Regulation of branched-chain alpha-ketoacid dehydrogenase complex by covalent modification.

The branched-chain alpha-ketoacid dehydrogenase complex, like the pyruvate dehydrogenase complex, is an intramitochondrial enzyme subject to regulation by covalent modification. Phosphorylation causes inactivation and dephosphorylation causes activation of both complexes. The branched-chain alpha-ketoacid dehydrogenase kinase, believed distinct from pyruvate dehydrogenase kinase, is an integral component of the branched-chain alpha-ketoacid dehydrogenase complex and is sensitive to inhibition by branched-chain alpha-ketoacids, alpha-chloroisocaproate, phenylpyruvate, clofibric acid, octanoate and dichloroacetate. Phosphorylation of branched-chain alpha-ketoacid dehydrogenase occurs at two closely-linked serine residues (sites 1 and 2) of the alpha-subunit of the decarboxylase. HPLC and sequence data suggest homology of the amino acid sequence adjacent to phosphorylation sites 1 and 2 of complexes isolated from several different tissues. Stoichiometry for phosphorylation of all of the complexes studies was about 1 mol P/mol alpha-subunit for 95% inactivation and 1.5 mol P/mol alpha-subunit for maximally phosphorylated complex. Site 1 and site 2 were phosphorylated at similar rates until total phosphorylation exceeded 1 mol P/mol alpha-subunit. The complexes from rabbit kidney, rabbit heart, and rat heart showed 30-40% additional phosphorylation of the alpha-subunit beyond 95% inactivation. Site specificity studies carried out with the kinase partially inhibited with alpha-chloroisocaproate suggest that phosphorylation of site 1 is primarily responsible for regulation of the complex. The capacity of the branched-chain alpha-ketoacid dehydrogenase to oxidize pyruvate (Km = 0.8 mM, Vmax = 20% of that of alpha-ketoisovalerate) interferes with the estimation of activity state of the hepatic pyruvate dehydrogenase complex. The disparity between the activity states of the two complexes in most physiologic states contributes to this interference. An inhibitory antibody for branched-chain alpha-ketoacid dehydrogenase can be used to prevent interference with the pyruvate dehydrogenase assay. Almost all of the hepatic branched-chain alpha-ketoacid dehydrogenase in chow-fed rats is active (greater than 90% dephosphorylated). In contrast, almost all of the hepatic enzyme of rats fed a low-protein (8%) diet is inactive (greater than 85% phosphorylated). Fasting of chow-fed rats has no effect on the activity state of hepatic branched-chain alpha-ketoacid dehydrogenase, i.e. greater than 90% of the enzyme remains in the active state. However, fasting of rats maintained on low-protein diets greatly activates the hepatic enzyme.(ABSTRACT TRUNCATED AT 400 WORDS)