J. W. FAHEY and P. TALALAY
The widespread belief that oxidative damage plays a major role in cancer, ageing, and in a number of chronic diseases has focused scientific and public attention on the possibility that antioxidants could prevent or at least retard these processes. Antioxidants are of two types: direct and indirect. Direct antioxidants [e.g. glutathione (GSH), toco- pherols, ascorbic acid and carotenoids] are substances that can participate in physiological, biochemical or cellular processes that inactivate free radicals or that prevent free-radical initiated chemical reactions. Direct antioxidants may also exhibit prooxidant effects under some experimental conditions, but whether these properties play a significant role in vivo remains unclear. In contrast, indirect antioxidants are not able to participate in radical or redox reactions as such, but they boost the antioxidant capacity of cells by a variety of mechanisms described below, and thereby afford protection against oxidative stress.