title: Mechanism of O2- generation in reduction and oxidation cycle of ubiquinones in a model of mitochondrial electron transport systems.
abstract: O2- 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 Q1-. and O2- during enzyme-catalyzed reduction and O2+ coenzyme Q1-. are in equilibrium with O2- + 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 O2-. 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 microM O2, pH 7.5 and 25 degrees C.
authors.affiliation: College of Medical Care and Technology, School of Medicine, Gunma University, Maebashi, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Sugioka
authors.forename: M
authors.initials: M
authors.lastname: Nakano
authors.forename: H
authors.initials: H
authors.lastname: Totsune-Nakano
authors.forename: H
authors.initials: H
authors.lastname: Minakami
authors.forename: S
authors.initials: S
authors.lastname: Tero-Kubota
authors.forename: Y
authors.initials: Y
authors.lastname: Ikegami
mesh_terms: D000693:Anaerobiosis; D000818:Animals; D002417:Cattle; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D005609:Free Radicals; D005650:Fumarates / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008928:Mitochondria / Q000378:metabolism*; D008929:Mitochondria, Heart / Q000378:metabolism; D008954:Models, Biological; D009249:NADP / Q000378:metabolism; D009251:NADPH-Ferrihemoprotein Reductase / Q000378:metabolism; D010084:Oxidation-Reduction; D010101:Oxygen Consumption; D011190:Potassium Cyanide / Q000494:pharmacology; D013367:Submitochondrial Particles / Q000378:metabolism; D013386:Succinates / Q000378:metabolism; D019802:Succinic Acid; D013482:Superoxide Dismutase / Q000378:metabolism; D013481:Superoxides / Q000378:metabolism*; D014451:Ubiquinone / Q000031:analogs & derivatives / Q000378:metabolism*; D014812:Vitamin K / Q000378:metabolism
chemical_list: D005609:Free Radicals; D005650:Fumarates; D013386:Succinates; D013481:Superoxides; D014812:Vitamin K; D014451:Ubiquinone; D009249:NADP; C032005:fumaric acid; D019802:Succinic Acid; D013482:Superoxide Dismutase; D009251:NADPH-Ferrihemoprotein Reductase; C003741:ubiquinol; D011190:Potassium Cyanide