Biosyntheticstudies have shown that pheomelanins, the distinctive pigments of red human hair, arise from oxidative polymerization of cysteinyldopas via 1,4-benzothiazinylalanine intermediates. However, the mode of formation of the pigment polymer remains controversial. To address this point, we have investigated the conversion of the major biosynthetic precursor 5-S-cysteinyldopa (2a) to pheomelanin
Under biologically relevant conditions, oxidation of 5-S-cysteinyldopa (1) to pheomelanins proceeds through the formation of the 1,4-benothiazine 7 along with the 3-carboxy analogue 6 in much smaller amounts, as evidenced by isolation of the reduced forms 5 and 4 and by deuterium labelling experiments.
New intermediates of phaeomelanogenesis in vitro beyond the 1,4-benzothiazine stage