Reduction of the Q-pool by duroquinol via the two quinone-binding sites of the QH2: cytochrome c oxidoreductase. A model for the equilibrium between cytochrome b-562 and the Q-pool.

The steady-state reduction of exogenous ubiquinone-2 by duroquinol as catalysed by the ubiquinol: cytochrome c oxidoreductase was studied in bovine heart mitoplasts. The reduction of ubiquinone-2 by duroquinol proceeds both in the absence of inhibitors of the enzyme, in the presence of outside inhibitors, e.g., myxothiazol, and in the presence of inside inhibitors, e.g., antimycin, but not in the presence of both inside and outside inhibitors. It is concluded that both the Qin-binding domain and the Qout-binding domain may independently catalyse this reaction. The rate of the reduction of ubiquinone-2 by duroquinol via the Qin-binding domain is dependent on the type of outside inhibitor used. The maximal rate obtained for the reduction of ubiquinone-2 by DQH2 via the Qout-binding domain, measured in the presence of antimycin, is similar to that catalysed by the Qin-binding domain of the non-inhibited enzyme and depends on the redox state of the high-potential electron carriers of the respiratory chain. The reduction of ubiquinone-2 by DQH2 via the Qin-binding domain can be described by a mechanism in which duroquinol reduces the enzyme, upon which the reduced enzyme is rapidly oxidized by ubiquinone-2 yielding ubiquinol-2. By determination of the initial rate under various conditions and simulation of the time course of reduction of ubiquinone-2 using the integrated form of the steady-state rate equation the values of the various kinetic constants were calculated. During the course of reduction of ubiquinone-2 by duroquinol in the presence of outside inhibitors only cytochrome b-562 becomes reduced. At all stages during the reaction, cytochrome b-562 is in equilibrium with the redox potential of the ubiquinone-2/ubiquinol-2 couple but not with that of the duroquinone/duroquinol couple. At low pH values, cytochrome b-562 is reduced in a single phase; at high pH separate reduction phases are observed. In the absence of inhibitors three reduction phases of cytochrome b-562 are discernible at low pH values and two at high pH values. In the presence of antimyin cytochrome b becomes reduced in two phases. Cytochrome b-562 is reduced in the first phase and cytochrome b-566 in the second phase after substantial reduction of ubiquinone-2 to ubiquinol-2 has occurred. In ubiquinone-10 depleted preparations, titration of cytochrome b-562, in the presence of myxothiazol, with the duroquinone/duroquinol redox couple yields a value of napp = 2, both at low and high pH.(ABSTRACT TRUNCATED AT 400 WORDS)