In the presence of sulfhydryl compounds such as cysteine, dopaquinone reacts to produce several cysteinyldopa (CD) isomers, 5-S-cysteinyldopa (5SCD) and 2-S-cysteinyldopa (2SCD) in 74% and 14% yields, respectively (Ito & Prota 1977, Thompson et al. 1985). 2,5-S,S'-dicysteinyldopa (DiCD) is produced in a 5% yield. Further oxidation of the thiol adducts leads to the formation of pheomelanin via benzothiazine intermediates.
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