The effect of carotenoids on the concentration of singlet oxygen in lipid membranes

An effect of beta-carotene and its polar derivative, zeaxanthin, on a concentration of singlet oxygen in lipid membranes was studied in a model system. The carotenoids were incorporated into the membranes of small unilamellar liposomes at a concentration of 0.15 mol% with respect to lipid. Singlet oxygen was generated in a liposome suspension via photosensitization of toluidine blue, and its concentration in a membrane was detected with application of a specific fluorescence probe (singlet oxygen sensor green reagent) located in the lipid bilayer. The results show the carotenoid-dependent decrease in the concentration of singlet oxygen in the membranes formed with unsaturated lipids (egg yolk phosphatidylcholine and digalactosyldiacylglycerol) but not in the case of the membranes formed with a saturated lipid (dimyristoylphosphatidylcholine). The effect of carotenoids was about twice as high as in the case of cholesterol present in liposomes at the same concentration. The results suggest that carotenoids protect membranes formed with unsaturated lipids against singlet oxygen through combined activity of different mechanisms: modification of structural properties of the lipid bilayers, physical quenching of singlet oxygen and chemical reactions leading to the pigment oxidation. The latter conclusion is based on the analysis of the absorption spectra of liposomes before and after light exposure. An importance of the different modes of protection by carotenoids against single oxygen toxicity towards bio-membranes is discussed.