Biphasic Effect of Simple Phenol Derivatives on Mushroom Tyrosinase Activity

In searching for inhibitors of mushroom tyrosinase, we observed that a number of phenol derivatives showed a biphasic effect on oxidation of the catechol DOPA. These compounds stimulated activity at low concentrations and inhibited at higher concentrations. The stimulatory effect is due to an increase in k cat ; the inhibition is due to an increase in K m (and is thus competitive). These compounds show only competitive inhibition when phenol is used as a substrate. The EC50 for compounds acting as activators of catechol oxidation show a strong positive correlation with their EC 50 as inhibitors of phenol oxidation. Taken together, these results suggest that mushroom tyrosinase contains two distinct binding sites for phenols: a high affinity site whose occupancy causes allosteric activation of catechol oxidation and competitive inhibition of phenol oxidation, and a second low affinity site whose occupancy causes competitive inhibition of catechol oxidation. The data also suggest that the active site for phenol oxidation may be distinct from the active site for catechol oxidation. Support from the Adrian Tinsley Program for Undergraduate Research is gratefully acknowledged.