Critical aspects of the antioxidant function of coenzyme Q in biomembranes

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Abstract
Coenzyme Q (ubiquinone, UQ) is increasingly considered as a significant natural antioxidant, which protects biomembranes in concert withα‐tocopherol. In vitro experiments demonstrated that reduced UQ (ubiquinol) can improve the chain‐breaking activities ofα‐tocopherol by recycling the antioxidant‐derived reaction product, the chromanoxyl radical, to the native antioxidant. Less attention, however, was devoted to the antioxidant‐derived reaction products of reduced UQ. Although bothα‐tocopherol and ubiquinol were found to be equally effective in scavenging chain‐propagating lipid radicals,α‐tocopherol protected lipid membranes from lipid peroxidation more efficiently than ubiquinol. The present study not only provides data which document this discrepancy but also contributes experimental data on the existence of ubiquinol derived pro‐oxidants, which give an explanation of this phenomenon.