ABSTRACTIn mammals, melanin formation occurs only in melanosomes within the melanocyte, where tyrosinases are most highly accumulated; however tyrosinases exist as precursors of melanosomal enzymes in microsomes, where the enzymes are inactivated by unknown mechanism(s). The Km value of melanosomal tyrosinase for tyrosine was 0.6 mM, but that of the microsomal enzyme was 1.3 mM. The Km for tyrosine of microsomal tyrosinase was lowered to that of melanosomal tyrosinase after incubation of the microsomal enzymes with a Golgi fraction isolated from mouse melanoma. Further, the molecular weight of microsomal tyrosinase appeared to increase after incubation with the Golgi fraction; microsomal tyrosinase showed a slower mobility in SDS Polyacrylamide gel electrophoresis after incubation with the Golgi fraction than before incubation. These data suggest the possibility that microsomal tyrosinase is processed in the Golgi area during transfer to the melanosome in the melanocyte.