title: Reduction of ubiquinone in membrane lipids by rat liver cytosol and its involvement in the cellular defence system against lipid peroxidation.
abstract: Rat liver homogenates reduced ubiquinone (UQ)-10 to ubiquinol (UQH2)-10 in the presence of NADPH rather than NADH. This NADPH-dependent UQ reductase (NADPH-UQ reductase) activity that was not inhibited by antimycin A and rotenone, was located mainly in the cytosol fraction and its activity accounted for 68% of that of the homogenates. Furthermore, the NADPH-UQ reductase from rat liver cytosol efficiently reduced both UQ-10 incorporated into egg yolk lecithin liposomes, and native UQ-9 residing in rat microsomes, to the respective UQH2 form in the presence of NADPH. The gross redox ratios of UQH2-9/(UQ-9 + UQH2-9) in individual tissues of rat correlated positively with the log of their respective cytosolic NADPH-UQ reductase activities, while the redox ratios in every intracellular fraction from liver were at about the same level, irrespective of NADPH-UQ reductase activities in the respective fractions. The combined addition of rat liver cytosol and NADPH inhibited to a great extent 2,2'-azobis(2,4-dimethyl-valeronitrile)-induced lipid peroxidation of UQ-10-fortified lecithin liposomes and completely inhibited such peroxidation in the liposomes in which UQH2-10 replaced UQ-10. The NADPH-UQ reductase activity was clearly separated from DT-diaphorase (EC 1.6.99.2) activity by means of Cibacron Blue-immobilized Bio-Gel A-5m chromatography. In conclusion, the NADPH-UQ reductase in cytosol, which is a novel enzyme to our knowledge, was presumed to be responsible for maintaining the steady-state redox levels of intracellular UQ and thereby to act as an endogenous antioxidant in protecting intracellular membranes from lipid peroxidation that is inevitably induced in aerobic metabolism.
authors.affiliation: Faculty of Pharmaceutical Sciences, Kobe Gakuin University, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Takahashi
authors.forename: T
authors.initials: T
authors.lastname: Yamaguchi
authors.forename: M
authors.initials: M
authors.lastname: Shitashige
authors.forename: T
authors.initials: T
authors.lastname: Okamoto
authors.forename: T
authors.initials: T
authors.lastname: Kishi
mesh_terms: D000818:Animals; D000975:Antioxidants / Q000494:pharmacology; D002850:Chromatography, Gel; D003600:Cytosol / Q000378:metabolism*; D015227:Lipid Peroxidation; D008081:Liposomes; D008099:Liver / Q000378:metabolism*; D008563:Membrane Lipids / Q000378:metabolism*; D016660:NAD(P)H Dehydrogenase (Quinone) / Q000302:isolation & purification / Q000378:metabolism; D015388:Organelles / Q000378:metabolism; D051381:Rats; D014451:Ubiquinone / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D000975:Antioxidants; D008081:Liposomes; D008563:Membrane Lipids; D014451:Ubiquinone; D016660:NAD(P)H Dehydrogenase (Quinone); C025204:Ubiquinone Q2