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 pos sess 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 mela nin, a representative eumelanin, and both red hail pheomelanin and syntheti c pheomelanin were employed in these studies. Both eumelanin and pheomelani n inhibited UVA/B- and UVA-induced liposomal lipid peroxidation in a concen tration-dependent manner as measured by inhibition of conjugated diene form ation. No change in protective properties of the melanins was observed in t he presence of saturating levels of O-2 during UVA irradiation, Pheomelanin irradiated with UVA/B or UVA induced superoxide-catalyzed reduction of nit roblue tetrazolium, whereas eumelanin did not. Melanins are known to bind v arious metals, and we examined the effect of iron on the photoproperties of melanins. Eumelanin complexed with Fe(III) did not inhibit UVA/B-induced l ipid peroxidation, whereas pheomelanin complexed with Fe(III) stimulated UV A/B-induced lipid peroxidation. Thus, complexation with iron reversed the a ntioxidant effect of eumelanin anti 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 indicat e that both eumelanin and pheomelanin exert antioxidant effects against UV- induced lipid peroxidation but that the prooxidant activities of pheomelani n result from pheomelanin-metal complexation.