Endogenous and exogenous regulation of redox-properties of coenzyme Q

Abstract

Ubiquinol (QH 2, reduced coenzyme Q) is increasingly reported to exert antioxidant functions besides its implication in mitochondrial energy metabolism. On the other hand ubisemiquinones (SQ • ) of the respiratory chain are considered to account for the production of superoxide radicals as a byproduct of cellular respiration. Since the formation of potentially prooxidative ubisemiquinones can be expected to result from the antioxidant activity of ubiquinol, the evaluation whether or not QH 2 exerts antioxidant activities depends on the fate of antioxidant-derived metabolites and the existence of a natural recycling system for oxidized QH 2. We have recently shown that SQ • increasingly undergo autoxidation when approaching the external more polar phase of the membrane. In contrast to mitochondria where the QH 2/SQ • /Q pools are dynamically kept in relatively stable relationships the fate of semi and fully oxidized QH 2 is not at all clear in LDL particles where QH 2 is suggested to exert important antioxidant functions. Therefore, the antioxidant-derived metabolites of QH 2 in liposomes following lipid peroxidation were studied with respect to their localization in the bilayer and the possibility to recycle oxidized QH 2 via dihydrolipoic acid (DHLA). The results revealed a considerable fraction of QH 2 existing in the outer membrane section where protons from the aqueous phase have access to allow autoxidation. DHLA was found to recycle oxidized QH 2 although due to slow partition equilibration the reduction velocity appears to be not sufficient for therapeutic application.