Effect of pH on the oxidation pathway of dopamine catalyzed by tyrosinase

Abstract The oxidation of 3,4-dihydroxyphenylethylamine (dopamine) by O 2 catalyzed by tyrosinase yields 4-(2-aminoethyl) -1,2-benzoquinone ( o -dopaminequinone), which evolves nonenzymatically through two branches or sequences of reactions, whose respective operations are determined by the pH of the medium. The cyclization branch of o -dopaminequinone takes place in the entire range of pH and is the only significant branch at pH ⩾ 6. The hydroxylation branch of o -dopaminequinone only operates significantly at pH p -topaminequinone), identified from cyclic voltammetry assays. The kinetic characterization of the hydroxylation branch of o -dopaminequinone has been carried out by spectrophotometric and oxymetric assays. The successful fitting of data to the kinetic behavior predicted by the kinetic analysis at both pH ⩾ 6 and pH p -topaminequinone, accumulated at the acidic pH characteristic of melanosomes and melanome cells.