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. I t has been proposed that superoxide dismutase (SOD) may protect QH(2) from autoxidation by acting either directly as a superoxide-semiquinon e oxidoreductase or indirectly by scavenging superoxide. In this study , such an interaction between QH(2) and SOD was tested by monitoring t he fluorescence of cis-parinaric acid (cPN) incorporated phosphatidylc holine (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'-azobi s (2-methylpropionate), while Q(6) or SOD alone had no inhibitory effe ct. Addition of either SOD or catalase to Q(6)H(2)-containing liposome s 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 th at reduces the effectiveness of QH(2) as an antioxidant and was not sl owed by either SOD or catalase. The in vivo interaction of SOD and QH( 2) was also tested by employing yeast mutant strains harboring deletio ns in either CuZnSOD and/or MnSOD. The sod mutant yeast strains contai ned the same percent Q(6)H(2) per cell as wild-type cells. These resul ts indicate that the autoxidation of QH(2) is independent of SOD.