Degree of Acyl Chain Unsaturation Modulates Cholesterol Solubility Threshold of Phospholipid Membranes

The deposition of minute cholesterol crystals (CHCs) in arteries is an early cause rather than a late consequence of inflammation, providing new insight into the pathogenesis of atherosclerosis. This research investigates the effect of phospholipid acyl chain degree of unsaturation in the formation of CHCs via the membranes. When the membranes are saturated with cholesterol, excess cholesterol is no longer incorporated within the membrane and forms CHCs outside the membrane. The cholesterol concentration at which these CHCs begin to form is the membranes\u27 cholesterol solubility threshold (CST). We used differential scanning calorimetry (DSC) to investigate the cholesterol/phosphatidylcholine (PC) membrane at varying cholesterol concentrations to estimate the CST of PC membranes of varying degrees of unsaturation (i.e., 16:0-20:4 PC, 18:2-18:2 PC, and 18:1-18:1 PC). The rapid solvent exchange method was used to prepare the multilamellar vesicles to ensure homogeneity throughout membrane suspension; these suspensions were then heated from 0 to 120 °C, and the CHCs peaks from DSC thermal scans were detected and used to determine the CST of the membranes. We estimate the CST for 16:0-20:4 PC, 18:2-18:2 PC, and 18:1-18:1 PC membranes to be ~ 43 mol%, ~ 46 mol%, and ~ 32 mol% respectively