Review Volume 3, Issue 5 pp 464—478

Figure 2. Catabolism of branched-chain amino acids. Starvation and exercise stimulate protein breakdown, thereby increasing the concentrations of branched-chain amino acids (BCAAs) in adipose and muscle cells. The BCAAs are transaminated in muscle mitochondria by branched-chain aminotransferase (BCAT), and branched-chain α-keto acids (BCKAs, especially α-keto acid from leucine) inhibit branched-chain α-keto acid dehydrogenase kinase, resulting in elevation of the active state of the rate limiting enzyme branched-chain α-keto acid dehydrogenase complex (BCKDH). Recent results indicate a novel mechanism for regulation of BCAA oxidation in adipose tissue, i.e. changes in the expression of BCAA enzymes, in contrast to altered BCKDH phosphorylation, which is the major mechanism regulating BCAA oxidation in muscle and liver [121]. Ala, alanine; α-KG, α-ketoglutarate; Glu, glutamate; Gln, glutamine; IB-CoA, isobutyryl-coenzyme A; IV-CoA, isovaleryl-coenzyme A; MB-CoA, α-methylbutyryl-coenzyme A; R-CoA, acyl-coenzyme A; TCA, tricarboxylic acid.