The effect of membrane potential on the redox state of cytochrome b561 in antimycin-inhibited submitochondrial particles.

The oxidation of cytochrome b561 by ATP was measured in submitochondrial particles inhibited by antimycin. The redox potential of the bulk (M phase) was controlled by the ratio of fumarate:succinate, and the oxidation of cytochrome b was calculated and expressed as a change in redox potential (Eh) measured in millivolts. The oxidation of cytochrome b561 is an energy-driven reaction affected only by the delta psi component of the proton motive force. The oxidation (measured in millivolts) is a function of the phosphate potential, reaching a maximal value of 40 mV at delta G'ATP less than - 12 kcal/mole. The maximal measured value of ATP-dependent delta psi was 100 mV. Thus only a fraction of the membrane potential effects the redox state of cytochrome b561. In contrast to the ATP-induced oxidation of cytochrome b561, cytochrome b566 is in redox equilibrium with fumarate succinate either in the presence or in the absence of ATP. The selective oxidation of b561 is explained within the term of the Q cycle as a reflection of delta psi on the electron electrochemical potential. The positive electric potential of the C phase causes cytochrome b566 to act as oxidant with respect to cytochrome b561. In the presence of antimycin cytochrome b561 cannot equilibrate with the quinone and undergoes oxidation, while cytochrome b566 reequilibrates with the quinone and thus regains redox equilibrium with the fumarate succinate redox buffer.