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The cellular pigments of the retinal pigment epithelium (RPE) have been shown to catalyze free radical activity, especially when illuminated with visible or ultraviolet light. This activity is sufficient to cause photooxidation of several major cellular components. The present investigation determined the relative ability of melanin, lipofuscin, and melanolipofuscin granules isolated from human and bovine eyes to oxidize polyunsaturated fatty acids, specifically linoleic and docosahexaenoic acids. The dark reactivity as well as the light-stimulated reactions were determined. The production of hydroperoxide derivatives of the linoleic and docosahexaenoic acids were determined by NADPH oxidation coupled to the activity of glutathione peroxidase, and also by production of thiobarbituric acid reactive substances. All RPE pigment granules stimulated fatty acid oxidation when irradiated with short wavelength (< 550 nm) visible light, with the melanosomes exhibiting the greatest light-induced activity. Only lipofuscin granules, however, caused peroxidation of fatty acids in the dark. These findings provide additional support for the role of RPE pigments in "blue light toxicity" as well as indicating that accumulation of lipofuscin may contribute to increased photooxidation in the aging RPE.

The kinetic properties of ascorbic acid oxidation by light-activated melanin granules demonstrate the presence of a specific reactive site on the melanin granule saturable by ascorbic acid. Increased light intensity increased the Vmax and reduced the Km of this reaction, indicating increased affinity of the active site for ascorbic acid. The kinetics of this reaction are not markedly changed in a reduced-oxygen environment. Ascorbic acid oxidation is competitively inhibited by isoascorbic acid, an epimer of ascorbic acid, while other tested reducing agents are inactive. The Ki for isoascorbic acid is 1 mM, about the same as the Km of ascorbic acid.