Regulatory mechanisms of tyrosinase activity in melanocyte. I.

The activity of tyrosinase (EC 1.10.3,1 o-diphenol:02 oxidoreductase) was found to be in melanosomes, smooth-surface membranes, rough-surface membranes , and ribosomes when they are isolated from Harding-Passey mouse mela-noma. In vivo incorporation studies of 14C-dopa into these cell particles were carried out in order to clarify the site of malanin formation in melanocytes. '4C-dopa incorporation occurred only in melanosomes. If the site of the dopa incorporation could be accepted as the site of melanin formation, it was concluded that melanin was synthesized only in melanosomes (1). Although, in mammals, tyrosinase is generally believed to occur only in the presence of melanin, and vice versa, ried out in order to clarify the site of melanin formation in melanocytes. C'C-dopa despite the presence of tyrosinase activity. It is considered, therefore, that the presence of tyrosinase activity is not necessarily correlated to the formation of melanin in melanocyte. How does tyrosinase exist in smooth-surface membranes, rough-surface membranes or ribosomes? Is it in a form of protyrosinase or is it inhibited in some way? The presence of naturally occurring inhibitors of tyrosinase has been reported in the serum (2) and melanoma tissues (3-5). Satoh et al. (4) found the presence of tyrosinase-inhibitory materials in the hamster melanomas, and they were shown to be heat stable, dialyzable and did not exhibit sulfhydryl properties. Chian et al. (5) have isolated the tyrosinase inhibitor from mouse melanomas, and it was found to be heat stable, dialyzable and could be inactivated by ultraviolet irradiation. The inhibitor isolated fully inhibited soluble tyrosinase but only partially inhibited melanosome tyrosinase. In the present studies, the regulatory mechanisms of tyrosinase activity in the melanocytes have been studied in vivo and in vitro. The tyrosinase-inhibitory material was found to be present in the soluble fraction of melanoma tissue. The inhibi-tory effect was examined in vitro on soluble tyrosinase and on tyrosinase associated with smooth-surface membranes and melanosomes obtained from melanomas, and also on melanogenesis in the melanocyte in vivo.