[Participation of the quinone acceptor in the transition of complex I from an inactive to active state]

Almost complete slow activation of the deactivated purified Complex I was observed after the steady-state NADH: cytochrome c reductase reaction turnovers catalyzed by the endogenous (tightly-bound) ubiquinone and contaminant Complex III. The rotenone-sensitive NADH oxidase was reconstituted from bovine heart Complex I and Escherichia coli quinol-oxidase. The ratio between active and inactive Complex I during the steady-state NADH oxidase reaction catalyzed by the reconstituted system was shown to be proportional to the rate of quinol oxidation. Analogous result was obtained for the NADH oxidase reaction catalyzed by the submitochondrial particles. It is concluded that the ratio between active and inactive Complex I depends on the quinone redox state. One of the functions of ubiquinone-binding site(s) in Complex I is the control of its active/inactive state.