Age-related macular degeneration (AMD) is the leading cause of blindness in the over 60 year old American population. Available evidence suggests that AMD results from a deterioration in function of the retinal pigment epithelium (RPE). the critical role played by the RPE in photoreceptor debris digestion, the light-rich and oxygen-rich location of the RPE which provides an environment for oxidative damage, and the accumulation of fluorescent pigments in the RPE with aging probably stimulated by oxidative processes combine to suggest oxidative damage as a potential cause of AMD. Antioxidant systems protect the RPE from the cellular damage that can be induced by oxidative processes. Glutathione (GSH) is a naturally occurring tripeptide which functions as an antioxidant by reacting with free radicals and peroxides to limit nonenzymatic oxidative processes. Through this detoxification, GSH plays a major role in protection against oxidative injury from chemicals or other sources in many cell systems, including the liver, lung, and kidney. Preliminary studies have demonstrated that exogenous GSH can protect cultured human RPE cells from oxidative injury.
The aims of this proposal are to characterize the ability of the RPE to transport and synthesize GSH, to determine how these functions change with age, and to better understand the role of GSH in the RPE defense against oxidative injury. The ability of RPE cells to transport GSH will be measured by he uptake of radiolabelled GSH. GSH transport will be characterized further by the study of the sodium dependence, pH dependence, concentration dependence, and specificity of the transport system. RPE synthesis of GSH will be measured by using HPLC determination after depletion of intracellular GSH with diethyl maleate (DEM) followed by incubation of cells with radiolabelled amino acid precursors of GSH. The rates of GSH transport and synthesis can be measured and compared with donor age. Further insight into the role of GSH in macular degeneration will be provided by with measurement of cell viability by trypan blue exclusion. Cultured cells will then be supplemented by GSH or the amino acid precursors to determine which better protects the RPE from oxidative injury. By understanding glutathione's role in protecting the retinal pigment epithelium from oxidative injury, these studies will provide insights into the pathogenesis of AMD and, possibly, suggest therapeutic direction for this devastating disease.
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