Cysteinyldopa

Cysteinyldopa refers to cysteine-conjugated forms of dopaquinone that arise during the biosynthesis of pheomelanin, the red-yellow pigment of hair and skin. The best-known species are 5-S-cysteinyldopa and 2-S-cysteinyldopa. They form when the enzyme tyrosinase oxidizes tyrosine to dopaquinone, which can then react with cysteine to yield cysteinyldopa adducts. These adducts are intermediates in the pheomelanin pathway and can further convert into benzothiazine intermediates that polymerize into pheomelanin.

In humans, eumelanin and pheomelanin production is influenced by genetic factors, UV exposure, and availability of substrates such as cysteine. Higher cysteine levels tend to promote pheomelanin formation, contributing to lighter or red-toned pigmentation in some individuals. Cysteinyldopa is detected in melanized tissues such as skin and hair and serves as a biochemical marker of pheomelanin synthesis. Its levels can vary with hair color and exposure to sunlight, reflecting the balance between eumelanin and pheomelanin in a given sample.

Analytically, cysteinyldopa and related adducts are measured in research settings using high-performance liquid chromatography (HPLC) with electrochemical or UV detection, or by mass spectrometry-based methods. Such measurements provide an indirect estimate of pheomelanin content in hair, skin, or excreta and are used to study pigment biology, pigmentation disorders, and the effects of UV exposure on melanin composition.

Cysteinyldopa thus represents a key intermediate and biomarker in the study of pheomelanin formation within the broader melanin biosynthetic pathway.