Influence of mercury (II), cadmium (II), methylmercury, and phenylmercury on the kinetic properties of rat liver glutathione peroxidase.

The effect of phenylmercury and methylmercury on rat liver glutathione peroxidase (GSH X Px) is investigated and compared with that of Hg(II) and with some previously reported results for Cd(II). Analysis of the kinetics of metal binding to the enzyme gives apparent inhibition rate constants: kc = 9.7 mM-1 min-1 for all three mercury compounds and 75 mM-1 min-1 for CdCl2. Glutathione (0.2 mM) protects the enzyme from metal inhibition, decreasing the apparent inhibition rate constants (kc) by 3.6 times for mercury compounds and 4.4 times for CdCl2. KI for the three mercury compounds is found to be 53 microM. It is unexpected that the same value of KI exists for all three forms of mercury studied and that inhibition of the enzyme by the metals is a relatively slow process. For Cd(II) the value of KI is 8.5 microM. It is suggested that inhibition of GSH X Px enzyme activity by cadmium, mercury, and organic mercury salts may not be due to simple complexation of the active site selenium moiety but may be due to a slower process, e.g., an alteration of the enzyme tertiary or quaternary structure.