Abstract
With aging, retinal pigment epithelium melanosomes, by fusion with the age pigment lipofuscin, form complex granules called melanolipofuscin. Lipofuscin granules may contain oxidized proteins and lipid hydroperoxides, which in melanolipofuscin could chemically modify melanin polymer, while transition metal ions present in melanin can accelerate such oxidative modifications. The aim of this research was to examine the effect of selected transition metal ions on melanin susceptibility to chemical modification induced by the water-soluble tert-butyl hydroperoxide used as an oxidizing agent. Synthetic melanin obtained by DOPA autooxidation and melanosomes isolated from bovine retinal pigment epithelium were analyzed. To monitor tert-butyl hydroperoxide-induced oxidative changes of DMa and BMs, electron paramagnetic resonance spectroscopy, UV-vis absorption spectroscopy, dynamic light scattering, atomic force microscopy and electron paramagnetic resonance oximetry were employed. These measurements revealed that both copper and iron ions accelerated chemical degradation induced by tert-butyl hydroperoxide, while zinc ions had no effect. Strong prooxidant action was detected only in the case of melanosomes and melanin degraded in the presence of iron. It can be postulated that similar chemical processes, if they occur in situ in melanolipofuscin granules of the human retinal pigment epithelium, would modify antioxidant properties of melanin and its reactivity.
Highlights
Melanin is a photoprotective pigment present in plants and animals
In the retinal pigment epithelium (RPE), melanin shows very little, if any, metabolic turnover [4], suggesting that with ageing the RPE melanin may undergo chemical modifications induced by oxygen, light, and transition metal ions accumulated during life
In another studies we showed that melanosomes isolated from bovine RPE (BMs) irradiated with visible light lost their antioxidant properties and even became prooxidant [8, 9]
Summary
Melanin is a photoprotective pigment present in plants and animals. Skin melanin efficiently screens solar radiation, especially its UV component [1]. In the retinal pigment epithelium (RPE), melanin shows very little, if any, metabolic turnover [4], suggesting that with ageing the RPE melanin may undergo chemical modifications induced by oxygen, light, and transition metal ions accumulated during life. Our previous studies showed that RPE melanosomes isolated from 60–90 years old human donors were phototoxic [7]. In another studies we showed that melanosomes isolated from bovine RPE (BMs) irradiated with visible light lost their antioxidant properties and even became prooxidant [8, 9]. RPE as metabolically active post-mitotic cells accumulate the age pigment lipofuscin. Melanosomes in the RPE, by fusion with the age pigment lipofuscin, form complex granules called melanolipofuscin [13]. It is tempting to speculate that in such a complex
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