Conjugation of dopa and 5-S-cysteinyldopa with cysteine mediated by superoxide radical.

cytotoxicity of catechols has been ascribed to their binding with proteins through sulfhydryl groups. Superoxide radical (O2-) generated in hypoxanthine-xanthine oxidase system at pH 7.4 mediated conjugation of dopa with cysteine to form cysteinyldopas. Similarly, 5-S-cysteinyldopa gave 2,5-S,S-dicysteinyldopa. The rates of oxidation of the catechols by O2- appear to be comparable to that of reduction of nitro-blue tetrazolium by O2-. These results suggest that catechols may exert cytotoxicity in cells where biochemical defence against O2- or the quinone oxidation products is not sufficient.