Hydrogen Peroxide Helps in the Identification of Monophenols as Possible Substrates of Tyrosinase

Abstract

Tyrosinase exists in three forms in the catalytic cycle depending on the oxidation state of the copper: met- (E_m), oxy- (E_ox), and deoxy- (E_d). When O-quinones, products of the enzymatic reaction, evolve chemically to generate an O-diphenol in the reaction medium, the enzyme acts on a monophenol with O-diphenol as reductant, converting E_m to E_d. The binding of E_d to molecular oxygen gives E_ox, which is active on monophenols, but when the O-quinone product does not generate O-diphenol through chemical evolution, the monophenol does not act as an enzyme substrate. The fact that E_ox can be formed from E_m with hydrogen peroxide can be used to help identify whether a monophenol is a substrate of tyrosinase. The results obtained in this study confirm that compounds previously described as inhibitors of the enzyme are true substrates of it.