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

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.