Solar light exposure induces pigmentary responses on the skin. UVA elicits immediate pigment darkening and persistent pigment darkening. These processes are thought to result from oxidation and/or polymerization of existing melanin and/or melanogenic precursors.
Melanocytes produce two types of pigment, eumelanin and pheomelanin. Eumelanin consists of 5,6-dihydroxyindole (DHI) and 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA), while pheomelanin consists of benzothiazine and benzothiazole units. Melanins can be analyzed through specific degradation products by HPLC. Alkaline hydrogen peroxide oxidation (AHPO) of eumelanin gives pyrrole-2,3,5-tricarboxylic acid (PTCA) and pyrrole-2,3-dicarboxylic acid (PDCA) as specific degradation products of DHICA and DHI moieties. Benzothiazole pheomelanin can be analyzed as thiazole-2,4,5-tricarboxylic acid (TTCA). Benzothiazine pheomelanin can be analyzed by reductive hydrolysis, as 4-amino-3-hydroxyphenylalanine (4-AHP) and 3‐amino‐4‐hydroxyphenylalanine (3‐AHP).
Both eumelanin and pheomelanin undergo modifications of their structures upon UVA exposure. Eumelanin exposed to UVA undergoes oxidative cleavage of the indolequinone moiety to free pyrrole‐2,3,5‐tricarboxylic acid (free PTCA) and cross-linking to form pyrrole-2,3,4,5-tetracarboxylic acid (PTeCA) upon AHPO. UVA exposure of pheomelanin induces oxidative conversion of the benzothiazine moiety to the benzothiazole moiety, as indicated by an increase in the TTCA/4‐AHP ratio. Nevertheless, these structural modifications have never been characterized in human skin.
In this study we exposed ex vivo skin to increasing UVA1 doses (60, 90 and 120 J/cm²) and characterized the induced pigment darkening before, immediately and two hours after exposure through colorimetry and HPLC. The results showed changes in the CIELAB colorimetric parameters, namely decrease in Luminance L*, yellow-blue component b* and Individual Typology Angle in UVA1-exposed samples, indicative of skin darkening. In parallel UVA1 exposure induced modifications of the levels of PTCA, TTCA, 4-AHP, and ratios of various markers, such as PTeCA/PTCA, free/total PTCA, and TTCA/4‐AHP, indicative of photooxidation/degradation of melanins. Our study shows first-time evidence of UVA-induced modifications of melanins associated with pigment darkening in human skin.