Inability of Murine Melanoma "Tyrosinase" (Dopa Oxidase) To Oxidize TYrosine in the Presence or Absence of Dopa or Dihydroxyfumarate Cofactor

Soluble “tyrosinase” was isolated from Harding Passey melanoma and its ability to oxidize tyrosine or dopa to melanin was studied by quantitative analysis of tyrosine consumption and dopa formation and by spectrophotometric recording of dopachrome formation.Using these methods soluble murine melanoma “tyrosinase” was shown to be an active dopa oxidase. but it had no ability to oxidize tyrosine in the presence or absence of either dopa or dihydroxyfumarate cofactor. In the system containing “tyrosinase,” tyrosine, and dopa. Tyrosine decreased the rate of dopa oxidation, presumably by competition for the active site on the enzyme. “Tyrosinase” did not show any evidence of tyrosine oxidation, using a range of tyrosine concentrations and tyrosine: dopa ratios.The results of this study support the proposal that mammalian “tyrosinase” is a dopa oxidase arid that hydroxylation ot tyrosine in mammalian melanogenesis is mediated by peroxidase