Branched-chain amino acids are main constituents of muscle proteins as well as serve as energy sources during exercise. Branched-chain amino acid metabolism is regulated by branched-chain a -keto acid dehydrogenase (BCKDH). The present study demonstrated that liver BCKDH activity was markedly lower in female rats than in male rats, suggesting that regulation of branched-chain amino acid metabolism is different between male and female. The activity of BCKDH kinase which is responsible for phosphorylation and inactivation of BCKDH was significantly higher in female than in male, suggesting that the gender difference of liver BCKDH activity is at least in part attributed to the kinase expression in the liver. Liver BCKDH activity in female rats fed a low protein diet was significantly increased by acute exercise, suggesting that branched-chain amino acid catabolism is promoted by the exercise despite under the condition of dietary protein insufficiency and that the amino acids may be useful energy sources during exercise. Dietary proteins contain varying amount of branched-chain amino acids ; e.g., milk protein contains 21.4% of the amino acid and soybean protein 18.4%. Serum branched-chain amino acid concentrations were higher in trained rats fed a 30% milk casein diet than in the animals fed a 30% soybean protein diet, even though both dietary groups of rats were not insufficient to dietary protein. These findings suggest that dietary protein rich in branched-chain amino acids may be beneficial to protein synthesis in the exercise-trained muscle.

DESCENTE SPORTS SCIENCE Vol.18/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE