Studies on the kinetics of oxidation of 4-hydroxyanisole by tyrosinase.

The potential use of 4-hydroxyanisole as a chemotherapeutic agent in the treatment of malignant melanoma led us to investigate the kinetics of oxidation of this tyrosine analogue by tyrosinase. We found that addition of amino acids accelerated the reaction, resulting in a reduction in length of the characteristic lag period of monohydric phenol oxidation. The lag period was abolished completely by an aliquot of exhausted 4-hydroxyanisole/tyrosinase reaction mixture and by very low concentrations of thiol-containing compounds. We conclude that the reaction-accelerating property of non-thiol amino acids is due to the reductive addition of the ortho-quinone reaction product to nucleophilic groups of the amino acids. The dihydric phenol product which results is capable of met-tyrosinase recruitment by electron donation to the cupric active site generating the cuprous form of the enzyme which binds oxygen and is able to oxidise monohydric phenols. Abolition of the lag period by an aliquot of exhausted reaction mixture is probably due to recruitment of the met-enzyme by catecholic oligomers of the quinone product. Thiol containing compounds are able to abolish the lag period due to the ability of these compounds to reduce met-tyrosinase directly.