Inhibitory activities of hydroxyphenolic acid–amino acid conjugates on tyrosinase

The inhibitory effects of hydroxyphenolic acid–aromatic amino acid conjugates on the activity of mushroom tyrosinase were investigated. Amongst the hydroxyphenolic acid–amino acid derivatives, protocatechuic acid–amino acid amide (PA-AA-NH2) showed highly increased tyrosinase inhibitory activity. The results show that it could strongly inhibit both the monophenolase and diphenolase activities of tyrosinase. The IC50 values, inhibition type, and KI values of these hydroxyphenolic acid derivatives (PA-F-NH2, PA-W-NH2, PA-Y-NH2) were evaluated and compared. Kinetic analyses of PA-AA-NH2 as a tyrosinase inhibitor revealed that it acted as a reversible mixed-І type inhibitor, possibly by chelating copper at the active site of tyrosinase. These results suggest that aromatic amino acid conjugation assisted PA in binding to the active site of mushroom tyrosinase where it interrupted access to the substrate.