Zinc-induced Structural Effects Enhance Oxygen Consumption and Superoxide Generation in Synthetic Pheomelanins on UVA/Visible Light Irradiation†

The abnormal susceptibility of red-haired individuals to UV-induced inflammation and skin cancers is commonly attributed to the marked photoreactivity of pheomelanin pigments, which would be responsible for the sustained generation of reactive oxygen species in the skin following sun exposure. The structural factors determining pheomelanin photolability remain mostly unknown. Here, we describe the effects of zinc ions, typically found at high levels in red hair, in enhancing both oxygen consumption and superoxide production in model pheomelanin pigments following irradiation with UVA and visible light. Electron paramagnetic resonance (EPR) oximetry and EPR-spin trapping experiments with synthetic pheomelanins, prepared by oxidation of dopa and cysteine or isomeric cysteinyldopas under different conditions, indicate a higher photoreactivity of the pigments prepared in the presence of zinc ions compared with those obtained in the absence of the metal. Quantitative analysis of thiazole-containing structural markers of the synthetic pheomelanins provides evidence that the effect of zinc ions is due to modification of the formation pathway and structural features of the pigments. Overall, these results point to a hitherto unrecognized critical role of zinc ions in pheomelanogenesis, which may affect the intrinsic photoreactivity of the pigment.