Regulation of substrate availability for the branched-chain alpha-keto acid dehydrogenase enzyme complex.

The tissue distribution of BCAT and BCKDH is largely responsible for the unique metabolism of branched-chain amino acids in rat tissues. Because BCKDH is a mitochondrial enzyme, tissue capacity for branched-chain amino acid oxidation will be a function of mitochondrial specific activity and tissue mitochondrial content, as well as the activity state of the BCKDH complex. In muscle tissues, the activity of the BCKDH appears to restrict branched-chain amino acid oxidation. Therefore, in muscle, transamination exceeds oxidation. Depending on muscle fiber type, the branched-chain alpha-keto acid transporter operates primarily as either an efflux or an exchange pathway and keto acids are released from the tissue. The liver contains very low cytosolic BCAT activity and no mitochondrial BCAT. Since the BCKDH is largely in the active state in hepatic tissue, the liver is a major site of branched-chain amino acid oxidation. Thus, control of the metabolism of these essential amino acids in vivo is achieved through distribution and regulation of the activity of the first two enzymes in the catabolic pathway.