Mammalian Tyrosinase Catalyzes Three Reactions in the Biosynthesis of Melanin

The biosynthesis of melanin is initiated by the catalytic oxidation of tyrosine to dopa by tyrosinase in a reaction that requires dopa as a cofactor. Tyrosinase then catalyzes the dehydrogenation of dopa to dopaquinone. The subsequent reactions can proceed spontaneously in vitro. Tyrosinase, purified from murine melanomas and the skins of brown mice, has now been shown to catalyze a third reaction in mammalian melanogenesis, namely the conversion of 5,6-dihydroxyindole to melanochrome. This reaction requires dopa as a cofactor and is inhibited by tyrosine. Conversely, 5,6-dihydroxyindole inhibits the oxidation of tyrosine to dopa, so that the relative concentrations of tyrosine and 5,6-dihydroxyindole within the mammalian pigment cell are capable of regulating melanogenesis in a previously unrecognized fashion. Tyrosinase has the unusual property of catalyzing three distinct reactions within a single biochemical pathway: the hydroxylation of a monophenol, the dehydrogenation of a catechol, and the dehydrogenation of a dihydroxyindole. The first and third of these reactions require dopa as a cofactor; in the second reaction, dopa is a substrate.