Photoprotective actions of natural and synthetic melanins.
Melanins are thought to be important modulators of photochemistry in skin. Eumelanin, a black-brown pigment, is believed to protect against UV-induced photodamage, whereas pheomelanin, a red-yellow pigment, is believed to possess photosensitizing properties. To investigate the hypothesized dichotomy of melanins as both photoprotectants and photosensitizers, we examined the effects of melanins on UV-induced liposomal lipid peroxidation. Sepia melanin, a representative eumelanin, and both red hair pheomelanin and synthetic pheomelanin were employed in these studies. Both eumelanin and pheomelanin inhibited UVA/B- and UVA-induced liposomal lipid peroxidation in a concentration-dependent manner as measured by inhibition of conjugated diene formation. No change in protective properties of the melanins was observed in the presence of saturating levels of O2 during UVA irradiation. Pheomelanin irradiated with UVA/B or UVA induced superoxide-catalyzed reduction of nitroblue tetrazolium, whereas eumelanin did not. Melanins are known to bind various metals, and we examined the effect of iron on the photoproperties of melanins. Eumelanin complexed with Fe(III) did not inhibit UVA/B-induced lipid peroxidation, whereas pheomelanin complexed with Fe(III) stimulated UVA/B-induced lipid peroxidation. Thus, complexation with iron reversed the antioxidant effect of eumelanin and converted pheomelanin into a prooxidant. Analysis of lipid peroxidation products indicated that the oxidation was mediated by free radicals rather than by singlet oxygen. These data indicate that both eumelanin and pheomelanin exert antioxidant effects against UV-induced lipid peroxidation but that the prooxidant activities of pheomelanin result from pheomelanin-metal complexation.