Study of alpha-methyldopa oxidation by tyrosinase.

The pathway for alpha-methyldopa oxidation to alpha-methyldopachrome, by mushroom tyrosinase, is proposed. Characterization of intermediates in this oxidative reaction and stoichiometry determination have both been undertaken. The steps for alpha-methyldopa transformation into its aminochrome would be: alpha-methyldopa----o-alpha-methyldopaquinone-H+----o-alpha- methyldopaquinone----leuko-alpha-methyldopachrome----alpha- methyldopachrome. The stoichiometry for this conversion corresponded to the equation: 2 o-alpha-methyldopaquinone-H+----alpha-methyldopa + alpha-methyldopachrome. At very acid pH values, another route implying the addition of water to the quinonic ring, competes with the first one. Two chemical pathways can be proposed from alpha-methyldopaquinone-H+, the relative importance of which is determined by the pH. A theoretical and experimental kinetic approach was applied to this oxidative reaction. Rate constants and thermodynamic activation parameters of the chemical steps, have been evaluated. The results obtained confirmed that alpha-methyldopa oxidation by tyrosinase followed a scheme similar to that established for L-dopa and alpha-methylnoradrenaline.