Neutral p H and copper ions promote eumelanogenesis after the dopachrome stage

The diversity of pigmentation in the skin, hair, and eyes of humans has been largely attributed to the diversity of p H in melanosomes with acidic p H being proposed to suppress melanin production. Tyrosinase has an optimum p H of 7.4 and its activity is suppressed greatly at lower p H values. The first step of eumelanogenesis is the oxidation of tyrosine to dopachrome ( DC ) via dopaquinone. However, how eumelanogenesis is controlled by p H beyond this stage is not known. In this study, we examined the effects of p H (5.3–7.3) on the conversion of DC to 5,6‐dihydroxyindole ( DHI ) and 5,6‐dihydroxyindole‐2‐carboxylic acid ( DHICA ) and the subsequent oxidation of DHI and DHICA to form eumelanin. The effects of Cu 2+ ions on those reactions were also compared. The results indicate that an acidic p H greatly suppresses the late stages of eumelanogenesis and that Cu 2+ ions accelerate the conversion of DC to DHICA and its subsequent oxidation.