Oxidation-Induced Increase In Photoreactivity of Bovine Retinal Lipid Extract

Abstract

The mammalian retina contains a high level of polyunsaturated fatty acids, including docosahexaenoic acid (22:6) (DHA), which are highly susceptible to oxidation. It has been shown that one of the products of DHA oxidation—carboxyethylpyrrole (CEP), generated in situ, causes modifications of retinal proteins and induces inflammation response in the outer retina. These contributing factors may play a role in the development of age-related macular degeneration (AMD). It is also possible that some of the lipid oxidation products are photoreactive, and upon irradiation with blue light may generate reactive oxygen species. Therefore, in this work we analysed oxidation-induced changes in photoreactivity of lipids extracted from bovine neural retinas. Lipid composition of bovine neural retinas closely resembles that of human retinas making the bovine tissue a convenient model for studying the photoreactivity and potential phototoxicity of oxidized human retinal lipids. Lipid composition of bovine neural retinas Folch’ extracts (BRex) was determined by gas chromatography (GC) and liquid chromatography coupled to an electrospray ionization source-mass spectrometer (LC-ESI-MS) analysis. Liposomes prepared from BRex, equilibrated with air, were oxidized in the dark at 37 °C for up to 400 h. The photoreactivity of BRex at different stages of oxidation was studied by EPR-oximetry and EPR-spin trapping. Photogeneration of singlet oxygen (1O2, 1Δg) by BRex was measured using time-resolved detection of the characteristic phosphorescence at 1270 nm. To establish contribution of lipid components to the analysed photoreactivity of Folch’ extract of bovine retinas, a mixture of selected synthetic lipids in percent by weight (w/w %) ratio resembling that of the BRex has been also studied. Folch’s extraction of bovine neural retinas was very susceptible to oxidation despite the presence of powerful endogenous antioxidants such as α-tocopherol and zeaxanthin. Non-oxidized and oxidized BRex photogenerated singlet oxygen with moderate quantum yield. Blue-light induced generation of superoxide anion by Folch’ extract of bovine neural retinas strongly depended on the oxidation time. The observed photoreactivity of the studied extract gradually increased during its in vitro oxidation.

Oxidized Lipids Decrease Phagocytic Activity of ARPE‐19 Cells In Vitro

“…We recently reported that oxidation of retinal lipids increased their photoreactivity, and endogenous antioxidants significantly reduced their ability to generate reactive oxygen species . Here, we aimed at determining if oxidized lipids could affect normal functions of RPE cells exposed in vitro to visible light.…”

“…The Folch’ extraction is regarded as one of the most reliable methods for isolation of a broad range of lipid classes from biological materials . The predominant component of the Folch’ extract of bovine retinas are lipids and other hydrophobic components such as retinoids, xanthophylls, or vitamin E . Previously we have shown that in vitro oxidation of BRex increases its photoreactivity.…”

“…Lipids stand for 25–30% of a dry weight of the mammalian retina and phospholipids (PL) are the main component of retinal lipids . PL accounts for 87% and 78% of total lipids in Folch’ extracts of human and bovine neural retinas, respectively . Most of phospholipids present in the neural retina contain unsaturated fatty acids esterified primarily in the SN‐2 position .…”

“…High reactivity of lipid oxidation products is mainly discussed in terms of their ability to cause irreversible modifications of cellular components. Although visible light induced photoreactivity of bovine neural retina was recently reported, its possible phototoxicity has not been discussed …”

“…Recently we have shown that Folch’ extract of bovine neural retinas (BRex) is photoreactive; that is, both, freshly prepared and oxidized BRex generated singlet oxygen and superoxide anion upon irradiation with blue light . Although the observed photoreactivity was rather moderate, it increased upon oxidation of the lipids.…”