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Mechanism of O2 generation in reduction and oxidation cycle of ubiquinones in a model of mitochondrial electron transport systems |
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| Authors: | Katsuaki Sugioka Minoru Nakano Hiroko Totsune-Nakano Hisanori Minakami Shozo Tero-Kubota Yusaku Ikegami |
| Institution: | a College of Medical Care and Technology, School of Medicine, Gunma University, Maebashi, Japan b Behavior Research Institute, School of Medicine, Gunma University, Maebashi, Japan c Department of Obstetrics and Gynecology, Jichi Medical School, Tochigi, Japan d Chemical Research Institute of Non-Aqueous Solution, Tohoku University, Sendai, Japan |
| Abstract: | 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 Q1 are in equilibrium with O?2 + coenzyme Q1 in the presence of enough O2. The coenzyme Q1 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 Q1 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. |
| Keywords: | Ubiquinone ESR O?2 generation Electron transport mitochondrial |
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