Comparable photoreactivity of hair melanosomes, eu- and pheomelanins at low concentrations: low melanin a risk factor for UVA damage and melanoma?

Melanin is known to be photoreactive and photoprotective, but its function in skin in vivo is still debated. Data is lacking of the effects of UVA irradiation on human skin melanosomes of different pigmentation, which have not been extensively degraded by isolation procedures. We have shown previously that melanosomes isolated from human oriental and black cat hair, and synthetic eumelanins, are photoreactive producing superoxide at low concentrations when exposed to UVA irradiation comparable to UK levels of sunlight. Here we investigated the UVA-irradiation of melanosomes, isolated from different colored human hair samples, using electron spin resonance spectroscopy and spin trapping. Comparable irradiation of synthetic pheomelanins synthesized from L-dopa and L-cysteine was also studied. An alkali method (5 min NaOH at 90 degrees C) could be used to isolate oriental hair melanosomes but was not suitable for auburn and blonde hair. Dithiothreitol and proteinase K resulted in melanin release from possible over-digestion of melanosomes; however, dithiothreitol and papain resulted in no melanin release and good melanosome yields with separation from residual keratin for brown, auburn and blonde hair. Melanosomes isolated by the latter method and synthetic pheomelanins were similar in UVA-photoreactivity at low concentrations, independent of hair color, and broadly comparable to synthetic melanins. Melanosome concentration at constant fluence may be more significant with respect to photodamage and UVA photocarcinogenesis (melanoma) via superoxide radical production than pigment type.