The role of DMQ(9) in the long-lived mutant clk-1

IntroductionUbiquinone (UQ) is a redox active lipid that transfers electrons from complex I or II to complex III in the electron transport chain (ETC). The long-lived Caenorhabditis elegans mutant clk-1 is unable to synthesize its native ubiquinone, and accumulates high amounts of its precursor, 5-demethoxyubiquinone-9 (DMQ₉). In clk-1, complexes I–III activity is inhibited while complexes II–III activity is normal. We asked whether the complexes I–III defect in clk-1 was caused by: (1) a defect in the ETC; (2) an inhibitory effect of DMQ₉; or (3) a decreased amount of ubiquinone.MethodsWe extracted the endogenous quinones from wildtype (N2) and clk-1 mitochondria, replenished them with exogenous ubiquinones, and measured ETC activities.ResultsReplenishment of extracted mutant and wildtype mitochondria resulted in equal enzymatic activities for complexes I–III and II–III ETC assays. Blue native gels showed that supercomplex formation was indistinguishable between clk-1 and N2. The addition of a pentane extract from clk-1 mitochondria containing DMQ₉ to wildtype mitochondria specifically inhibited complexes I–III activity. UQ in clk-1 mitochondria was oxidized compared to N2.DiscussionOur results show that no measurable intrinsic ETC defect exists in clk-1 mitochondria. The data indicate that DMQ₉ specifically inhibits electron transfer from complex I to ubiquinone.