Effect of thiohydroxyl compounds on tyrosinase: inactivation and reactivation study.

An unusual thioether bridge (Cys-His) has been detected at the active site of mushroom tyrosinase, and the effects of thiohydroxyl compounds such as dithiothreitol (DTT) and beta-mercaptoethanol (beta-ME) on Cu2+ at the active site have been elucidated. Treatment with DTT and beta-ME on mushroom tyrosinase completely inactivated 3,4-dihydroxyphenylalanine oxidase activity in a dose-dependent manner. Sequential kinetic studies revealed that DTT and beta-ME caused different mixed-type inhibition mechanisms: the slope-parabolic competitive inhibition (Ki = 0.143 mM) by DTT and slope-hyperbolic noncompetitive inhibition (Ki = 0.0128 mM) by beta-ME, respectively. Kinetic Scatchard analysis consistently showed that mushroom tyrosinase had multiple binding sites for DTT and beta-ME with different affinities. Reactivation study of inactivated enzyme by addition of Cu2+ confirmed that DTT and beta-ME directly bound with Cu2+ at the active site. Our results may provide useful information regarding interactions of tyrosinase inhibitor for designing an effective whitening agent targeted to the tyrosinase active site.