Dopa oxidation and tyrosine oxygenation by human melanoma tyrosinase.

A tyrosinase purified from cultured human melanoma cells was studied for dopa oxidation and tyrosine oxygenation. Km for oxidation of L-dopa was 0.5 mM, and for D-dopa 3 mM. L-tyrosine was oxygenated only in the presence of a cosubstrate. L-Dopa was superior to D-dopa, dopamine, L- and D-alpha-methyldopa, dopac, and 5,6-dihydroxyindole-2-carboxylic acid as cosubstrate. Ascorbic acid, 5-S-cysteinyldopa, and 5-OH-dopa did not function as cosubstrates. The rate of tyrosine hydroxylation was much lower than that of dopa oxidation. Tyrosine inhibits dopa oxidation, and dopa in high concentrations inhibits tyrosine hydroxylation. The cosubstrate function of dopa, the substrate functions of dopa and tyrosine, and the mutual inhibition of tyrosinase by these compounds are explained in three equations.