Inactivation of human tyrosinase by cysteine. Protection by dopa and tyrosine.

Human tyrosinase prepared from cultured melanoma cells is inactivated by 10 mM cysteine. The inactivation of the enzyme by cysteine is less pronounced in the presence of catalase and superoxide dismutase. Thus, oxygen radicals and/or hydrogen peroxide may contribute to the inactivation of human tyrosinase by cysteine. Dopa and/or tyrosine protects tyrosinase against inactivation by cysteine. The protection observed with tyrosine alone indicates that oxidation of substrate is not necessary for the protection. The effect of dopa and/or tyrosine is probably due to steric hindrance at the active site preventing the access of cysteine to the copper.