The main findings in the liver were decreased concentrations of methionine, aspartate, and glycine and increased alanine. In muscles of HIS-consuming animals increased alanine and glutamine. In high HIS group (in SOL and TIB) increased chymotrypsin-like activity of proteasome (indicates increased proteolysis); in SOL decreased anserine (beta-alanyl-N1-methylhistidine). We conclude that HIS supplementation increases ammonia production, alanine and glutamine synthesis in muscles, affects turnover of proteins and HIS-containing peptides, and increases requirements for glycine and methionine.

Effects of histidine load on ammonia, amino acid, and adenine nucleotide concentrations in rats

The unique capability of proton buffering is the rationale for using histidine (HIS) as a component of solutions for induction of cardiac arrest and myocardial protection in cardiac surgery. In humans, infusion of cardioplegic solution may increase blood plasma HIS from ~ 70 to ~ 21,000 µM. We examined the effects of a large intravenous dose of HIS on ammonia and amino acid concentrations and energy status of the body. Rats received 198 mM HIS intravenously (20 ml/kg) or vehicle. Samples of blood plasma, urine, liver, and soleus (SOL) and extensor digitorum longus (EDL) muscles were analysed at 2 or 24 h after treatment. At 2 h after HIS load, we found higher HIS concentration in all examined tissues, higher urea and ammonia concentrations in blood and urine, lower ATP content and higher AMP/ATP ratio in the liver and muscles, higher concentrations of almost all examined amino acids in urine, and lower glycine concentration in blood plasma, liver, and muscles when compared with controls. Changes in other amino acids were tissue dependent, markedly increased alanine and glutamate in the blood and the liver. At 24 h, the main findings were lower ATP concentrations in muscles, lower concentrations of branched-chain amino acids (BCAA; valine, leucine, and isoleucine) in blood plasma and muscles, and higher carnosine content in SOL when compared with controls. It is concluded that a load of large HIS dose results in increased ammonia levels and marked alterations in amino acid and energy metabolism. Pathogenesis is discussed in the article.

Metabolism and functions of L-glutamate in the epithelial cells of the small and large intestines

l-Glutamate is one of the most abundant amino acids in alimentary proteins, but its concentration in blood is among the lowest. This is largely because l-glutamate is extensively oxidized in small intestine epithelial cells during its transcellular journey from the lumen to the bloodstream and after its uptake from the bloodstream. This oxidative capacity coincides with a high energy demand of the epithelium, which is in rapid renewal and responsible for the nutrient absorption process. l-Glutamate is a precursor for glutathione and N-acetylglutamate in enterocytes. Glutathione is involved in the enterocyte redox state and in the detoxication process. N-acetylglutamate is an activator of carbamoylphosphate synthetase 1, which is implicated in l-citrulline production by enterocytes. Furthermore, l-glutamate is a precursor in enterocytes for several other amino acids, including l-alanine, l-aspartate, l-ornithine, and l-proline. Thus, l-glutamate can serve both locally inside enterocytes and through the production of other amino acids in an interorgan metabolic perspective. Intestinal epithelial cell capacity to oxidize l-glutamine and l-glutamate is already high in piglets at birth and during the suckling period. In colonocytes, l-glutamate also serves as a fuel but is provided from the bloodstream. Alimentary and endogenous proteins that escape digestion enter the large intestine and are broken down by colonic bacterial flora, which then release l-glutamate into the lumen. l-Glutamate can then serve in the colon lumen as a precursor for butyrate and acetate in bacteria. l-Glutamate, in addition to fiber and digestion-resistant starch, can thus serve as a luminally derived fuel precursor for colonocytes.

Splanchnic utilization of enteral alanine in humans

The splanchnic bed extracts the majority of the enteral nonessential amino acids glutamine and glutamate, while extracting a much smaller proportion of essential amino acids such as leucine and phenylalanine. Alanine is an abundant nonessential amino acid that plays an important role in hepatic gluconeogenesis and ureagenesis. However, its enteral fate has not been studied. Twelve normal healthy postabsorptive adults received a 7-hour infusion of [1-13C]alanine, 3.5 hours intravenously (IV) and 3.5 hours via a nasogastric tube (NG). The order of infusion was randomized among subjects. Alanine kinetics were calculated from the enrichments of plasma alanine 13C and expired 13CO2. The alanine appearance rate (Ra), measured during the IV tracer infusion, was 279+/-17 micromol/kg/h; 92%+/-2% of the IV-infused and 86%+/-2% of the NG-infused [1-13C]alanine tracer was recovered as 13CO2. From the difference in plasma alanine 13C enrichment between IV-infused and NG-infused tracers, we determined that the splanchnic bed extracted 69%+/-1% of the enterally delivered alanine tracer on the first pass during absorption. Only one third of the enteral alanine passed intact through the splanchnic bed and was made available to systemic tissues. Of the enteral alanine extracted, 83%+/-3% of the carboxyl-carbon label was recovered as CO2, leaving only 17% of the sequestered alanine available for use in splanchnic protein synthesis. Thus, the splanchnic bed, presumably the liver, extracts and metabolizes most of the enterally delivered alanine.

Can a glutamate-enriched diet counteract glutamine depletion in endotoxemic rats?

The study evaluated whether a glutamate-enriched diet would restore glutamine tissue pools and maintain tissue trophicity in endotoxemic rats. For this purpose, young male Sprague-Dawley rats received an intraperitoneal injection of lipopolysaccharide (LPS) from Escherichia coli at 3 mg/kg body weight. After 24 hours of food deprivation, the rats were enterally refed for 48 hours using Osmolite enriched with glutamate at 4 g/kg/d (LPS-Glu group, n = 7) or glycine isonitrogenous to glutamate (LPS-Gly group, n = 7). A control group (healthy group, n = 7) had free access to a standard rodent diet. Tissue weights and protein contents were significantly lower in both LPS-treated groups than in the healthy group. No plasma or tissue accumulation of glutamate was observed except in the liver. Glutamine concentrations were increased in the jejunum, liver, and plasma in the LPS-Glu group versus the other two groups (P < 0.05). Conversely, they were depleted in muscles of the endotoxemic groups versus the healthy group (P < 0.05). Villus height was significantly greater in the LPS-Glu group than in the LPS-Gly group in the jejunum (P < 0.05), but not in the ileum. In conclusion, a glutamate-enriched diet administered enterally to endotoxemic rats can counteract glutamine depletion in the splanchnic area but not in muscles. In addition, glutamate displayed a trophic effect restricted to the jejunum.

Glutamate and the UMAMI taste: sensory, metabolic, nutritional and behavioural considerations. A review of the literature published in the last 10 years

Monosodium glutamate (MSG) is used increasingly often in processed foods and in home cooking in the Western world. This substance is responsible for a pleasurable taste sensation, the Umami taste. This review covers recent developments in sensory studies of glutamate effects, and traces the Umami taste from sensory receptors on the tongue to the brain. The metabolism of glutamic acid, as revealed from recent literature, is described. A specific section is devoted to safety issues. In addition, effects of glutamic salts on nutrition and ingestive behaviours are shown to be potent. Animal and human works are treated separately, with special attention to the specific methods used in both cases. Future areas of research include further investigation of sensory physiology, role of glutamate as an excitatory substance in the brain, acquisition of food likes and impact on long-term food selection, food intake, and body weight control.

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.