The structure of melanins and melanogenesis—III The structure of sepiomelanin

New degradation products of sepiomelanin have been obtained. Alkali fusion yields, in addition to other compounds, 5,6-dihydroxyindole, 5,6-dihydroxyindole-2-carboxylic acid, 4-methyl-cathechol and a compound, which is probably 5,6-dihydroxyindole-4,7-dicarboxylic acid. These products constitute the first proof of the indole structure of a natural melanin. The carboxyl group of 5,6-dihydroxyindole-2-carboxylic acid is not formed during alkali fusion, but pre-exists in the macromolecule. Cysteic acid, taurine, glycine and aspartic acid were obtained by oxidation of sepiomelanin with hydrogen peroxide in acetic acid. The formation of cysteic acid indicates that sepiomelanin is bound to the protein by means of cysteine. Taurine is clearly an artifact generated by decarboxylation of cysteine. Glycine and aspartic acid probably are derived from the pyrrole moiety of the indole units: they also result from the oxidation of 5,6-dihydroxyindole-2-carboxylic acid.

Oxidation of methylated sepiomelanin yields 3-carbomethoxypyrrole-2,5-dicarboxylic acid and 5-carbomethoxypyrrole-2,3-dicarboxylic acid; isolation of the former further proves the presence of pyrrole units in sepiomelanin, whereas formation of the latter is further evidence that some indole (probably dopachrome) units of the macromolecule have a carboxyl group in position 2.