effect of mercuric chloride on kinetic properties of horseradish peroxidase

mercury is one of the three major environmental metal poisons, and mercuric chloride is a highly reactive compound which can harm cells by a variety of mechanisms including direct interaction with sulphydryl groups of proteins and enzymes, therefore affecting the enzymatic activity. this study focused on the effect of hg++ on horseradish peroxidase (donor: hydrogen peroxide oxidoreductase, ec 1.11.1.7) (hrp) (isoenzyme c) activity. in the presence of 88 mm hydrogen peroxide km for o-dianisidine oxidation was 0.05 millimolar and vmax was 8.5 m.s-1. incubation of the enzyme with 1 to 100 millimolar mercuric chloride for 5-20- and 60 min resulted in progressive inhibition of the enzymatic activity. at low hg++ concentrations the inhibition was reversible by excess substrate, while at high hg++ concentration the inhibition was not reversible. results also indicated that the type of inhibition depended on the duration of incubation of the enzyme with metal ion and on the hg++ concentration. so we could conclude that the type of inhibition changed from noncompetitive to mix with increased incubation time and increased metal concentration.