[Hysteresis behavior of complex I in delta mu H+-dependent reduction of NAD+ succinate]

It was shown that the membrane-bound complex I is fully inactive in the absence of NADH during the reverse electron transfer from succinate to NAD+. The enzyme activation is attained by preincubation of submitochondrial particles with low concentrations of NADH; the activating effect persists after a complete oxidation of the latter during long-term (several hours) aerobic incubation. The experimental results suggest that complex I contains a redox component, whose reduction by NADH and aerobic oxidation are not involved in the overall catalytic reaction. An experimental scheme is proposed, according to which the key role of such a component is ascribed to the tightly bound ubiquinone; the activation and inactivation of the enzyme are due to a slow reversible redox conversion (ubiquinone in equilibrium ubisemiquinone), whereas the catalytic act involves a rapid reversible conversion (ubisemiquinone in equilibrium ubiquinol). It was demonstrated that the "redox" mechanism of the inactivation-activation reaction determines the strong dependence of activity of the reverse electron transfer on the mode of preparation of submitochondrial particles. The coupling properties of the submitochondrial particulate membrane and the activities of enzymes involved in the reverse electron transfer are stable at room temperature for over 14 hours.