Relative photoreactivity of the pigment inclusions of the retinal pigment epithelium

The cellular pigments of the retinal pigment epithelium are photoactive; that is, they promote free radical oxidative reactions when illuminated with visible or ultraviolet light. This activity is sufficient to cause photo-oxidation of several major cellular components such as antioxidants, dinucleotide cofactors, proteins and fatty acids. The present investigation determined the relative ability of melanin, lipofuscin, and melanolipofuscin granules isolated from human and bovine eyes to oxidize linoleic and docosahexaenoic acids, which are polyunsaturated fatty acids. The dark reactivity as well as the light-stimulated reactions were determined. All RPE pigment granules stimulated fatty acid oxidation when irradiated with the blue-green (488.1 and 514.5 nm) emission of the Argon-ion laser. Only lipofuscin, however, caused peroxidation of fatty acids in the dark. These findings not only suggest that accumulation of lipofuscin in the aging eye may contribute to increased photooxidative stress to the retina and RPE, but also that the photoactive RPE pigments might serve as endogenous photosensitizers for therapeutic applications.