Biosynthetic pathway to neuromelanin and its aging process.

Using model compounds of the melanic component of neuromelanin (NM) prepared by tyrosinase oxidation at various ratios of dopamine (DA) and cysteine (Cys) under physiological conditions, we examined a biosynthetic pathway to NM and its aging process by following the time course of oxidation to NM and the subsequent structural modification of NM under various heating conditions. Chemical degradation methods were applied to the synthetic NM. 4-Amino-3-hydroxyphenylethylamine (4-AHPEA) and thiazole-2,4,5-tricarboxylic acid (TTCA) were used as markers of benzothiazine and benzothiazole units, respectively. By following the time course of the biosynthetic pathway of synthetic NM, we found that neurotoxic molecules are trapped in NM. An aging simulation of synthetic NM showed that benzothiazine units in NM are gradually converted to benzothiazole during the aging process. Thus, natural NM was found to be similar to aged (heated) NM prepared from a 2:1 molar ratio of DA and Cys.