Pro-oxidant activation of ocular reductants. 2. Lens epithelial cell cytotoxicity of a dietary quinone is associated with a stable free radical formed with glutathione in vitro.

The ability of 2,6-dimethoxyquinone (DMQ) to impair 86Rb uptake by bovine lens epithelial cells was found to be independent of exogenous ascorbate in contrast to the impairment induced by Fe/Cu or riboflavin plus light. The cytotoxicity was associated with an electron spin resonance (ESR) detectable singlet radical (g = 2.0062) which also formed on incubation of DMQ with glutathione (GSH) or gamma-crystallin in vitro. Formation of the stable free radical appeared to require conjugation of DMQ by peptidyl thiol and required transition metal catalysis. A structure for the DMQ-glutathione free radical conjugate is proposed. Redox activity of quinone conjugates is suggested to be of relevance to an oxidative damage hypothesis of cataract.