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Results for “AMINO ACID METABOLISM”

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At least 811 records · Page 45Linked to original sources
PubMed1999

Abnormalities in branched-chain amino acid metabolism in cirrhosis: influence of hormonal and nutritional factors and directions for future research.

Plasma branched-chain amino acid (BCAA) levels are decreased in patients with liver cirrhosis, owing to an increase in BCAA tissue uptake and/or catabolism and a decrease in BCAA production from proteins. Non-specific factors such as malnutrition worsen this picture. Studies of BCAA fluxes and protein turnover in cirrhotic patients have given conflicting results due to patient heterogeneity, differences in method and bias in the expression of results. In well compensated cirrhosis, muscle wasting is moderate and probably due more to decreased protein synthesis than to increased protein catabolism. Hyperinsulinemia has been suggested as the main cause of decreased BCAA levels, by increasing BCAA uptake in muscle and additionally in adipose tissue. However, as depletion of fat stores is frequent in cirrhosis, this effect is certainly quantitatively weak. Also, there is no correlation between state of hyperinsulinemia and decrease in BCAA levels. An effect of cytokines (IL1 and TNF) on muscle BCAA catabolism is a possibility. Until recently, the contribution of the liver to abnormal BCAA metabolism has been underestimated. In cirrhotic liver an increase in liver transamination of branched-chain keto acids (BCKAs) has been suggested and may result from inhibition of liver BCKA dehydrogenase. A modification of protein turnover in cirrhotic liver must be also considered. Lastly, the contribution of non-hepatocyte liver cells, which are activated in cirrhosis, remains to be assessed.

Amino Acids, Branched-Chain↗
PubMed1981

The effects of hyperalimentation and infused leucine on the amino acid metabolism in sepsis: an experimental study in vivo.

Liver blood flow and exchange of oxygen, glucose, lactate, and amino acids were measured in pigs at the same time as the peripheral arteriovenous (A-V) difference of these substances was determined. Four groups of animals were studied; they were normal postabsorptive, septic fasted, and septic infused either with complete parenteral nutrition (4.25% mixed amino acid solution with 25% glucose) or an isocaloric solution of 1.8% leucine with glucose. Sepsis in the pig caused a rise in arterial concentration of all essential amino acids except tryptophan and a decrease of most of the others. The liver uptake of the sum of all amino acids rose from nonsignificant values to 26.03 mumol/min/kg at the same time as the peripheral A-V difference changed from +20.4 to -678.0 mumol/l. Hyperalimentation increased arterial amino acid concentration, whereas peripheral A-V difference decreased to -132.3 mumol/l. The total liver uptake of amino acids was 24.80 mumol/min/kg but with a higher proportion of essential amino acids than in the fasted septic state suggesting increased liver protein synthesis. When leucine and glucose were infused the peripheral A-V difference of the sum of all amino acids was only -45.6 mumol/l indicating an almost complete cessation of muscle proteolysis. The arterial plasma concentration of all amino acids except leucine, glutamine, and glutamate were markedly reduced. Although hepatic clearance rate of amino acids fell only slightly, due to the low plasma concentrations, the liver uptake decreased substantially to 7.37 mumol/min/kg suggesting a decreased liver protein synthesis which could be deleterious in the presence of sepsis.

Amino Acids↗
PubMed2002

Role of amino acid metabolism in an air-breathing catfish, Clarias batrachus in response to exposure to a high concentration of exogenous ammonia.

The air-breathing ureogenic walking catfish (Clarias batrachus) faces various environmental constraints throughout the year leading to the problem of accumulation of toxic ammonia. In the present study, the possible role of conversion of accumulated ammonia to various non-essential free amino acids (FAAs) was tested in this fish under hyper-ammonia stress caused by exposing the fish at 25 mM NH(4)Cl for 7 days. Significant accumulation of ammonia of approximately two- to threefold was observed in different tissues (except in the brain), which was accompanied with the significant accumulation of non-essential FAAs in the NH(4)Cl-exposed fish. There was approximately two- to threefold increase of non-essential FAAs in different tissues and in the plasma of the NH(4)Cl-exposed fish compared to the control fish after 7 days of exposure, which was mainly attributable to the increase of Asp, Ala, Gly, Glu, Gln and taurine (Tau) concentrations in general, with certain tissue-specific variations. This was also accompanied with significant increase of activity of certain amino acid metabolism-related enzymes such as the glutamine synthetase (approx. two- to threefold), glutamate dehydrogenase (ammonia utilizing direction) (approx. twofold), aspartate and alanine aminotransaminases (approx. twofold) mainly in the liver, kidney and muscle of the NH(4)Cl-exposed fish. Thus, it appears that the walking catfish has the capacity of active conversion of accumulated ammonia to non-essential FAAs under condition of high concentrations of external ammonia. However, the increase of urea excretion rate due to active conversion of ammonia to urea via the induced urea cycle appears to be quantitatively much more important pathway than the increase of tissue levels of FAAs in dealing with a severe ammonia load.

Alanine Transaminase↗
PubMed1986

Activities of enzymes of amino acid metabolism in rat brown adipose tissue.

There was a nil arginase and serine dehydratase activities in interscapular brown adipose tissue, but the activity of adenylate deaminase, glutamine synthetase, glutamate dehydrogenase and the aspartate, alanine and branched chain amino acid transaminases was higher than those of white adipose tissue; the differences were diminished when expressed per unit of protein weight. Brown adipose tissue enzyme activities were in a range between those of liver and muscle. The high amino acid handling capabilities, together with its physiological role, suggest that brown adipose tissue can metabolize significant amounts of amino acids, its enzyme pattern being different both from white adipose tissue, as well as of liver and muscle.

AMP Deaminase↗
PubMed1988

Amino acid metabolism in tumour-bearing mice.

Mice bearing the Lewis lung carcinoma showed a high tumour glutaminase activity and significantly higher concentrations of most amino acids than in both the liver and the skeletal muscle of the host. Tumour tissue slices showed a marked preference for glutamine, especially for oxidation of its skeleton to CO2. It is proposed that the metabolism of this particular carcinoma is focused on amino acid degradation, glutamine being its preferred substrate.

Amino Acids↗
PubMed1984

Amino acid metabolism in hepatocytes isolated from lactating rats.

Parenchymal hepatocytes isolated from lactating rats had similar rates of amino acid incorporation into protein, but increased rates of urea formation compared to hepatocytes from non-lactating rats. The increased urea formation may be due to increased amino acid transport and degradation. The liver contributes to the increased utilization of amino acids during lactation.

Amino Acids↗
PubMed1971

Amino acid metabolism in exercising man.

Arterial concentration and net exchange across the leg and splanchnic bed of 19 amino acids were determined in healthy, postabsorptive subjects in the resting state and after 10 and 40 min of exercise on a bicycle ergometer at work intensities of 400, 800, and 1200 kg-m/min. Arterio-portal venous differences were measured in five subjects undergoing elective cholecystectomy. In the resting state significant net release from the leg was noted for 13 amino acids, and significant splanchnic uptake was observed for 10 amino acids. Peripheral release and splanchnic uptake of alanine exceeded that of all other amino acids, accounting for 35-40% of total net amino acid exchange. Alanine and other amino acids were released in small amounts (relative to net splanchnic uptake) by the extrahepatic splanchnic tissues drained by the portal vein. During exercise arterial ananine rose 20-25% with mild exertion and 60-96% at the heavier work loads. Both at rest and during exercise a direct correlation was observed between arterial alanine and arterial pyruvate levels. Net amino acid release across the exercising leg was consistently observed at all levels of work intensity only for alanine. Estimated leg alanine output increased above resting levels in proportion to the work load. Splanchnic alanine uptake during exercise exceeded that of all other amino acids and increased by 15-20% during mild and moderate exercise, primarily as a consequence of augmented fractional extraction of alanine. For all other amino acids, there was no change in arterial concentration during mild exercise. At heavier work loads, increases of 8-35% were noted for isoleucine, leucine, methionine, tyrosine, and phenylalanine, which were attributable to altered splanchnic exchange rather than augmented peripheral release. The data suggest that (a) synthesis of alanine in muscle, presumably by transamination of glucose-derived pyruvate, is increased in exercise probably as a consequence of increased availability of pyruvate and amino groups; (b) circulating alanine serves an important carrier function in the transport of amino groups from peripheral muscle to the liver, particularly during exercise; (c) a glucose-alanine cycle exists whereby alanine, synthesized in muscle, is taken up by the liver and its glucose-derived carbon skeleton is reconverted to glucose.

Abdomen↗
PubMed2005

Branched-chain [corrected] amino acid metabolism: implications for establishing safe intakes.

There are several features of the metabolism of the indispensable BCAAs that set them apart from other indispensable amino acids. BCAA catabolism involves 2 initial enzymatic steps that are common to all 3 BCAAs; therefore, the dietary intake of an individual BCAA impacts on the catabolism of all 3. The first step is reversible transamination followed by irreversible oxidative decarboxylation of the branched-chain alpha-keto acid transamination products, the branched chain alpha-keto acids (BCKAs). The BCAA catabolic enzymes are distributed widely in body tissues and, with the exception of the nervous system, all reactions occur in the mitochondria of the cell. Transamination provides a mechanism for dispersing BCAA nitrogen according to the tissue's requirements for glutamate and other dispensable amino acids. The intracellular compartmentalization of the branched-chain aminotransferase isozymes (mitochondrial branched-chain aminotransferase, cytosolic branched-chain aminotransferase) impacts on intra- and interorgan exchange of BCAA metabolites, nitrogen cycling, and net nitrogen transfer. BCAAs play an important role in brain neurotransmitter synthesis. Moreover, a dysregulation of the BCAA catabolic pathways that leads to excess BCAAs and their derivatives (e.g., BCKAs) results in neural dysfunction. The relatively low activity of catabolic enzymes in primates relative to the rat may make the human more susceptible to excess BCAA intake. It is hypothesized that the symptoms of excess intake would mimic the neurological symptoms of hereditary diseases of BCAA metabolism.

Amino Acids, Branched-Chain↗
PubMed1978

Branched chain amino acid metabolism in the retina of diabetic rats.

Diabetes is known to produce increased levels of the branched chain amino acids in plasma, heart and muscle as well as increased oxidation of [14C]-leucine by nerves and muscles from rats. Plasma and retinas from streptozotocin diabetic rats had significant elevations in branched chain amino acid levels compared to control. Retinas from diabetic rats have been found to oxidize significantly more of the branched chain amino acids, leucine, isoleucine and valine than did control retinas when incubated in media containing 16.5 mmol/l glucose. Neither the extracellular space nor the tissue pool of leucine was significantly different in the two groups. The addition of 19 amino acids, at normal plasma concentrations, to the incubation media resulted in 80 percent suppression of leucine oxidation without significant change in incorporation of [14C] into protein. These results suggest that the major role for the branched chain amino acids in the rat retina is in protein synthesis which is not affected by short-term diabetes.

Amino Acids↗
PubMed1998

Regulation of branched-chain amino acid metabolism in the lactating rat.

There is evidence that during lactation, uptake of the essential branched-chain amino acids (BCAA) by mammary glands exceeds their output in milk protein. In this study, we have measured the potential of lactating rats to catabolize BCAA. The activity, relative protein and specific mRNA levels of the first two enzymes in the BCAA catabolic pathway, branched-chain aminotransferase (BCAT) and branched-chain alpha-keto acid dehydrogenase (BCKD), were measured in mammary gland, liver and skeletal muscle obtained from rat dams at peak lactation (12 d), from rat dams 24 h after weaning at peak lactation and from age-matched virgin controls. Western analysis showed that the mitochondrial BCATm isoenzyme was found in mammary gland. Comparison of lactating and control rats revealed that tissue BCATm activity, protein and mRNA were at least 10-fold higher in mammary tissue during lactation. Values were 1.3- to 1. 9-fold higher after 24 h of weaning. In mammary gland of lactating rats, the BCKD complex was fully active. In virgin controls and weaning dams, only about 20% of the complex was in the active state. Hypertrophy of the liver and mammary gland during lactation resulted in a 73% increase in total oxidative capacity in lactating rats. The results are consistent with increased expression of the BCATm gene in the mammary gland during lactation, whereas oxidation appears to be regulated primarily by changes in activity state (phosphorylation state) of BCKD.

3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)↗