CHEMICAL DEGRADATION OF MELANINS - APPLICATION TO IDENTIFICATION OF DOPAMINE-MELANIN

Melanocytes produce two chemically distinct types of melanin pigments, eumelanins and pheomelanins. These pigments can be quantitatively ana lyzed by acidic KMnO4 oxidation or reductive hydrolysis with hydriodic acid (HI) to form pyrrole-2,3,5-tricarboxylic acid (PTCA) or aminohyd roxyphenylalanine (AHP), respectively. Dark brown melanin-like pigment s are also widespread in nature, for example, in the substantia nigra of humans and primates (neuromelanin), in butterfly wings and in the f ungus Cryptococcus neoformans. To characterize such diverse types of m elanins, we have improved the alkaline H2O2 oxidation method of Napoli tano et al. (Tetrahedron, 51:5913-5920, 1995) and re-examined the HI h ydrolysis method of Wakamatsu et al. (Neurosci. Lett., 131:57-60, 1991 ). The results obtained with H2O2 oxidation show that 1) pyrrole-2,3-d icarboxylic acid (PDCA), a specific marker of 5,6-dihydroxyindole unit s in melanins, is produced in yields ten times higher than by acidic K MnO4 oxidation, and 2) PTCA is artificially produced from pheomelanins . The results with HI hydrolysis show that dopamine-melanin produces a 1:1 mixture of 3-amino and 4-amino isomers of aminohydroxyphenylethyl amine, while the isomer ratio is about 0.2 in melanins prepared from d opamine and cysteine. These results indicate that alkaline H2O2 oxidat ion is useful in characterizing synthetic and natural eumelanins and t hat reductive hydrolysis with HI can be applied to analyzing oxidation products of dopamine such as neuromelanin.