Modulation of leucine transaminase activity by dietary means.

The specific activity of leucine transaminase was measured in supernatants of liver, skeletal muscle (gastrocnemius), and kidney homogenates obtained from fed, starved, and protein-deprived rats. After 12 h of starvation, there were slight reductions in leucine transaminase activity of both muscle and kidney tissues. When starvation was prolonged to 1 full day, the activity of this enzyme increased by approximately twofold in both muscle and kidney. Prolongation of fasting to 5 days resulted in an additional increase in specific activity of leucine transaminase in muscle. During the entire 5 days of starvation, leucine transaminase activity remained unaltered in liver of starved rats. Protein deprivation for 1 or 5 days resulted in significant reductions in specific activity of leucine transaminase in skeletal muscle. Protein deprivation did not produce a remarkable effect on the activity of this enzyme in kidney or liver tissue. The results of this study, together with those previously obtained, indicate that within our experimental conditions increased oxidation of leucine in skeletal muscle of starved rats is not initially related to an alteration in activity of leucine transaminase. When caloric deficiency is prolonged, the potential for transamination is also increased. These adaptive changes increase the ability of skeletal muscle to use leucine as a metabolic fuel.