Physical effects of biologically formed cholesterol oxidation products on lipid membranes investigated with fluorescence depolarization spectroscopy and electron spin resonance.

Planar oriented membranes of 1-palmitoyl, 2-oleoyl-phosphatidylcholine (POPC) containing cholesterol, 19-hydroxycholesterol, 22S-hydroxycholesterol, or 25-hydroxycholesterol in concentrations up to 5 mol % were investigated with angle-resolved fluorescence depolarization and electron spin resonance measurements. Analyses of the data with the Brownian diffusion model show that the oxysterols have structural effects similar to those of cholesterol: an increase in molecular order and no change in the rotational diffusion coefficients of the probe molecules. Time-resolved fluorescence anisotropy measurements on diphenylhexatriene (DPH) in small unilamellar vesicles of POPC and DOPC were performed using oxysterols commonly found in oxidized low density lipoproteins (LDL) in comparison to membranes containing cholesterol or no sterols. Analyses using the Brownian rotational diffusion model show that most LDL-oxysterols affect the vesicle physical structure in a manner similar to cholesterol, viz. an increase in molecular order and a decrease in the dynamics. Cholesterol-alpha-epoxide has a much smaller ordering effect than cholesterol in POPC-vesicles. A similar effect was found for 7 beta-hydroxycholesterol in DOPC-vesicles. The tendency of the oxysterols to influence the molecular order as compared to pure cholesterol may contribute to cell membrane permeability changes affecting crucial cell functions and events leading to vascular cell injury. Increased LDL oxysterol levels may account for some of the structural changes noted for oxidatively modified LDL as well as its toxicity to vascular cells.