Reactivity of Hg(II) with superoxide: evidence for the catalytic dismutation of superoxide by Hg(II).

Mercuric ion, a well-known nephrotoxin, promotes oxidative tissue damage to kidney cells. One principal toxic action of Hg(II) is the disruption of mitochondrial functions, although the exact significance of this effect with regard to Hg(II) toxicity is poorly understood. In studies of the effects of Hg(II) on superoxide (O2-) and hydrogen peroxide (H2O2) production by rat kidney mitochondria, Hg(II) (1-6 microM), in the presence of antimycin A, caused a concentration-dependent increase (up to fivefold) in mitochondrial H2O2 production but an apparent decrease in mitochondrial O2- production. Hg(II) also inhibited O(2-)-dependent cytochrome c reduction (IC50 approximately 2-3 microM) when O2- was produced from xanthine oxidase. In contrast, Hg(I) did not react with O2- in either system, suggesting little involvement of Hg(I) in the apparent dismutation of O2- by Hg(II). Hg(II) also inhibited the reactions of KO2 (i.e., O2-) with hemin or horseradish peroxidase dissolved in dimethyl sulfoxide (DMSO). Finally, a combination of Hg(II) and KO2 in DMSO resulted in a stable UV absorbance spectrum [currently assigned Hg(II)-peroxide] distinct from either Hg(II) or KO2. These results suggest that Hg(II), despite possessing little redox activity, enhances the rate of O2- dismutation, leading to increased production of H2O2 by renal mitochondria. This property of Hg(II) may contribute to the oxidative tissue-damaging properties of mercury compounds.