B ie Vitamin B Glutathionethione peroxidse, catalase, ascorbic acid (vitamin C),>> a-tocopherol (vit E) and )3-carotene (precursor of Vit. A) protect lens against reactive oxygen species produced during photochemical reactions in the lensQ. So vitamins CE>>A has antioxidant action in lens and deficiency of these antioxidants causes opacification of lens fibers with fibrous metaplasia (risk factor for cataract) Reactive Oxygen Species (free radicals) - Include superoxide anion, hydroxyl free radical, hydroperoxyl radicals, singlet oxygen & hydrogen peroxide (H202) - Originate cell metabolism or photo chemical reaction and also can enter from aqueous. - Photochemical damage occurs when light is absorbed by a photosensitizer, aqueous humor. - Chromophore, which forms an excited triplet state producing free radicals. - Preferential absorption of shoer wave lengths (295-400nm) makes lens tissue paicularly susceptible to photochemical reactions. - Major ultraviolet (UV) absorber in lens are free or bound aromatic amino acid (eg. tryptophan), numerous pigments (eg. 3- hydroxykynurenine) and fluorophores. Antioxidant Mechanism - Superoxide dismutase or ascorbate conve superoxide anion into H202. - H202 is detoxified by catalase or glutathione peroxidase (glutathione system) - Catalse is present in epithelial cells but found at very low levels in fiber, whereas glutathione peroxidase is found in significant amounts in both, although the highest levels are found in epithelial cells. - Glutathione system provides impoant protection against H202 & lipid peroxide. - Ascorbic acid (vit C) plays a major role in antioxidants system in lens. It is present at high levels in outer layers of lens & viually absent in nucleus. It reacts with superoxide anion, peroxide radicals, hydroxyl radicals, thiol radicals, singlet oxygen & prevents lipid peroxidation. - Low levels of antioxidants a-tocopherol and 13-carotene also are present in aqueous humorQ. - Lutein & zeaxathine are naturally occurring carotenoids in crystalline lens (moleculear cousins of p carotene & vit A). Selenium, zinc, copper & very rarely riboflavin, p12 as well as amino acid cysteine may play a role (+- evidence).