Luminal and metabolic regulation of jejunal amino acid absorption in the rat

Studies were carried out to determine the role of luminal amino acids and metabolic balance in in vivo amino acid absorption. Previous in vitro studies have shown that adaptation of amino acid transport is a complex phenomenon. In the first series of experiments, parenterally nourished rats received a 7-day jejunal infusion of either 3% aspartic acid, glutamine, lysine, valine, or mixed amino acids. A single-pass perfusion was performed to determine the effects of infusates on 5 mM valine, aspartic acid, and lysine absorption. Compared with controls receiving luminal saline, prior glutamine infusion increased valine absorption; prior valine, glutamine, and aspartic acid infusion significantly increased aspartic acid absorption; and prior valine and lysine infusion significantly increased lysine absorption. The mixed amino acid solution had no effect. The effects of metabolic balance were examined by comparing fasted rats with parenterally fed and orally fed rats. Within 24 h fasting significantly increased valine and aspartic acid absorption, despite a significant decrease in intestinal mass.     

Relationship of a Membrane-Bound D-(-)-Lactic Dehydrogenase to Amino Acid Transport in Isolated Bacterial Membrane Preparations

The conversion of D-lactate to pyruvate in isolated membrane preparations of E. coli ML 308-225 markedly stimulates the transport of proline, glutamic acid, aspartic acid, aspargine, tryptophan, lysine, serine, alanine, and glycine. The uptake of histidine, phenylalanine, tyrosine, leucine, isoleucine, and valine by the membranes is also markedly stimulated by this conversion, although these amino acids are taken up much less effectively than those mentioned previously. The uptake of arginine, methionine, cystine, and cysteine is enhanced only about twofold in the presence of D-(-)-lactate, and these amino acids are not concentrated well by the membranes. With the exception of glutamate, asparate, asparagine, and methionine, which are converted to other metabolites to varying extents in the intramembranal pool, each of the other amino acids was recovered from the membranes as the unchanged amino acid. Succinate, L-(+)-lactate, D,L-alpha-hydroxybutyrate, and DPNH partially replace D-(-)-lactate but are less effective.     

Evolution and relatedness in two aminoacyl-tRNA synthetase families.

Sequence segments of about 140 amino acids in length, each containing a selected consensus region, were used in alignments of the aminoacyl-tRNA synthetases with the aim of discerning their evolutionary relationships. In all cases tested, enzymes specific for the same amino acid from a variety of organisms grouped together, reinforcing the supposition that the aminoacyl-tRNA synthetases are very ancient enzymes that evolved to include the full complement of 20 amino acids long before the divergence leading to prokaryotes and eukaryotes. The enzymes are divided into two mutually exclusive groups that appear to have evolved from independent roots. Group I, for which two sequence segments were analyzed, contains the enzymes specific for glutamic acid, glutamine, tryptophan, tyrosine, valine, leucine, isoleucine, methionine, and arginine. Group II enzymes include those activating threonine, proline, serine, lysine, aspartic acid, asparagine, histidine, alanine, glycine, and phenylalanine. Both groups contain a spectrum of amino acid types, suggesting the possibility that each could have once supported an independent system for protein synthesis. Within each group, enzymes specific for chemically similar amino acids tend to cluster together, indicating that a major theme of synthetase evolution involved the adaptation of binding sites to accommodate related amino acids with subsequent specialization to a single amino acid. In a few cases, however, synthetases activating dissimilar amino acids are grouped together.     

Amino acids are not all initially attached to the same position on transfer RNA molecules.

Escherichia coli tRNA has been modified by replacement of the 3'-terminal AMP with either 3'-amino-3'-deoxy AMP of 2'-amino-2'-deoxy AMP. These tRNA analogs have enabled us to determine the initial site of enzyme-catalyzed aminoacylation of different tRNAs by the formation of aminoacyl-tRNA molecules in which the amino acid is linked to the 3'-terminal ribose through a stable amide bond. The tRNA species specific for glutamic acid, glutamine, leucine, phenylalanine, tyrosine, and valine are all aminoacylated on the 2'-hydroxyl group. The tRNA species specific for alanine, asparagine, aspartic acid, glycine, histidine, lysine, and threonine are aminoacylated on the 3'-hydroxyl group. The amino acids arginine, isoleucine, methionine, proline, serine, and tryptophan form stable amide bonds with both amino tRNA analogs. This might suggest that the synthetases for these amino acids can acylate both the 2'- and 3'-hydroxyl groups, but it is more likely that these enzymes can acylate both hydroxyl and amino groups at either the 2' or 3'-position of the tRNA. These results clearly illustrate a fundamental heterogeneity which is apparent in the mechanism of action of aminoacyl-tRNA synthetases.     

Atypical Pattern of Utilization of Amino Acids for Mitochondrial Protein Synthesis in HeLa Cells

The capacity of HeLa cell mitochondria, either isolated or in intact cells, to incorporate different labeled amino acids into proteins was investigated. Eight amino acids (alanine, arginine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, and lysine), which include most of the charged polar ones, showed a very low amount, if any at all, of chloramphenicol-sensitive incorporation, relative to that expected for an "average" HeLa-cell protein. By contrast, the most hydrophobic amino acids (leucine, isoleucine, valine, phenylalanine, and methionine) were the most actively incorporated by HeLa mitochondria. The available evidence suggests that pool effects cannot account for this general pattern of utilization of amino acids; furthermore, this pattern is in good agreement with the known hydrophobic properties of proteins synthesized in mitochondria.     

Brain uptake and release of amino acids in nondiabetic and insulin-dependent diabetic subjects: important role of glutamine release for nitrogen balance.

We measured net uptake and release of amino acids in the brain of 7 nondiabetic and six diabetic subjects. Duration of insulin-dependent diabetes (IDDM) was 19.4 +/- 2.1 years. Arteriojugular vein measurements were performed before and after 120 minutes of insulin infusion and ensuing Biostator-regulated normoglycemia. Cerebral blood flow was measured during normoglycemia by 11-CH3-F and positron emission tomography. During hyperglycemia in the IDDM subjects, arterial concentrations of valine and leucine were higher, and those of glutamic acid and arginine lower, than in nondiabetic subjects. Insulin infusion lowered levels of most amino acids in both groups. Insulin treatment did not significantly affect the uptake or release of amino acids. Significant net uptake of branched-chain amino acids was noted in both groups, as well as uptake of lysine and phenylalanine in the IDDM subjects. The sum of measured differences was not different from zero in either group. Nitrogen balance depended on impressive release of glutamine from the brain (-963 +/- 147 and -960 +/- 303 nmol/100 g/min), which amounted to 73% and 69% of net release in nondiabetic and IDDM subjects, respectively. We conclude that balance between uptake and release of amino acids is similar in nondiabetic and in long-term IDDM subjects.     

Citrulline blood levels as indicators of residual intestinal absorption in patients with short bowel syndrome

Plasma citrulline is known to be a marker of absorptive enterocyte mass in humans. We evaluated whether citrulline and other blood amino acids are indicators of residual small intestinal length and therefore potential predictors of dependence on parenteral nutrition in the long term. We studied 25 patients with short bowel syndrome (SBS) after at least 18 months since last digestive circuit modification; 24 of them were again evaluated 1 year later. Ten patients were weaned off parenteral nutrition and 15 were dependent on parenteral nutrition. Fifty-four healthy volunteers (28 women and 26 men) served as controls. Amino acid levels were determined on serum with high-performance liquid chromatography (HPLC) as well as on blood and serum with tandem mass spectrometry analysis. Five amino acids (citrulline, leucine, isoleucine, valine and tyrosine) were significantly lower in all SBS patients than in controls, whereas glutamine, measured only by HPLC, was significantly higher. Nevertheless, only serum citrulline measured with HPLC was significantly related to small bowel length. We conclude that HPLC remains the reference methodology to evaluate blood or serum amino acid levels in adult population with SBS.     

Plasma concentrations and tissue uptake of free amino acids in dogs in sepsis and starvation: effects of glucose infusion--some effects of low alimentation

The plasma concentrations of substrates, together with transhepatic and transgut balances, have been studied in six control and eight septic awake fasted dogs. Four severely ill septic dogs (typically fluid in chest and/or abdomen, extensive peritonitis, respiratory difficulties) had high concentrations of threonine, glycine, tyrosine, lysine, histidine, tryptophan, and triglycerides (p less than or equal to 0.05). The other septic dogs (less severely ill) showed fewer and less pronounced alterations in the plasma substrates (aspartate and tryptophan were elevated, p less than or equal to 0.05). The infusion of glucose increased the concentration of glucose, lactate, and pyruvate and depressed the concentrations of most amino acids in both normal and septic dogs. Threonine, asparagine, glutamine, leucine, isoleucine, alpha-aminobutyrate, and tyrosine were significantly depressed in the severely ill septic dogs (p less than or equal to 0.05). In the normal dogs most amino acids were removed by the liver, with alanine accounting for approximately 40% of the total. Glutamine removal was negligible. In the septic dogs hepatic removal of amino acids was variable; livers of two severely ill septic dogs did not remove amino acids. In the control dogs glucose infusion (0.015--0.017 g/kg/min) tended to lower hepatic removal of amino acids. Hepatic dye removal in the septic dogs was always very poor. In the gut glutamine was removed and alanine, glutamate, glycine, and ammonia produced, but the overall sum of amino acid uptake was negligible in both the control and septic dogs. The ratio of tryptophan to the sum of valine, isoleucine, leucine, tyrosine, and phenylalanine concentrations was greatly elevated in all septic dogs in which it was measured. The free concentrations of amino acids in the liver, heart, and muscle tissues were grossly elevated in the low intravenous alimented septic state relative to the fasted normal state, whereas the tissue concentrative ability as measured by nonmetabolizable amino acids, alpha-aminoisobutyrate and cycloleucine, was not similarly increased. Sepsis clearly alters plasma and tissue concentrations, and in some instances hepatic uptake of amino acids.     

Glutamine for the gut: Mystical properties or an ordinary amino acid?

Glutamine is a nonessential amino acid that can be synthesized from glutamate and glutamic acid by glutamine synthetase. It is the preferred fuel for the small intestine in the rat. Results from animal studies suggest that both glutamine-supplemented parenteral nutrition and enteral diets may prevent bacterial translocation. This effect may be modulated through the preservation and augmentation of small bowel villus morphology, intestinal permeability, and intestinal immune function. The existing data from studies with humans are less compelling. What, if any, intestinal deficits actually occur during provision of exclusive parenteral nutrition remains unclear. Furthermore, the clinical significance of these changes is largely undefined. Nevertheless, glutamine and glutamine supplementation appear to be important for the normal maintenance of intestinal morphology and function, intestinal adaptation following resection, and prevention of clinical infection related to bacterial translocation. The existing data on the use of parenteral and enteral glutamine for preservation of intestinal morphology and function and prevention of bacterial translocation in humans are reviewed in this article. Pertinent animal data are also described.     

Influence of glutamine and branched chain amino acids on the jejunal atrophy associated with parenteral nutrition

Infusions of conventional parenteral nutrients (CPN) are associated with gut atrophy. This may be due to the absence of glutamine in such solutions. Although glutamine is a preferred gut nutrient, it is excluded from CPN because it is unstable at room temperature. This problem may be circumvented either directly by the infusion of fresh solutions of glutamine, or indirectly by the infusion of branched chain amino acids (BCAA). We evaluated the effect of infusing either glutamine, BCAA, or glutamine plus BCAA-enriched CPN on the rat jejunum. Sixty male Wistar rats were randomized to receive 6 days of either conventional parenteral nutrition (CPN), CPN plus 1.5% glutamine (GLN), CPN plus 2% BCAA (BCAA), CPN plus 0.8% BCAA and 1.0% glutamine (GLN/BCAA), or a normal oral diet (Chow). Standardized segments of jejunum were then removed for assessment. Compared with the CPN group, both the GLN/BCAA and the BCAA groups had greater mucosal weights (P less than 0.05) and mucosal protein concentrations (P less than 0.05), the GLN/BCAA group had greater jejunal weights (P less than 0.05), and the GLN group had an increased jejunal weight (P less than 0.05) and a higher crypt cell production rate (P less than 0.05). We conclude that the infusion of glutamine or BCAA-enriched parenteral nutrition improves jejunal morphology compared with conventional parenteral nutrition.     

Amino acid metabolism during exercise in trained rats: the potential role of carnitine in the metabolic fate of branched-chain amino acids

The influence of endurance training and an acute bout of exercise on plasma concentrations of free amino acids and the intermediates of branched-chain amino acid (BCAA) metabolism were investigated in the rat. Training did not affect the plasma amino acid levels in the resting state. Plasma concentrations of alanine (Ala), aspartic acid (Asp), asparagine (Asn), arginine (Arg), histidine (His), isoleucine (Ile), leucine (Leu), lysine (Lys), methionine (Met), phenylalanine (Phe), proline (Pro), serine (Ser), threonine (Thr), and valine (Val) were significantly lower, whereas glutamate (Glu), glycine (Gly), ornithine (Orn), tryptophan (Trp), tyrosine (Tyr), creatinine, urea, and ammonia levels were unchanged, after one hour of treadmill running in the trained rats. Plasma concentration of glutamine (Glu), the branched-chain keto acids (BCKA) and short-chain acyl carnitines were elevated with exercise. Ratios of plasma BCAA/BCKA were dramatically lowered by exercise in the trained rats. A decrease in plasma-free carnitine levels was also observed. These data suggest that amino acid metabolism is enhanced by exercise even in the trained state. BCAA may only be partially metabolized within muscle and some of their carbon skeletons are released into the circulation in forms of BCKA and short-chain acyl carnitines.     

The effect of hemorrhagic shock on intestinal amino acid absorption in vivo.

Hemorrhagic shock causes a disproportionate decrease in portal blood flow which may adversely affect the barrier and absorptive functions of the intestine. The absorptive capacity of the small intestine was studied during shock induced by mild and severe hemorrhage in the rat by measurement of the uptake of a radiolabelled amino acid analog (aminoisobutyric acid: AIB). Hemorrhage resulted in a significant reduction in systemic blood pressure, portal blood flow, and the absorption of AIB from the small intestine. Resuscitation restored both blood pressure and portal blood flow. But a significant reduction in absorption of AIB persisted. This suggests that hemorrhage results in an inhibition of amino acid intestinal active transport which is not dependent on the mesenteric circulation. The reduced intestinal absorptive function has important implications for the route of administration of nutrition following hemorrhage and trauma.     

Nutritional management of short bowel syndrome

Short-bowel syndrome refers to malabsorption, diarrhea, and weight loss following an extensive resection of small bowel. A main consequence is malabsorption of macro- and micronutrients. Nutritional outcome after intestinal resection depends on the extent and location of resection, presence of ileocecal valve and a colon, functional status of the residual intestine, and adaptation. Intraluminal nutrients and trophic factors are critical for intestinal adaptation. The dietary management is focused on the enhancement of intestinal adaptation and optimal caloric intake. Patients with short-bowel syndrome require an individualized diet, and some may require total parenteral nutrition indefinitely. Growth hormone, glutamine, and GLP-2 are reviewed with emphasis on their current use in clinical practice. The nutritional aspect of short-bowel syndrome is complex, with the ultimate goal of weaning the patients from parenteral nutrition. Intestinal transplant is a treatment option for select patients.     

Post-feeding hyperammonaemia in patients with transjugular intrahepatic portosystemic shunt and liver cirrhosis: role of small intestinal ammonia release and route of nutrient administration

BACKGROUND: Hyperammonaemia is a pathogenetic factor for hepatic encephalopathy that may be augmented after a transjugular intrahepatic portosystemic shunt (TIPS). Experimental data suggest that hyperammonaemia may be caused to a large extent by metabolism of small intestinal enterocytes rather than colonic bacteria. AIMS: To evaluate if ammonia release and glutamine metabolism by small intestinal mucosa contribute to hyperammonaemia in vivo in patients with liver cirrhosis. METHODS: Using TIPS to examine mesenteric venous blood, we measured mesenteric venous-arterial concentration differences in ammonia and glutamine in patients with liver cirrhosis before, during, and after enteral (n = 8) or parenteral (n = 8) isonitrogenous infusion of a glutamine containing amino acid solution. RESULTS: During enteral nutrient infusion, ammonia release increased rapidly compared with the post-absorptive state (65 (58-73) v. 107 (95-119) micromol/l after 15 min; mean (95% confidence interval)) in contrast with parenteral infusion (50 (41-59) v. 62 (47-77) micromol/l). This resulted in a higher portal ammonia load (29 (21-36) v. 14 (8-21) mmol/l/240 minutes) and a higher degree of systemic hyperammonaemia (14 (11-17) v. 9 (6-12) mmol/l/240 minutes) during enteral than parenteral infusion. The mesenteric venous-arterial concentration difference in glutamine changed from net uptake to release at the end of the enteral infusion period (-100 (-58 to -141) v. 31 (-47-110) micromol/l) with no change during parenteral nutrition. CONCLUSIONS: These data suggest that small intestinal metabolism contributes to post-feeding hyperammonaemia in patients with cirrhosis. When artificial nutrition is required, parenteral nutrition may be superior to enteral nutrition in patients with portosystemic shunting because of the lower degree of systemic hyperammonaemia.     

Effect of different protein diets on the distribution of amino acids in plasma, liver and brain in the rat.

The distribution of amino acids between plasma, liver and brain was studied in adult male rats, fed a diet containing 8.7, 17 (control animals), 32 and 51% of protein during 15 days. The caloric intake was nearly equal in all groups. The highest food intake was observed in the animals on the low protein diet. Changes in plasma amino acids were variable. In contrast to the behavior of most amino acids in plasma, the branched chain amino acids were highest in the animals fed the 51% protein diet. Despite the low protein intake in the animals fed a 8.7% protein diet, the concentration of serine, glutamic acid, glutamine, glycine, alanine, methionine, isoleucine, leucine, phenylalanine and ornithine were significantly higher compared to control animals, whereas in those receiving a high protein diet, valine, leucine, tyrosine, tryptophan and histidine increased in relation to the increased protein and amino acid intake. The plasma amino acid patterns are not greatly influenced by the amino acid distribution in the food and the amount ingested. Alanine aminotransferase, aspartate aminotransferase, glutamate dehydrogenase and cholinesterase showed a two- to fivefold increased activity in the liver of animals consuming a high protein diet. In the brain, the concentration of valine, leucine, isoleucine, phenylalanine and tyrosine in animals receiving the low protein diet was higher than in controls and increased further with increasing protein content of the diet. Glutamine was increased in all dietary groups. The predicted influx of amino acids showed increasing influx rates in dependence of the plasma amino acid concentration. The entry of tyrosine and tryptophan and their brain concentration was inversely proportional to the protein content of the diet. In the present study which considers long-term adaptation to an increasing protein and amino acid intake in comparison to a balanced control protein diet, the levels of the indispensable amino acids were maintained within narrow limits in the brain and liver. The results indicate that inspite of a variable protein intake, the body tends to keep organ amino acids in relatively narrow limits favoring in this way amino acid homeostasis.     

Decreased concentration of free histidine in serum in rheumatoid arthritis, an isolated amino acid abnormality not associated with generalized hypoaminoacidemia.

The serum concentrations of 12 free amino acids (alanine, arginine, glycine, histidine, isoleucine, leucine, lysine, phenylalanine, serine, threonine, tyrosine, and valine) were measured in 26 patients with rheumatoid arthritis and in 12 control subjects. Patients with rheumatoid arthritis had a low serum histidine concentration (P equals 0.002) but no abnormality of any other amino acid concentration or of the combined concentration of the measured amino acids, excluding histidine. These data and 22 other reported studies provide strong evidence for the presence of hypohistidemia, not associated with generalized hypoaminoacidemia, in patients with rheumatoid arthritis. (J Rheumatol 2: 384-392, 1975).     

Selective release of gastric inhibitory polypeptide by intraduodenal amino acid perfusion in man

Intraduodenal amino acids are known to stimulate the release of gastric inhibitory polypeptide and cholecystokinin. In order to separate and quantitate gastric inhibitory polypeptide secretion selectively, 12 normal subjects received an intraduodenal perfusion of a mixed amino acid solution (158 mM) containing either methionine, phenylalanine, tryptophan, and valine (perfusate 1), or an amino acid solution containing arginine, histidine, isoleucine, leucine, lysine, and threonine (perfusate 2). Serum concentrations of gastric inhibitory polypeptide and insulin were significantly greater in the group receiving perfusate 2 (P less than 0.001). In contrast, after administration of amino acid perfusate 1, there was only a slight increase in serum gastric inhibitory polypeptide concentration and insulin secretion increased only slightly. Mean trypsin and bilirubin outputs in the group receiving perfusate 1 were nearly 3 times greater than the outputs of the group receiving the other amino acid mixture. This study expands the importance of intraduodenal amino acid mixtures in stimulating secretion of gastric inhibitory polypeptide and insulin and quantitatively separates gastric inhibitory polypeptide release from release of hormones that stimulate pancreatic enzyme secretion, such as cholecystokinin.     

Inhibition of amino acid transport into lymphoid cells by the glutamine analog L-2-amino-4-oxo-5-chloropentanoate.

Transport of L-glutamine and of the chloroketone glutamine analog L-2-amino-4-oxo-5-chloropentanoate into lymphoid cells is mediated by the same system. Arginine and a number of other amino acids (e.g., glutamate, aspartate, and lysine) are transported to a much lesser extent by this system. However, after uptake of the chloroketone into the cells, the transport of glutamine, arginine, and other amino acids is markedly inhibited, due evidently to reaction of the chloroketone with intracellular components that are involved in amino acid transport. The chloroketone acts more effectively on growing than on resting cells. Treatment of lymphoid cells with the chloroketone or with L-chloro-2,4-dinitrobenzene leads to rapid and complete depletion of intracellular glutathione without affecting cell viability. These reagents appear to be useful experimental tools for studies of glutathione function and metabolism.     

An apparent inhibition of insulin biosynthesis resulting from inhibition of transport of neutral amino acids by arginine

Summary At concentrations higher than 10 mM, the cationic amino acid, arginine, inhibited the incorporation of the neutral amino acids such as alanine, threonine, valine and leucine into insulin in the presence of glucose. This inhibitory effect probably did not result from the stimulatory effect of arginine on insulin release because, in the absence of glucose, arginine failed to stimulate insulin release but nevertheless inhibited the incorporation of leucine into insulin. This inhibitory effect of arginine was shared by another basic amino acid, histidine, but not by lysine. Arginine inhibited the incorporation of leucine not only into insulin but also into other islet proteins. This inhibition was not accompanied by any disturbance of glucose metabolism in the islet cells. Further studies indicated that the inhibition of incorporation resulted primarily from the interference of uptake of the neutral amino acids by arginine.     

Excretion of uric acid and amino acids during diuresis in the adult female Glossina morsitans.

Radiometric analysis was carried out on the urine collected for one hour following feeding of the adult female Glossina morsitans on day 1 of a pregnancy cycle, which had previously received haemocoelic injections of U-14C labelled arginine, histidine, leucine, lysine, phenylalanine, threonine, tyrosine or valine. Mean radioactivity in the urine was quite high after labelled arginine (17.4% of injected activity) and histidine (21.8%) administration, most of the activity being in the amino acid fractions. With the remaining six labelled amino acids, mean radioactivity in the urine varied between 1.6 and 7.2% of injected activity, most of this activity occurred in a non-amino acid fraction (probably uric acid), though low radioactivity was also detected in a range of essential as well as non-essential amino acids.