Thermotropic Properties and Molecular Dynamics of Cholesteryl Ester Rich Very Low Density Lipoproteins: Effect of Hydrophobic Core on Polar Surface

Cholesteryl ester rich very low density lipoproteins (CER-VLDL), isolated from the plasma of rabbits fed a hypercholesterolemic diet, have been studied by differential scanning calorimetry (DSC), ¹³C nuclear magnetic resonance (NMR), and spin-label electron paramagnetic resonance (EPR) to determine the temperature-dependent dynamics of cholesteryl esters in the hydrophobic core and of phospholipids on the polar surface. Intact CER-VLDL exhibit two DSC heating endotherms; these occur at 40-42 and 45-48°C. Cholesteryl esters isolated from CER-VLDL also exhibit two DSC endotherms; these occur at 50.0 and 55.1°C and correspond to the smectic → cholesteric and cholesteric → isotropic liquid-crystalline phase transitions. A model mixture containing cholesteryl linoleate, oleate, and palmitate in a ratio (0.21, 0.51, and 0.28 mol fraction) similar to that in CER-VLDL exhibited comparable DSC endotherms at 45.2 and 51.5°C. CER-VLDL at 37°C gave ¹³C NMR spectra that contained no resonances assignable to cholesteryl ring carbons but detectable broad resonances for some fatty acyl chain carbons, suggesting the cholesteryl esters were in a liquidcrystalline state. When the mixture was heated to 42°C, broad ring carbon resonances became detectable; at 48°C, they became narrow, indicating the cholesteryl esters were in an isotropic, liquid-like state. With increasing temperature over the range 38-60°C, the resonances for cholesteryl ring carbons C3 and C6 in CER-VLDL narrowed differentially. Similar spectral changes were observed for the synthetic cholesteryl ester mixture, except they occurred at temperatures about 10°C higher. These results indicate that the two DSC transitions in CER-VLDL do not directly correlate with the smectic → cholesteric and cholesteric → isotropic transitions exhibited by pure cholesteryl esters. (5-Doxylpalmitoyl)-phosphatidylcholine (5-DP-PC) and (12-doxylstearoyl)phosphatidylcholine (12-DS-PC) were used to probe the polar surface monolayer of CER-VLDL; the corresponding cholesteryl esters (5-DP-CE and 12-DS-CE) were used to probe the hydrophobic core. None of these probes in CER-VLDL detected an abrupt change in EPR order parameters, S, or maximum splitting, 2T_max, over the temperature range 20-58 °C even though 12-DS-PC and 5-DP-PC can detect phase transitions in phospholipid bilayers and 12-DS-CE and 5-DP-CE can detect phase transitions in neat cholesteryl esters. However, 12-DS-CE and 5-DP-CE did detect a much greater acyl chain order for the neutral lipids of CER-VLDL than for those of normal triglyceride-rich VLDL. In addition, 12-DS-PC and 5-DP-PC did detect significantly greater acyl chain order for the phospholipids of CER-VLDL than for those of normal VLDL. The latter results suggest that the organization of lipids in the hydrophobic core of a lipoprotein can directly affect the dynamics of lipids in the polar surface.