CATALYTIC METALS, ASCORBATE AND FREE-RADICALS - COMBINATIONS TO AVOID

Trace levels of transition metals can participate in the metal-catalyz ed Haber-Weiss reaction (superoxide-driven Fenton reaction) as well as catalyze the oxidization of ascorbate. Generally ascorbate is thought of as an excellent reducing agent; it is able to serve as a donor ant ioxidant in free radical-mediated oxidative processes. However, as a r educing agent it is also able to reduce redox-active metals such as co pper and iron, thereby increasing the pro-oxidant chemistry of these m etals. Thus ascorbate can serve as both a pro-oxidant and an antioxida nt. In general, at low ascorbate concentrations, ascorbate is prone to be a pro-oxidant, and at high concentrations, it will tend to be an a ntioxidant. Hence there is a crossover effect. We propose that the ''p osition'' of this crossover effect is a function of the catalytic meta l concentration. In this presentation, we discuss: (1) the role of cat alytic metals in free radical-mediated oxidations; (2) ascorbate as bo th a pro-oxidant and an antioxidant; (3) catalytic metal catalysis of ascorbate oxidation; (4) use of ascorbate to determine adventitious ca talytic metal concentrations; (5) use of ascorbate radical as a marker of oxidative stress; and (6) use of ascorbate and iron as free radica l pro-oxidants in photodynamic therapy of cancer. (C) 1996 by Radiatio n Research Society