Stimulation of porphyrinogen oxidation by mercuric ion. I. Evidence of free radical formation in the presence of thiols and hydrogen peroxide.

The etiology of mercury-induced porphyrinuria was investigated by testing the hypothesis that mercuric ions (Hg2+) promote free radical-mediated oxidation of reduced porphyrins (porphyrinogens) by compromising the antioxidant potential of endogenous thiols, particularly GSH. Studies in vitro demonstrated that porphyrinogens (uroporphyrinogen and coproporphyrinogen) readily undergo H2O2-dependent oxidization in the presence of Fe3(+)-EDTA and that this action is attenuated by GSH at biologically relevant concentrations (0.5-10 mM). At low concentrations, Hg2+ complexes with GSH in a 1:2 molar ratio to decrease the antioxidant effect of GSH. However, at Hg2+ concentrations approaching saturation-complexation with available GSH, stimulation of porphyrinogen oxidation to 2 to 3 times that mediated by the H2O2/Fe3(+)-dependent system alone is observed. Stimulation of porphyrinogen oxidation by Hg2+ plus GSH increases in a dose-related manner with the concentration of H2O2 in the reaction mixture but is independent of the presence of iron. No porphyrinogen oxidation is observed in reaction mixtures containing H2O2 and either Hg2+ or GSH alone or when Hg+ is substituted for Hg2+. Studies with reactive oxidant scavengers and ESR spectroscopy suggest the participation of free radical species in Hg:GSH-mediated porphyrinogen oxidation. A mechanism involving ligand exchange between Hg2+ and GSH, which leads to formation of GS radicals and subsequent propagation of reactive oxygen-based radical species, is proposed. These studies support the view that Hg2+ both compromises the antioxidant potential of GSH and promotes formation of reactive species via thiol complexation. These findings suggest a mechanistic basis underlying the porphyrinogenic as well as tissue-damaging properties of mercuric ions.