Pro-oxidant Activation of Ocular Reductants. 1. Copper and Riboflavin Stimulate Ascorbate Oxidation Causing Lens Epithelial Cytotoxicity in Vitro

Ascorbic acid may not only be involved as an antioxidant in the aqueous humour and lens but under certain conditions may also undergo 'pro-oxidant conversion' leading to the generation of oxidants. It is demonstrated that the bovine aqueous humour contains electron spin resonance (ESR) detectable levels of ascorbyl semiquinone free radical associated with the transition metal-catalysed oxidation of ascorbate. Cu2+ was found to be a more efficient catalyst of ascorbate oxidation than Fe2+ and was similarly more potent in inhibiting 86Rb uptake of lens epithelial cells in the presence of ascorbate. The metal-chelating agent diethylenetriaminepentaacetic acid (DETAPAC) inhibited metal-catalysed ascorbate oxidation and lens epithelial cytotoxicity. Riboflavin in the presence of light stimulates the formation of ESR-detectable ascorbyl semiquinone radicals and stimulates O2 consumption (oxidation) by ascorbate in a manner dependent upon the concentration of riboflavin. It is proposed that photoexcited riboflavin can perform a one-electron oxidation of ascorbate generating a riboflavin radical. This radical may then autoxidize generating superoxide anion. The riboflavin-mediated photoimpairment of 86Rb uptake of bovine lens epithelial cells was found to be dependent upon the presence of ascorbate. This 'pro-oxidant' activation of ascorbate by riboflavin is discussed in the context of previous studies on riboflavin phototoxicology.