Ubiquinone-Dependent Recycling of Vitamin E Radicals by Superoxide

Superoxide-driven reduction of α-tocopheroxyl radical in the presence of ubiquinone-10 has been demonstrated in dimethylsulfoxide. Our HPLC measurements showed that ubiquinone-10 protected α-tocopherol against oxidation by KO2in a concentration-dependent manner. α-Tocopherol was oxidized by KO2to form ESR-detectable radicals of tocopherol ubisemiquinone. In the presence of ubiquinone-10, neither these radicals nor α-tocopheroxyl radicals (generated by uv light or PbO2) could be detected in ESR spectra. Instead, ESR signals of ubisemiquinone-10 radicals were observed. Formation of ubisemiquinone-10 radicals from ubiquinone-10 upon addition of KO2was ascertained by their characteristic ESR and uv–vis spectra. α-Tocopherol caused a concentration-dependent decrease of the ubisemiquinone-10 radical steady-state concentration. We conclude that one-electron reduction of ubiquinone-10 by superoxide ion resulting in the formation of ubiquinone-10 radicals caused redox-cycling of α-tocopherol from its phenoxyl radical, thus preventing loss of α-tocopherol. This suggests that coenzyme Q may have another physiological function, i.e., protection of α-tocopherol against superoxide-driven oxidation.