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.
Highlights
Retina, being a part of a central nervous system, shares its unique lipid composition [1,2,3]
Representative chromatograms at 295 nm of non-oxidized bovine neural retinas Folch’ extracts (BRex) and α-TOH used as a standard, are shown in Fig. 1 Retention time of α-TOH was about 16 min, consistent with literature data [35]
The level of αtocopherol (1.5–3 μM) in BRex is comparable to that reported by Dilley and McConnell [36] and reaches 0.1% mol relative to phospholipids [36, 37]
Summary
Retina, being a part of a central nervous system, shares its unique lipid composition [1,2,3]. Up to 40% of phospholipids present in the retina contain polyunsaturated fatty acids (PUFAs) esterified mainly in the SN-2 position, and, to much lesser extent, in the SN-1 position [4,5,6,7]. The most abundant PUFAs in the outer retina, especially in photoreceptor outer segments (POS), are docosahexaenoic acid (DHA (22:6)) and arachidonic acid (ARA (20:4)), which account for 21 and 10% of total fatty acids residues, Cell Biochem Biophys (2017) 75:443–454 respectively [1, 4, 8]. The outer retina, being exposed to intense irradiation from focused light, high oxygen concentration [11] and the presence of endogenous sensitizers, such as rhodopsin photobleached products [12, 13] or age pigment—lipofuscin [14, 15], is at elevated risk of oxidative stress
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