5-S-cysteinyldopa as a substrate for tyrosinase.

Oxidation of 5-S-cysteinyldopa by mushroom tyrosinase was studied by measuring 5-S-cysteinyldopa consumption with HPLC. 5-S-cysteinyldopa was found to be a substrate for tyrosinase, but our results suggests that a self-catalysed oxidation induced by enzymatically formed 5-S-cysteinyl dopaquinone also takes place. Dopa oxidation by tyrosinase was determined by measuring substrate consumption with HPLC. The oxidation of 5-S-cysteinyldopa was markedly accelerated in the presence of dopa. This could be explained by dopaquinone oxidation of 5-S-cysteinyldopa, which has a lower oxidation potential than dopa.