Hematoxylin, an Alternative Substrate of Tyrosinase

Mushroom tyrosinase from <i>Agaricus bisporus</i> (<i>ab</i>TYR) is often used during the development of tyrosinase inhibitors for medicinal and cosmetic purposes. In the search for novel tyrosinase inhibitors, this study identified hematoxylin as an alternative substrate for <i>ab</i>TYR. The interaction of hematoxylin with <i>ab</i>TYR was investigated through spectrophotometric and chromatographic analyses. The results showed that hematoxylin acted as an <i>ab</i>TYR substrate and exhibited Michaelis-Menten kinetic behaviour at concentrations below 1.25 mM. The substrate properties of hematoxylin were similar to the natural tyrosinase substrate, L-3,4-dihydroxyphenylalanine (L-DOPA), with regards to K_m, while V_max was eightfold lower. The main oxidation product formed during the reaction of <i>ab</i>TYR with hematoxylin was identified as hematein. This is the first report of the interaction of hematoxylin with <i>ab</i>TYR.