Transamination Of Branched-chain Amino Acids Research Articles

Enzymic determination of branched-chain amino acids.

Transamination of branched-chain amino acids to (alpha-oxoglutarate, catalyzed by leucine aminotransferase coupled to the glutamate dehydrogenase reaction, is used in an enzymic assay for determination of branched-chain amino acids in serum and tissue homogenates. The coefficients of variation of the method within-day and day-to-day are 2.4 and 6.5%, respectively. Analytical recovery of physiological concentrations of branched-chain amino acids added to serum is near 100%. Measurements in serum of healthy subjects revealed normal values similar to those found by use of other methods. During prolonged fasting the concentration of these amino acids in serum first increases, reaching a maximum by three days, followed by a successive decline.

Leucine degradation and release of glutamine and alanine by adipose tissue.

Adipose tissue from fed rats rapidly degrades [l14C]leucine to 14C02. This process is stimulated by glucose (but not by pyruvate) and by insulin, which also enhances the degradation of isoleucine and valine. Fasting and protein deficiency markedly reduced the catabolism of [l-‘4C]leucine in epididymal fat pads by decreasing the decarboxylation of a-ketoisocaproic acid and the transamination of leucine. In tissue homogenates, the rates of these reactions decreased by 63 and 47%, respectively, after 2 days of food deprivation. Within only 1 day of food deprivation, total decarboxylation of leucine in tissue homogenates was 73% slower. In adipose tissue from the fasted animals, leutine oxidation was also stimulated by glucose but not by insulin. In fat pads from fed (but not fasted) rats, leucine decreased the oxidation of pyruvate and thus may spare use of glucose-derived acetyl coenzyme A for lipogenesis. When fed and fasted rats were injected with [1-14CJleucine, their rates of expiration of 14C02 did not differ, but when they were injected with p-‘4C]leucine, 14C02 expiration was greater in the fasted rats. Therefore, food deprivation did not affect the total amount of leucine degradation by the organism but seemed to increase the oxidation of leucine by muscle, while decreasing its conversion to triglycerides. The incubated fat pads appear to synthesize and release large amounts of glutamine and smaller quantities of alanine and glutamate. Addition of leucine increased release into the medium of glutamine and, to a lesser extent, that of alanine and glutamate (without affecting tissue protein balance or the release of other amino acids). In fasting, when degradation of leucine fell, the accumulation of glutamine and alanine in the medium decreased. Isoleucine and valine were degraded more slowly than leucine and stimulated glutamine and alanine release to a lesser extent. In adipose tissue, as in muscle, production of glutamine, as well as alanine and glutamate, seems to be the route for disposal of amino groups released in the transamination of branched-chain amino acids.

Gluconeogenesis, ureagenesis, and ketogenesis during sepsis.

Abstract : Acute sepsis is characterized by an accelerated catabolism of somatic proteins, an increased rate of oxidation and transamination of branched-chain amino acids within muscle, an enhanced formation of lactic acid, alanine and glutamine in muscle, an enhanced flux of these substrates from muscle to liver, and an acceleration of the synthesis and release of glucose from the liver. Endocrine responses which influence these metabolic changes include an enhanced secretion of both insulin and glucagon from pancreatic islet cells and a resultant decrease in the molar ratio of insulin to glucagon in plasma. The secretion of the adrenal glucocorticoids and growth hormone are increased as may be the secretion of catecholamines. The accelerated release of glucose from the liver is generally accompanied by an increase in glucose pool size and an increase in the rate of glucose utilization as a cellular fuel. The hormonal, enzymatic, and substrate interrelationships within the liner combine in their effects to inhibit Ketogenesis and stimulate hepatic lipogenesis during acute sepsis.