Expansion of antioxidant function of vitamin E by coenzyme Q.

Abstract

Coenzyme Q (Q) is an integral redox and proton translocating component of the mitochondrial electron transport chain [5,8]. It is, however, widely distributed in other subcellular membranes [4,6] and even serum lipoproteins [7] where its role in electron transport is not apparent. Moreover, its concentration in some membranes, including those of the Golgi and lysosomes, exceed that found in mitochondria. This raises the question of what functions coenzyme Q may play in the biology of the cell. Coenzyme Q may exist in one of three oxidation states; the fully reduced ubiquinol form (QH2), the fully oxidized ubiquinone form (Q) and the radical semiquinone (Q·) intermediate [15]. The radical form tends to be stabilized by delocalization of the unpaired electron about the fully substituted benzoquinone ring but can undergo a disproportionation reaction to yield equal amounts of fully reduced and fully oxidized products. Another feature of coenzyme Q is the relatively hydrophobic character of the molecule. This is due largely to the long polyisoprene substituent of the benzoquinone ring which, in biological membranes, varies in length from 6 to 10 or more isoprene units. The polarity of the molecule increases in the order oxidized > semiquinone > reduced and this is believed to influence the location of the benzoquinone ring with respect to the aqueous membrane interface [1,21]. The reduced form of coenzyme Q is known to have direct antioxidant action in which radical chain reactions are terminated [2,10,11,16,20,24]. Coenzyme Q is also present in membranes with another lipid antioxidant, vitamin E. The relative amount of each varies from one membrane to another and together they represent a formidable defense against oxidative attack [9,12]. It should be added parenthetically that coenzyme Q in its semiquinone form can generate superoxide from oxygen and this, in turn, can result in oxidation of biomolecules [3,25]. Recent studies in our laboratories have indicated that coenzyme Q and vitamin E, rather than acting independently in protecting biological membranes from oxidative attack, are integrated into a regenerative cycle. In this cycle the more powerful antioxidant, vitamin E, which is converted in the first instance to its phenoxyl radical (tocopheroxyl radical), is reconverted by reduced forms of coenzyme Q to recover