Tyrosinase Autoactivation and the Chemistry of ortho-Quinone Amines

Tyrosinase oxidizes tyrosine to dopaquinone, which undergoes nonenzymatic reactions leading to precursors of melanin pigments. Cyclization of dopaquinone gives cyclodopa, which participates in redox exchange with dopaquinone to give the eumelanin precursor dopachrome plus dopa. The indirect formation of the catechol (dopa) from the phenol (tyrosine) leads to unusual enzyme kinetics. Using a combination of enzyme oximetry, pulse radiolysis, and chemical oxidation, the study of structurally modified dopaquinones provides firm evidence of nonenzymatic catechol formation during tyrosinase oxidation of phenols and reveals significant differences in their modes of reaction.