Phospholipid-Cholesterol Bilayers, Cholesterol Bilayer Domains, and Cholesterol Crystals were Detected in Lipid Dispersion Prepared from Lipids Extracted from Lens Nucleus of Old Human Donors

Abstract

Human lens lipid membranes prepared using a rapid solvent exchange method from the total lipids extracted from the clear lens cortex and nucleus of 61- to 70-year-old donors were investigated. The measured cholesterol-to-phospholipid (Chol/PL) molar ratio in these preparations was extremely high, showing values of 1.8 and 4.4 for cortex and nucleus, respectively. We expect that at this elevated Chol content, the entire membrane will become saturated with Chol (as in the case of the cortical membrane) and the excess of Chol will form Chol crystals, presumably outside the membrane (as in the case of the nuclear membrane). Properties and organization of the lipid bilayer were investigated using electron paramagnetic resonance spin-labeling methods. Formation of Chol crystals was confirmed using the differential scanning calorimetry. We showed that in the lipid dispersion prepared from nuclear lipids Chol exists in three distinguished environments: (1) Chol dispersed in PL bilayer, (2) Chol in non-crystalline membrane domains (cholesterol bilayer domains, CBDs), and (3) Chol in crystals. In cortical membranes, because of the lower Chol content, Chol crystals were not detected. Amounts of Chol in CBDs were almost the same in cortical and nuclear membranes which indicates that Chol content in both membranes is close or exceeds the Chol solubility thresholds in these membranes. Profiles of cortical and nuclear membrane properties (alkyl-chain order, fluidity, oxygen transport parameter, and hydrophobicity) were very similar to each other and to those reported for cortical and nuclear lens lipid membranes of 41- to 60-year-old donors reported earlier. This confirms our earlier statement that saturation with Chol determines properties of the PL bilayer with the minor effect of the PL composition.