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T. J. Sarna, A. Zadlo, G. Szewczyk, M. B. Rozanowska, J. M. Burke; The Effect of Photobleaching of Bovine RPE Melanosomes on Their Photoprotective and Antioxidant Properties. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5079.
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RPE melanosomes are believed to be involved in protection of the outer retina against oxidative stress. However, these pigment granules are exposed to significant fluxes of visible light and high oxygen concentration that may modify their antioxidant properties. Here we studied the effect of experimental photobleaching of bovine RPE melanosomes on their efficiency to inhibit peroxidation of lipids.
: Melanosomes isolated form bovine RPE were photobleached by irradiation with intense visible light. The degree of the melanosome photobleaching was determined by electron spin resonance (ESR) spectroscopy at 77K. Liposomes, prepared with lipids extracted from bovine eye retinas by the Foloch method, were subjected to peroxidation of lipids induced by iron/ascorbate. Progress of lipid peroxidation, was monitored by measuring oxygen consumption using ESR-oximetry, and by determining accumulation of lipid hydroperoxides using the iodometric assay.
Untreated melansomes inhibited peroxidation of lipids in a dose-dependent manner. The inhibitory effect of melanosomes was related to their ability to sequester iron ions. Melanosomes saturated with iron or other transition metal ions did not inhibt iron-ion-induced peroxidiation of lipids. Photobleached melanosomes showed a reduced antioxidant capacity, and even became prooxidant. This, in part, was due to a decreased ability of the photobleached melanosomes to bind iron ions.
Photobleached bovine RPE melanosomes exhibit significantly modified physicochemical properties that resemble those of aged human RPE melanosomes. It can be postulated that the enhanced photoreactivity and reduced antioxidant capacity of RPE melanosomes from aged human donors result from chronic photooxidation that causes partial photobleaching of the pigment granules.
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