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T.J. Sarna, A. Wielgus, C.M. Skumatz, J.M. Burke; Antioxidant Properties of Human Iridial Melanin . Invest. Ophthalmol. Vis. Sci. 2003;44(13):425.
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Purpose: To examine if human iridial melanin exhibits in vitro antioxidant properties and to determine whether such properties depend on age of the donors and color of their irises. Methods: Synthetic or natural melanin, homogenates of human irises from donors of different age or of bovine irises (used as a control), were added to samples in which oxidation of selected substrates, induced by photosensitized reaction or iron/ascorbate, was monitored by electron spin resonance (ESR) oximetry, iodometric assay for lipid hydroperoxides and thiobarbituric acid test for malondialdehyde. Two photosensitizing dyes were used -- the positively charged methylene blue and negatively charged rose bengal. The efficiency of melanin and iris homogenates to decompose hydrogen peroxide was measured by oxidase electrode. The amount and type of iridial melanin was determined by ESR spectroscopy at 77 K. Results: Human iridial melanin was determined to be predominantly eumelanin. Melanin content in human irises, normalized to dry mass of the iridial tissue or its protein content, was about 40% higher in brown eyes compared to blue eyes and showed little change with age of the donors. Iridial homogenates and, to a lesser extent, purified melanin or synthetic melanin, stimulated decomposition of hydrogen peroxide, and inhibited, in a dose-dependent manner, photosensitized oxidation of histidine and peroxidation of lipids induced chemically or photochemically in micelles made of linolenic acid methyl ester. The inhibitory effect of iridial homogenates decreased with age of the donors and was independent on the iris color. Conclusions: Antioxidant action of iridial melanin can be explained by its ability to sequester redox-active metal ions and certain photosensitizing dye molecules. Binding by melanin of iron or methylene blue results in a dramatic decrease in their efficiency to generate reactive oxygen species by Fenton reaction and photosensitized oxidation reaction, respectively. The observed reduction in antioxidant efficiency of human iridial melanin with age may be due to an age-dependent modifications of the iridial melanin, in which chronic photooxidation of the pigment may play a role.
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