Mechanism of O−2 generation in reduction and oxidation cycle of ubiquinones in a model of mitochondrial electron transport systems

O−2 generation in mitochondrial electron transport systems, especially the NADPH-coenzyme Q10 oxidoreductase system, was examined using a model system, NADPH-coenzyme Q1-NADPH-dependent cytochrome P-450 reductase. One electron reduction of coenzyme Q1 produces coenzyme Q⨪1 and O−2 during enzyme-catalyzed reduction and O2 + coenzyme Q⨪1 are in equilibrium with O−2 + coenzyme Q1 in the presence of enough O2. The coenzyme Q⨪1 produced can be completely eliminated by superoxide dismutase, identical to bound coenzyme Q10 radical produced in a succinate/fumarate couple-KCN-submitochondrial system in the presence of O2. Superoxide dismutase promotes electron transfer from reduced enzyme to coenzyme Q1 by the rapid dismutation of O2− generated, thereby preventing the reduction of coenzyme Q1 by O−2. The enzymatic reduction of coenzyme Q1 to coenzyme Q1H2 via coenzyme Q⨪1 is smoothly achieved under anaerobic conditions. The rate of coenzyme Q1H2 autoxidation is extremely slow, i.e., second-order constant for [O2][coenzyme Q1H2] = 1.5 M−1 · s−1 at 258 μM O2, pH 7.5 and 25°C.