Oxidation and reduction of soluble cytochrome c by membrane-bound oxidase and reductase systems.

The kinetics of the reduction of soluble ferricytochrome c by the membrane-bound succinate-cytochrome c reductase system on fragments of inner mitochondrial membrane has been studied spectrophotometrically for comparison with the oxidation of ferrocytochrome c by the membrane-bound oxidase. The time course of the reduction is composed of a part proceeding at constant rate (zero order part), where the rate-limiting step is somewhere on the substrate side of cytochrome cl, then a part where the reaction is first order in ferricytochrome c. During the latter part the reaction with soluble ferricytochrome c is rate-limiting. As previously observed with the oxidase (SMITH, L., AND CONRAD, H. (1956) Arch. Biochem. Biophys. 63,403) the first order rate constant varies with the total concentration of cytochrome c in the reaction mixture. However the properties of the first order reductase reaction differ considerably from those of the first order oxidase reaction, implicating reaction of the reductase with different groups on cytochrome c.