Interactions between metal ions and different forms of melanin play significant roles in melanin biochemistry. The binding properties of natural melanin and related synthetic materials can be exploited for nonbiological applications, potentially including water purification. A method for investigating metal ion-melanin interactions on solid support is described, with lead as the initial target. 2.5 cm discs of the hydrophobic polymer PVDF were coated with synthetic eumelanin from the tyrosinase-catalyzed polymerization of L-dopa, and with melanin extracted from human hair. Lead (Pb2+) binding was quantified by atomic absorption spectroscopy (flame mode), and the data was well fit by the Langmuir model. Langmuir affinities ranged from 3.4 · 103 to 2.2 · 104 M−1. At the maximum capacity observed, the synthetic eumelanin coating bound ~9% of its mass in lead. Binding of copper (Cu2+), zinc (Zn2+), and cadmium (Cd2+) to the synthetic-eumelanin-coated discs was also investigated. Under the conditions tested, the Langmuir affinities for Zn2+, Cd2+, and Cu2+ were 35%, 53%, and 77%, respectively, of the Langmuir affinity for Pb2+. The synthetic-eumelanin-coated discs have a slightly higher capacity for Cu2+ on a per mole basis than for Pb2+, and lower capacities for Cd2+ and Zn2+. The system described can be used to address biological questions and potentially be applied toward melanin-based water purification.

Melanoma tumors have an unusually high rate of metaluptake and a poor ability to mediate oxidative stress, both of which may be attributed to constitutive changes in their intracellular melanin. Melanin pigments are fundamentally composed of polymeric catechols, and as such have inherent redox equilibria between quinole, semi-quinone and quinone oxidation states. This redox reactivity is inherent in the ability of melanins to act as buffers against oxidative and photochemical stress in vivo. We have investigated the speciation, metal-binding affinity and redox reactivity of these catecholic subunits in synthetic melanins formed from dihydroxyindole. The binding of various metal ions can generate a pro-oxidant response, making the melanin more easily oxidized, and susceptibility to reaction with oxygen to form superoxide and other cytotoxic reactive oxygen species. Similar pro-oxidant behavior has been seen in melanoma cell lines, and we hypothesized that this activity is due to the melanin within the melanoma cells. In line with this idea, several families of lipophilic metal compounds with antimelanoma activity have been identified. Our initial screening of the pro-oxidant response of synthetic melanin to various metal ions has identified new potential candidates for metalbased chemotherapy. IL-11ANTIOXIDANT AND PHOTOPROTECTIVE PROPER-TIES OF HUMAN IRIDIAL MELANIN. T. Objectives: To analyze the effects of human iridial melanin on formation and decay of reactive oxygen species, and to determine whether antioxidant properties of the iridial melanin change with age of donors and color of their irises. Methods: Content and type of melanin in human and bovine iris homogenates was determined by electron paramagnetic resonance (EPR) spectroscopy and HPLC detection of characteristic melanin degradation products. Human eyes were obtained from Wisconsin Lion Eye Bank. Iridial samples were pooled into four age groups (8-34; 37-54; 57-75 and 76-98 yr) and four color groups (blue, medium light, medium light and brown). Antioxidant and photoprotective properties of radial melanin were analyzed by EPR-asymmetry, EPR-spin trapping, oxidize electrode and iodometric assay of lipid hydroperoxides. Reactive oxygen species, in the absence and presence of iridial homogenates, were generated by UV radiation and photosensitized oxidation reactions. Results: On average, human brown irises contain about 50% more melanin than human blue irises. Human iridial melanin, regardless the iris color and age of donors, is predominantly eumelanin. Age of donors seems to have little effect on either the content or type of the iridial melanin. Irradiation of iridial homogenates with light, particularly with UV radiation, induced the formation of superoxide anion and hydrogen peroxide. Such an aerobic photoreactivity of the iridial melanin was independent of age of donors and color of their irises; however, it could be enhanced by exogenous ascorbate. Iridial melanin decomposed, in a dose-dependent manner, hydrogen peroxide and inhibited peroxidation...

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