The Chemistry of Melanin Iii. Mechanism of the Oxidation of Dihydroxyphenylalanine by Tyrosinase* by Howard S. Mason

When 3,4-dihydroxyphenylalanine is oxidized in the presence of phenol oxidases (2,3) or by silver oxide (4), a red pigment is formed. Enzymically, this pigment is converted to a melanin. In the absence of enzyme, it has been shown that the pigment may rearrange to either Zcarboxy-5,6-dihydroxyindole or to 5,6-dihydroxyindole (5). The structure of the red pigment has therefore been inferred to be either 2-carboxy-2,3dihydroindole-5,6-quinone or the tautomeric Zcarboxy-2,3-dihydro-6-hydroxyindole-1,8quinonimine (5-7), and on this basis Raper has proposed the accompanying hypothesis for the enzymic conversion of 3 ,Cdihydroxyphenylalanine to synthetic dopa melanin (5, 6). This study was undertaken to determine whether or not the postulated reactions beyond the formation of the red pigment do take place in the presence of oxygen and enzyme. The enzymic oxidation of 3,4-dihydroxyphenylalanine was followed spectrophotometrically. The process was observed to proceed in three chromophoric phases, the first corresponding to the formation of the red pigment, the second to an intermediate purple pigment, and the third to the formation of melanin. By comparison of the observed spectra with those of known substances, it was possible to show that a rearrangement of the red pigment does occur during the enzymic oxidation, that synthetic dihydroxyphenylalanine melanin is probably a polymer of indoled, 6-quinone, and that the inferred o-quinonoid formulation of the red pigment is correct.