TYROSINASE KINETICS - FAILURE OF ACCELERATION IN OXIDATION OF RING-BLOCKED MONOHYDRIC PHENOL SUBSTRATE

When 2,5,6-trimethyl-4-hydroxyanisole is used as substrate for mushroo m tyrosinase the oxidation rate is slow and the kinetics do not exhibi t an initial acceleration (lag period), in contrast to the kinetics of oxidation of the parent compound, 4-hydroxyanisole. This finding is i nterpreted as evidence that the acceleration of oxidation of 4-hydroxy anisole is indirectly contingent on a reductive nucleophile addition t o the orthoquinone product of the monohydric phenol, which is prevente d by ring methylation. Such a view is consistent with the proposal tha t the lag-phase characteristic of the kinetics of monohydric phenol ox idation by tyrosinase is due to the activation of previously inactive enzyme by electron donation from an orthodiphenol substrate formed fro m the orthoquinone oxidation product.