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.