Synthesis and hydrolysis of ADP-arsenate by beef heart submitochondrial particles

The kinetic parameters for inorganic phosphate and inorganic arsenate as substrates for the synthesis of ATP and ADP-arsenate, respectively, by beef heart submitochondrial particles have been determined. The Vm and Km values for arsenate and phosphate, as well as the Km values for ADP in the two reactions, are the same within experimental error of the measurements. These data are consistent with covalent bond formation not being the rate-limiting step for either ATP or ADP-arsenate synthesis. The hydrolysis of ATP and of ADP-arsenate was studied under conditions of net synthesis of ATP or ADP-arsenate. The apparent first order rate constant for ATP hydrolysis increased with submitochondrial particle concentration, indicating that hydrolysis of ATP was catalyzed by the submitochondrial particle preparation. Nonenzymic hydrolysis of ATP was negligible compared to enzymic hydrolysis. The apparent first order rate constant for ADP-arsenate hydrolysis did not vary with submitochondrial particle concentration, indicating that ADP-arsenate hydrolysis was nonenzymic. Enzymic hydrolysis of ADP-arsenate was too slow, compared with its rapid nonenzymic hydrolysis, to be detected. The first order rate constant for ADP-arsenate hydrolysis at pH 7.5, 30 degrees C, was determined to be greater than 5 min-1 and was estimated to be 70 min-1. These data confirm previous suggestions that arsenate "uncouples" oxidative phosphorylation by a mechanism involving synthesis of ADP-arsenate, followed by its rapid nonenzymic hydrolysis.