Pure Cholesterol Bilayer Domains are Formed at Cholesterol Contents Significantly Lower than Cholesterol Solubility Thresholds in Phospholipid Membranes: EPR and DSC Studies

Recently, using spin-labeled cholesterol analogs and saturation-recovery EPR approaches along with DSC measurements, we were able to show that formation of cholesterol bilayer domains (CBDs) precedes formation of cholesterol crystals in dimyristoylphosphatidylcholine membranes [1]. Here, we demonstrated that this is also true for other phospholipid membranes. Because our research interests focused on fiber-cell plasma membranes that build human eye lenses, we investigated membranes made of phosphatidylserine (PS), phosphatidylethanolamine (PE), and sphingomyelin (SM). These phospholipids, together with phosphatidylcholine (PC), are major components of fiber-cell plasma membranes. Our recent and previous results showed that CBDs begin to form at ∼33, ∼46, ∼48, and ∼50 mol% cholesterol for PE, SM, PS, and PC membranes, respectively. It was confirmed by DSC that the cholesterol solubility threshold is detected at ∼50, ∼66, ∼70, and ∼66 mol% cholesterol for PE, SM, PS, and PC membranes, respectively. To preserve compositional homogeneity throughout the membrane suspension, lipid multilamellar dispersions were prepared using a rapid solvent exchange method. Phospholipids contained both saturated acyl chains, or one acyl chain was mono-unsaturated. These findings help us better understand the lateral organization of membranes overloaded with cholesterol (such as the fiber-cell plasma membranes of the eye lens) and form new conclusions about the function of cholesterol in these membranes. Acknowledgements: Supported by NIH grants R01EY015526, R01EB002052, and P41EB001980. Literature: [1] L. Mainali, M. Raguz, W.K. Subczynski, J. Phys. Chem. B. (2013), 117:8994-9003.