Thermotropic properties and molecular dynamics of cholesteryl ester rich very low density lipoproteins: effect of hydrophobic core on polar surface

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Abstract
Cholesteryl ester rich very low density lipoproteins (CER-VLDL), isolated from the plasma of rabbits fed a hypercholesterolemic diet, were studied by differential scanning calorimetry (DSC), 13C-NMR, and spin-label 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 2 DSC heating endotherms; these occur at 40.degree.-42.degree. and 45.degree.-48.degree. C. Cholesteryl esters isolated from CER-VLDL also exhibit 2 DSC endotherms; these occur at 50.0.degree. and 55.1.degree. C and correspond to the smectic .fwdarw. cholesteric and cholesteric .fwdarw. 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.degree. and 51.5.degree. C. CER-VLDL at 37.degree. 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 liquid-crystalline state. When the mixture was heated to 42.degree. C, broad ring carbon resonances became detectable; at 48.degree. C, they became narrow, indicating the cholesteryl esters were in an isotropic, liquid-like state. When increasing temperature over the range 38.degree.-60.degree. 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.degree. C higher. These results indicate that the 2 DSC transitions in CER-VLDL do not directly correlate with the smectic .fwdarw. cholesteric and cholesteric .fwdarw. 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, 2Tmax, over the temperature range 20.degree.-58.degree. 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, 2-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.