APPARENT ATP-LINKED SUCCINATE THIOKINASE ACTIVITY AND ITS RELATION TO NUCLEOSIDE DIPHOSPHATE KINASE IN MITOCHONDRIAL MATRIX PREPARATIONS FROM RABBIT

The relative abilities of ATP and GTP to support succinyl-CoA synthesis by mitochondrial matrix fractions prepared from rabbit heart and liver mitoplasts were investigated. The activity supported by ATP in rabbit heart preparations was less than 15% of that obtained with GTP, while no ATP-supported activity was observed in rabbit liver preparations. However, the addition of 30 micromolar GDP to matrix fractions from either heart or liver stimulated the ATP-supported activity to 40% of that observed with GTP, and the further addition of bovine liver nucleoside diphosphate kinase in the presence of 8-mu-M added GDP increased the activity to near that observed with GTP. The specific activity of nucleoside diphosphate kinase assayed directly in mitochondrial matrix from heart was about 15% of the specific activity of ATP-supported succinate thiokinase induced upon adding GDP. Evidence for a complex between nucleoside diphosphate kinase and succinate thiokinase in mitochondrial matrix from rabbit heart was obtained by glycerol density gradient centrifugation. It is proposed that binding of nucleoside diphosphate kinase to succinate thiokinase activates the former enzyme, accounts for the ATP-supported succinyl-CoA synthetase activity observed, and is involved in the channeling of high energy phosphate from GTP produced in the Krebs cycle to the ATP pool.