Monovalent cations and inorganic phosphate alter branched-chain alpha-ketoacid dehydrogenase-kinase activity and inhibitor sensitivity.

Potassium ion protects the branched-chain alpha-ketoacid dehydrogenase complex against inactivation by thermal denaturation and protease digestion. Rubidium was effective but sodium and lithium were not, suggesting that the ionic size of the cation is important for stabilization of the enzyme. Thiamine pyrophosphate stabilization of the complex [Danner, D. J., Lemmon, S. K., and Elsas, S. J. (1980) Arch. Biochem. Biophys. 202, 23-28] was found dependent on the presence of potassium ion. Studies with resolved components indicate that the thiamine pyrophosphate-dependent enzyme of the complex, i.e., the 2-oxoisovalerate dehydrogenase (lipoamide) (EC 1.2.4.4), is the component stabilized by potassium ion. Branched-chain alpha-ketoacid dehydrogenase-kinase activity measured by inactivation of the branched-chain alpha-ketoacid dehydrogenase complex was maximized at a potassium ion concentration of 100 mM. Stimulation of kinase activity was also found with rubidium ion but not with lithium and sodium ions. All salts tested increased the efficiency of inactivation by phosphorylation, i.e., decreased the degree of enzyme phosphorylation required to cause inactivation of the complex. The effectiveness and efficacy of alpha-chloroisocaproate as an inhibitor of branched-chain alpha-ketoacid dehydrogenase kinase were enhanced by the presence of monovalent cations, and further increased by inorganic phosphate. These findings suggest that monovalent cations and anions, particularly potassium and phosphate, cause structural changes in the dehydrogenase-kinase complex that alter its susceptibility to phosphorylation and responsiveness to kinase inhibitors.