Autoxidation of ubiquinol-6 is independent of superoxide dismutase

Ubiquinone (Q) is an essential, lipid soluble, redox component of the mitochondrial respiratory chain. Much evidence suggests that ubiquinol (QH(2)) functions as an effective antioxidant in a number of membrane and biological systems by preventing peroxidative damage to lipids. It has been proposed that superoxide dismutase (SOD) may protect QH(2) from autoxidation by acting either directly as a superoxide-semiquinone oxidoreductase or indirectly by scavenging superoxide. In this study, such an interaction between QH(2) and SOD was tested by monitoring the fluorescence of cis-parinaric acid (cPN) incorporated phosphatidylcholine (PC) liposomes. Q(6)H(2) was found to prevent both fluorescence decay and generation of lipid peroxides (LOOH) when peroxidation was initiated by the lipid-soluble azo initiator DAMP, dimethyl 2,2'-azobis (2-methylpropionate), while Q(6) or SOD alone had no inhibitory effect. Addition of either SOD or catalase to Q(6)H(2)-containing liposomes had little effect on the rate of peroxidation even when incubated in 100% O-2. Hence, the autoxidation of QH(2) is a competing reaction that reduces the effectiveness of QH(2) as an antioxidant and was not slowed by either SOD or catalase. The in vivo interaction of SOD and QH(2) was also tested by employing yeast mutant strains harboring deletions in either CuZnSOD and/or MnSOD. The sod mutant yeast strains contained the same percent Q(6)H(2) per cell as wild-type cells. These results indicate that the autoxidation of QH(2) is independent of SOD.