Mechanisms of Photooxidative Stress in Retinal Pigment epithelium: Is Melanin a Photosensitizer?.

The cellular pigments of the rectinal 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, including proteins, fatty acids, as well as antioxidants, and other small molecules. Similar reactivity of melanin granules has been demonstrated in intact RPE cells by the use of fluorescent oxidation-sensitive vital probes. Experiments have been conducted in cultures of bovine and baboon RPE cells exposed to quantum-equivalent, 488, 514.5 or 647.1 nm emissions from Argon and Krypton ion CW lasers. Based on fluorescence microscopy and biochemical analysis, the blue-green wavelengths, on a quantal basis, most efficiently induced photooxidative stress in the pigmented cells. The fluorescence signal after laser exposure, corresponding to areas of greatest oxidative stress, was restricted to the cells cytoplasm. These findings indicate that the melanosomes of pigment cells are involved in intracellular photooxidative reactions, and indeed, may be considered as photosensitzers possibly contributing to light-induced, oxidative damage to ocular tissue.