Composition of Mammalian Eumelanins: Analyses of DHICA-Derived Units in Pigments From Hair and Melanoma Cells

The proportions in which two eumelanin monomers, namely 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and 5,6-dihydroxyindole (DHI), compose the eumelanin polymer are believed to determine properties of the pigment including its color. These proportions are, however, not well elucidated for naturally occurring eumelanins, largely because of methodological difficulties. In this study we estimate the content of DHICA-derived units in mammalian eumelanins using a combination of two analytical techniques: 1) quantitation of DHICA-derived eumelanin by measuring the yield of pyrrole-2,3,5-tricarboxylic acid (PTCA index) and 2) spectrophotometrical quantitation of total (DHI + DHICA) eumelanin at 350 nm (A₃₅₀ index). The ratio of PTCA/A₃₅₀ measured for melanins synthesized from DHI and DHICA mixed in various molar proportions correlates well with the content of DHICA in synthetic polymers. Using this relationship as a standard curve we estimated the proportion of DHICA-derived units in mammalian eumelanins from hair and melanoma cells and found it to be much higher in rodent pigments (58.8%-98.3%; two species, mouse and hamster were examined) as compared to human eumelanins (19.2%-41.8%; one Caucasian and one Oriental individual were examined). No relationship between proportion of DHICA-derived units in eumelanin and hair color is found. The latter seems to be determined predominantly by the ratio of pheo- to eumelanin synthesis.