Cholesterol may act as an antioxidant in lens membranes

Oxidative damage has been considered as a major factor involved in cataract formation. We have recently shown that cholesterol oxides accumulate in human cataractous lenses. The biological significance of accumulation of oxysterols in the lens is still poorly understood. However, it has been proposed that cholesterol may act as an antioxidant. This study was designed to establish whether cholesterol may act as an antioxidant in the lens.

Bovine lens membranes (BLM) were oxidised by incubation with an azo-compound. Lipid hydroperoxides were measured by the FOX-assay, vitamin E was determined by HPLC, cholesterol and cholesterol oxides were isolated in a C18 column and quantified by gas chromatography. Susceptibility of liposomes and BLM to oxidation was determined by the fluorescence quenching of parinaric acid.

Oxidation of BLM results in the production of lipid hydroperoxides, consumption of endogenous vitamin E and formation of cholesterol oxides. Cholesterol presents some important characteristics generally ascribed to an antioxidant molecule: its presence in liposomes increases the vesicle resistance to oxidation and its oxidised forms are stable as they are unable to stimulate further propagation of peroxidation reactions. Moreover, the protective effect of cholesterol in liposomes is comparable to that of vitamin E, suggesting that cholesterol possibly acts by intercepting the peroxyl radicals formed during lipid peroxidation.

Although cholesterol oxides may present a variety of noxious effects in the cells its presence in lens membrane is likely to be associated with the expression of its antioxidant effect, contributing to maintain lens transparency.