An analysis of the role of coenzyme Q in free radical generation and as an antioxidant.

The vital role of coenzyme Q in mitochondrial electron transfer and its regulation, and in energy conservation, is well established. However, the role of coenzyme Q in free oxyradical formation and as an antioxidant remains controversial. Demonstration of the existence of the semiquinone form of coenzyme Q during electron transport, coupled with recent evidence that hydrogen peroxide (but not molecular oxygen) may act as an oxidant of the semiquinone, suggests that the highly reactive OH. radical may be formed from the semiquinone. On the other hand, data exist implicating the Fe-S species as the source of electron transfer chain, free radical production. Additional data exist suggesting instead that the unpaired electron of the coenzyme Q semiquinone most likely dismutases superoxide radicals. These concepts and those arising from observations at several levels of organization including subcellular systems, intact animals, and human subjects in the clinical setting, supporting the concept of reduced coenzyme Q as an antioxidant, will be presented. The results of recent studies on the interaction between the two-electron quinone reductase--DT diaphorase and coenzyme Q10 will be presented. The possibility that superoxide dismutase may interact with reduced coenzyme Q, in conjunction with DT diaphorase inhibiting its autoxidation, will be described. The regulation of cellular coenzyme Q concentrations during oxidative stress accompanying aerobic exercise, resulting in increased protection from free radical damage, will also be presented.