Dependency on Ca ++ of ATP-stimulated uncoupled oxidation of succinate in rat liver mitochondria

Abstract

Rat liver mitochondria depleted of endogenous substrates and high-energy phosphate bonds require exogenous ATP for the maximal uncoupled oxidation of succinate. The present experiments using a mannitol sucrose medium indicate that the effect of ATP is dependent upon the presence of low concentrations of Ca ++ (less than 5 × 10 −6 M). The requirement for Ca ++ was obvious irrespective of the presence or absence of Mg ++ , and of the species of anions (Cl − , HPO −− 4 , HAsO −− 4 , or acetate) in the buffer. Sr ++ , but not Mn ++ could replace Ca ++ . The Ca ++ -ATP effect was not obtained in the uncoupled oxidation of NAD-dependent substrates or of ascorbate in the presence of tetramethyl-P-phenylenediamine. The results of a kinetic analysis of the uncoupled succinate oxidation and of experiments with reagents which interfere with the mitochondrial Ca ++ translocation suggested that, in the presence of ATP, Ca ++ after binding on the outer surface of the inner membrane enhanced the mitochondrial succinoxidase activity by lowering the apparent Km for succinate.