New regulatory mechanisms in the biosynthesis of pheomelanins: rearrangement vs. redox exchange reaction routes of a transient 2H-1,4-benzothiazine-o-quinonimine intermediate

Pheomelanins, the typical epidermal pigments of red haired, Celtic-type Cau casians, arise from oxidative cyclization of cysteinyldopas, mainly the 5-S -isomer CD, via 1,4-benzothiazines. However, the mechanism and the relative yields of formation of these intermediates have remained poorly defined. W e have now examined the course of the oxidation of CD at physiological pHs, under different reaction conditions. Surprisingly, a consumption of CD far exceeding the stoichiometry of the oxidant was observed at low oxidant-to- substrate ratios, low temperatures and high substrate concentrations. The y ields of the 3,4-dihydro-1,4-benzothiazine-3-carboxylic acid DHBCA vs. the non-carboxylated analogue DHB in the oxidation mixture, after NaBH4 reducti on, were also found to depend markedly on the reaction conditions. Based on these and other results, a reaction scheme is proposed involving a transie nt o-quinonimine generated by oxidative cyclization of CD to which three di fferent paths are offered, namely redox exchange with CD to give DHBCA (pat h A) or intramolecular rearrangement with (path B) or without (path C) deca rboxylation, leading to the benzothiazine BTZ and the 3-carboxy analogue BT ZCA, respectively. The relative operation of path A vs. path C was assessed by deuterium labeling experiments. These findings point to new mechanisms of regulation of the initial steps of pheomelanogenesis, bearing significan t implications on the structure of the final pigment. (C) 2000 Elsevier Sci ence B.V. All rights reserved.