Nature of tyrosinase inactivation in melanosomes

The decrease in tyrosinase activity following incubation of melanosomes isolated from mouse melanoma with dopa appeared to be related to the degree of in vitro melanization of melanosomes caused by incubation with dopa. The inactivation similarly occurred in the dopa-tyrosinase reaction system containing ascorbic aicd, although in the presence of ascorbic acid the dopa-quinone is immediately converted back to dopa and therefore melanin formation does not take place. The mode of inactivation of tyrosinase in melanosomes appeared to be similar to the reaction inactivation which is known to occur in the reaction of plant plant tyrosinase. Superoxide anion (O-2) and singlet oxygen (1O2) were estimated but undetectable during the dopa-tyrosinase reaction. Thus the inactivation of tyrosinase is not caused by the reaction products or the oxygen radicals.