Lipid peroxide and reactive oxygen species generating systems of the crystalline lens

Abstract

Lipid peroxidation (LPO) could be one of the mechanisms of cataractogenesis, initiated by enhanced production of oxygen free radicals in the eye fluids and tissues and impaired enzymatic and non-enzymatic defences of the lens. The increased concentrations of primary molecular LPO products (diene conjugates, lipid hydroperoxides) and end fluorescent LPO products were detected in the lipid moiety of the aqueous humor samples obtained from patients with cataract as compared to normal donors. Isolated human transparent and cataractous lenses and normal mouse and rabbit lenses were incubated with liposomes in organ culture in the presence and absence of LPO inhibitors, free radical scavengers and enzymes (catalase, superoxide dismutase (SOD)) in order to examine the potential of the lenses to induce LPO in the surrounding medium. LPO assayed spectrophotometrically were diene and triene conjugates, and malonaldehydes (MDA) were determined as thiobarbituric acid-reactive material. A chemiluminescence detection catalysed by peroxidase was used to measure H2O2 and O2-. was assayed spectrophotometrically using cytochrome C reduction. The level of lipid peroxides in liposomes was significantly (2.5-4.5-fold) higher after 3 h of incubation of the transparent lenses (or the lenses at the initial stage of cataract) than after the proper time of incubation of human mature cataractous lenses and virtually no oxidation of liposomes was detected in the absence of the lens. LPO in this system was decreased in the presence of free radical scavengers and enzymes that degrade H2O2 (EDTA, SOD, L-carnosine, chelated iron and catalase). The most effective agent was EDTA which chelates the free metal cations required to generate O2-. radicals that initiate the free radical process culminating in LPO. Lenses generated more H2O2 into the medium in the presence of exogenous ascorbate. Release of the oxidants, (O2-., H2O2, OH. and lipid hydroperoxides) by the intact lenses in the absence of respiratory inhibitors indicates that these metabolites are normal physiological products inversely related to the lens life-span potential (maturity of cataract) generated, probably, through the metal-ion catalysed redox-coupled pro-oxidant activation of the lens reductants (ascorbic acid, glutathione).