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An improved modification of permanganate oxidation of eumelanin that gives a constant yield of pyrrole-2,3,5-tricarboxylic acid.

Microanalysis of eumelanin is based on the formation of pyrrole-2,3,5-tricarboxylic acid (PTCA) on permanganate oxidation followed by its HPLC determination. A problem in this method was that the oxidation gave concave, exponential curves when the amounts of PTCA formed were plotted against the amounts of sample oxidized. The problem has been mostly overcome by adding a homogenate of 5 mg of a mouse liver to the oxidation medium. Sepia melanin, C57BL black mouse hair, B16 mouse melanoma, and MM418 human melanoma cells were oxidized in the absence or presence of the liver homogenate. The yields of PTCA increased about 1.5-fold by adding the liver homogenate and the calibration curves became linear or almost linear. With the improved method the PTCA values from various types of samples can be reliably compared.