Monovalent cation requirement for ADP inhibition of pyruvate dehydrogenase kinase

ADP is a competitive inhibitor with respect to ATP for pyruvate dehydrogenase kinase. Evidence is presented that K+ or NH4+ ions are required for inhibition of the kinase by ADP. K+ at 30–90 mM and NH4+ at 1–5 mM decrease markedly the apparent Ki of bovine kidney pyruvate dehydrogenase kinase for ADP and also decrease, to a lesser extent, the apparent Km for ATP. Na+ is less effective and, in addition, inhibits kinase activity. Since K+ and NH4+ are not required for kinase activity, their effect appears to be primarily of regulatory significance. K+ and NH4+ have little effect, if any, on pyruvate dehydrogenase phosphatase activity. When both the kinase and the phosphatase are present and functional, the near steady state activity of the pyruvate dehydrogenase complex is affected significantly by varying the concentration of K+ or NH4+ at a fixed ADP/ATP concentration ratio and by varying the ADPATP ratio at a fixed concentration of monovalent cation.