Zeaxanthin but not Lutein or Beta Carotene Destabilizes with the Presence of Higher Level of Cholesterol in the Liposomal Model Membrane: Absorption and Fluorescence Studies

To facilitate better knowledge of the physiological behavior of some dietary carotenoids in native membranes in the presence of cholesterol, the effects of incorporated 1.5 mol % beta-carotene (BC), lutein (LUT), zeaxanthin (ZEA), and colorant in diet canthaxanthin (CTX) on Egg Yolk phosphatidylcholine (EYPC) model liposomes which contain cholesterol at different concentrations (50 and 75 mol %) were investigated. We applied UV-Vis spectroscopy, laurdan fluorescence quenching efficiencies, and membrane rigidity as estimated by a generalized polarization parameter derived from laurdan emission spectra. The incorporation yield percentage (IY%) of polar carotenoids LUT and ZEA into lipid membranes was reduced when cholesterol was present at higher levels in the phospholipid bilayer. The decline in incorporation was the strongest for ZEA. The non-polar carotenoid BC was incorporated with the lowest IY% in the presence of different concentrations of cholesterol. LUT and CTX revealed higher quenching efficiencies, showing that they interact stronger with Laurdan in the membrane than ZEA did in the presence of lower and higher amounts of cholesterol. A rigidifying effect was observed for ZEA in membranes prepared with lower levels of cholesterol which was not observed in membranes prepared with higher levels of cholesterol. This weaker adaptation of ZEA with cholesterol could be explained by the high competition between them to reside in the membranes, whereas LUT is more adaptable within the liposomes. The above data indicate that ZEA is excluded from the cholesterol-rich membrane domains, which is consistent with its poor solubility in membranes with a high cholesterol content.