Effect of metal ions on the kinetics of tyrosine oxidation catalyzed by tyrosinase

The conversion of tyrosine into dopa is the rate-limiting step in the biosynthesis of melanins catalyzed by tyrosinase. This hydroxylation reaction is characterized by a lag period, the extent of which depends on various parameters, notably the presence of a suitable H donor, such as dopa or tetrahydropterin. Here, it is reported that catalytic amts. of Fe2+ have the same effect as dopa in stimulating the tyrosine hydroxylase activity of the enzyme. Kinetic expts. showed that the redn. of the induction time depended on the concn. of the added metal and the nature of the buffer system used and was not suppressed by superoxide dismutase, catalase, formate, or mannitol. Notably, Fe3+ had only a small delaying effect on tyrosinase activity. Among the other metal ions tested, Zn2+, Co2+, Cd2+, and Ni2+ had no detectable influence, whereas Cu2+ and Mn2+ exhibited a marked inhibitory effect on the kinetics of tyrosine oxidn. These findings are discussed in the light of the commonly accepted mechanism of action of tyrosinase.