The effect of membrane potential on the redox state of cytochromeb 561 in antimycin-inhibited submitochondrial particles

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Abstract
The oxidation of cytochromeb 561 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 cytochromeb was calculated and expressed as a change in redox potential (E h) measured in millivolts. The oxidation of cytochromeb 561 is an energy-driven reaction affected only by the Δψ 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 ΔG′ATP<−12 kcal/mole. The maximal measured value of ATP-dependent Δψ was 100 mV. Thus only a fraction of the membrane potential effects the redox state of cytochromeb 561. In contrast to the ATP-induced oxidation of cytochromeb 561, cytochromeb 566 is in redox equilibrium with fumarate succinate either in the presence or in the absence of ATP. The selective oxidation ofb 561 is explained within the term of theQ cycle as a reflection of Δψ on the electron electrochemical potential. The positive electric potential of theC phase causes cytochromeb 566 to act as oxidant with respect to cytochromeb 561. In the presence of antimycin cytochromeb 561 cannot equilibrate with the quinone and undergoes oxidation, while cytochromeb 566 reequilibrates with the quinone and thus regains redox equilibrium with the fumarate succinate redox buffer.