title: Aggregation of eumelanin mitigates photogeneration of reactive oxygen species.
abstract: Melanins protect tissue by absorption and rapid nonradiative, nonreactive dissipation of ultraviolet (UV) light. However, melanins also produce reactive oxygen species (ROS) upon UV illumination. A chemical understanding of this dichotomy of photoprotection and phototoxicity has not been established. Herein this issue is examined by studying the UV-B induced oxidation and reduction of cytochrome c by ROS generated by different aggregation states of eumelanin. The quantum yield for superoxide anion by unaggregated oligomers is 7.4 x 10(-3), an order of magnitude greater than that characteristic of the bulk pigment. The quantum efficiency of hydrogen peroxide production by oligomers is 5.7 x 10(-3), and its production is attributed to reaction between superoxide anion and hydroquinone groups on eumelanin oligomers. Aggregation of oligomers results in a reduction of these quantum yields, having a significantly greater effect on the efficiency of hydrogen peroxide production. This effect is attributed to the decrease in surface concentration of hydroquinone sites upon aggregation. The effect of aggregation on the photogeneration of ROS serves to provide a foundation for the understanding of the dichotomy of photoprotective and phototoxic properties of melanin.
authors.affiliation: Department of Chemistry, Duke University Medical Center, Durham, NC 27708-0347, USA.
authors.forename: J Brian
authors.initials: JB
authors.lastname: Nofsinger
authors.forename: Yan
authors.initials: Y
authors.lastname: Liu
authors.forename: John D
authors.initials: JD
authors.lastname: Simon
mesh_terms: D000818:Animals; D002374:Catalase / Q000378:metabolism; D002417:Cattle; D003574:Cytochrome c Group / Q000378:metabolism*; D006736:Horses; D008353:Mannitol / Q000494:pharmacology; D008543:Melanins / Q000378:metabolism* / Q000528:radiation effects; D010084:Oxidation-Reduction; D017382:Reactive Oxygen Species / Q000378:metabolism* / Q000528:radiation effects; D013482:Superoxide Dismutase / Q000378:metabolism; D013481:Superoxides / Q000378:metabolism; D014466:Ultraviolet Rays
chemical_list: D003574:Cytochrome c Group; D008543:Melanins; D017382:Reactive Oxygen Species; D013481:Superoxides; C041877:eumelanin; D008353:Mannitol; D002374:Catalase; D013482:Superoxide Dismutase