Retinal Photooxidative Stress and Its Modifiers

Retinal damage is classified into three types based on the mechanisms of the damage, i.e., photothermal, photomechanical, and photochemical, with the last likely the most common form of light-induced damage, since the natural range of light energy sufficient causative. Photochemical damage is caused by reactive oxygen species and free radicals generated by light exposure, also called photooxidative stress. Although different types of photooxidative stress have been proposed, they sometimes cannot be differentiated clearly in various experimental settings. Epidemiologic studies have suggested a correlation between environmental light exposure and development/progression of human retinal degeneration such as age-related macular degeneration and retinitis pigmentosa. The double bonds in long-chain polyunsaturated fatty acids (PUFAs), which are highly enriched in the retinal membranes, could be target substrates to propagate photooxidative stress in the photoreceptors. Posttranslational modification of retinal proteins by PUFA-degraded molecules, such as 4-hydroxynonenal and 4-hydroxyhexenal, causes retinal degeneration and neuroprotection depending on the timing and level of formation.