Branched-chain Amino Acid Oxidation in Skeletal Muscle -Physiological and Clinical Importance of its Modulation by Reactant Availability

Enhanced oxidation of branched-chain amino acids (BCAA; valine, leucine and isoleucine) in skeletal muscle is a typical metabolic alteration associated with activated synthesis of glutamine and development of muscle wasting in cachectic illness. The paper demonstrates that the rates of BCAA transamination and decarboxylation in skeletal muscle respond rapidly to changes in BCAA and glutamine availability. Enhanced BCAA availability increases BCAA oxidation and glutamine synthesis, while enhanced availability of glutamine suppresses BCAA oxidation. In physiological conditions, this enables compensation in alterations of amount and composition of food in order to keep optimal levels of BCAA and glutamine in the body. In cachectic illness, enhanced availability of BCAA (resulting mainly from activated proteolysis) and decreased availability of glutamine (resulting from its enhanced release by muscle) account significantly for activated BCAA oxidation and glutamine synthesis in muscle. The clinical importance of the opposite effect of BCAA and glutamine supplementation on BCAA oxidation in the treatment of muscle wasting disorders should be elucidated.