Tocopherylquinone and tocopherylhydroquinone.

Tocopherylquinone (TQ) is formed in the antioxidant action of tocopherol (T). TQ was found in human subjects and it was observed that the ratio alphaTQ/ alphaT increased in general with increasing oxidative stress. TQ is reduced to tocopheryl hydroquinone (TQH2) but the ratio TQH2/TQ in vivo has not been reported. TQH2 acts as a potent radical-scavenging antioxidant. alphaTQH2 is more reactive toward radicals than ubiquinol, a reduced form of coenzyme Q, and alphaT. The overall efficacy of TQH2 as an antioxidant is determined by the fate of semiquinone radical formed from TQH2 as well as the reactivity toward oxygen radicals. Partly substituted gammaTQ, but not alphaTQ, exerts cytotoxicity by both redox cycling and reaction with protein thiols and glutathione.