4-Hydroxyphenylacetic Acid as a Monophenolase Inhibitor and a Diphenolase Activator on Mushroom Tyrosinase

4-Hydroxyphenylacetic acid (4-HPA) can inhibit monophenolase (IC
₅₀
=0.79 mM) and activate diphenolase of mushroom tyrosinase with its increasing concentrations. The activation kinetics, analyzed by Lineweaver-Burk plots, indicated 4-HPA to be a competitive and uncompetitive mixed-type activator of diphenolase when
_L
-DOPA was used as substrate. 4-HPA can not only inhibit the oxy form but also activate the met form of mushroom tyrosinase.