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J. Biol. Chem., Vol. 255, Issue 2, 454-458, 01, 1980

Partial inactivation of cytochrome c oxidase by nonpolar mercurial reagents

AJ Mann and HE Auer

Purified beef heart cytochrome c oxidase is inactivated to the extent of 35 to 50% by the nonpolar mercurial reagents mercuric chloride and ethylmercuric chloride. The inactivation is complete within 5 min. In titrations of activity, the plateau level of inactivation is attained at added ethylmercuric chloride:heme a ratios of about 1:1. Up to 3 mercury atoms/heme a are bound to the oxidase, although only the first of these affects its enzymatic activity. Incubation of the ethylmercury- modified oxidase with sulfhydryl compounds reverses the inactivation, with 2,3-dimercaptopropanol being most effective of the reagents tested. Spectrophotometric and polarographic assays of enzymatic activity show that Km values for the native and the ethylmercury- modified enzymes are practically indistinguishable, and that the partial inactivation observed for the latter is reflected exclusively in a lower value of Vmax compared to that of the native enzyme. Based on these results, we propose that ethylmercuric chloride reacts with a single crucial--SH group per heme a, and that electron transfer processes in the modified product are partially inhibited.
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