Reassembled plasma low density lipoproteins. Phospholipid-cholesterol ester-apoprotein B complexes

Reassembled low density lipoprotein (LDL) complexes have been prepared by the interaction of lipid-free sodium deoxycholate-solubilized apoprotein B (apoB) of native human LDL with preformed, 200 A in diameter, microemulsions of cholesteryl oleate (CO), surface-stabilized by either egg yolk phosphatidylcholine ( EYPC ) or dimyristoyl phosphatidylcholine (DMPC). Gel chromatography of PC/CO/apoB complexes shows co-elution of the complex at 43% PC, 43% CO, and 14% apoB. Negative stain electron microscopy shows the particles to be circular, homogeneous, and approximately 200 A in diameter. PC/CO/apoB complexes exhibit beta-migration on agarose gels and show one high molecular weight protein band on 3.0% sodium dodecyl sulfate-polyacrylamide gels. Differential scanning calorimetry and x-ray scattering show the lipids in the complexes to undergo at least two specific thermal transitions depending on lipid composition, one associated with the core-located cholesterol esters similar to LDL and the protein-free microemulsions and the other from the phospholipid forming the surface monolayer. In addition, particle disruption-protein unfolding/denaturation occur irreversibly at 80-85 degrees C. At 4 degrees C, the secondary structure of apoB on complexes of EYPC /CO/apoB is similar to that of native LDL. For complexes of DMPC/CO/apoB, the secondary structure shows less alpha-helix which correlates with the difference in surface lipid environment. The reassembled complexes of PC/CO/apoB provide a defined system in which the components may be varied systematically in order to study the molecular organization, molecular interactions, and metabolism of LDL.

Physical properties of cholesteryl esters

Cholesteryl esters, the intracellular storage form and intravascular transport form of cholesterol, can exist in crystal, liquid crystal and liquid states. The physical state of cholesteryl esters at physiologic temperatures may be a determinant of their pathogenicity. This review has surveyed saturated aliphatic cholesteryl esters of chain length 1 to 24 carbons and a series of medium-chained unsaturated cholesteryl esters from chain lengths 14 to 24 carbons. A systematic study of transition temperatures by polarizing microscopy and enthalpies by differential scanning calorimetry has provided unifying concepts concerning the phase behavior as a function of chain length and unsaturation. Neat cholesteryl esters show chain-length dependence of transition temperature and enthalpy of both the crystal and liquid crystal transitions. Double bond position along the fatty acyl chain affected stability of the liquid crystal phases; a smectic phase was not observed for any cholesteryl ester with a double bond more proximal than delta 9. 13C NMR spectroscopy in the isotropic liquid phase has provided evidence suggesting a balance of ring-ring vs. chain-chain interactions as a determinant for isotropic liquid----cholesteric vs. isotropic liquid----smectic transitions. Specifically, anisotropic molecular motions of the steroid ring are greater for cholesteryl esters forming a cholesteric phase than a smectic phase from the melt. Chain-chain interactions apparently predominate in smectic phase formation. The X-ray diffraction patterns of cholesteryl esters as a function of chain length reveal several isostructural series and known single crystal data are presented. A chain length depending on the periodicity of the smectic phase is observed which may be different for saturated vs. unsaturated esters. In summary, the phase behavior of cholesteryl ester molecules is complex and cannot be determined a priori from the phase behavior of component cholesterol and fatty acid. The data presented here should provide insight into the biological behavior of this lipid class.

Temperature-dependent molecular motions of cholesterol esters: a carbon-13 nuclear magnetic resonance study

Carbon-13 NMR spectroscopy at 50.3 MHz has been used to study four long-chain cholesterol esters with a double bond in the omega-9 position: cholesteryl oleate, C18:1, omega-9; cholesteryl linoleate, C18:2, omega-6,9; cholesteryl erucate, C22:1, omega-9; cholesteryl nervonate, C24:1, omega-9. The linoleate and oleate esters exhibit two metastable liquid-crystalline phases (cholesteric and smectic), whereas the longer chain esters form a stable smectic phase but no cholesteric phase [Ginsburg, G. S., & Small, D. M. (1981) Biochim. Biophys. Acta 664, 98-107]. Line widths (nu 1/2), spin--lattice relaxation times (T1), and nuclear Overhauser enhancements (NOE) were measured for all well-resolved resonances from ring and fatty acyl (FA) carbons at different temperatures in the isotropic liquid of each ester. T1 and NOE values of FA resonances were constant between the FA-2 carbon and olefinic region of each acyl chain and increased markedly for carbons near the chain terminus. FA carbon motions are thus restricted and/or highly correlated in the region between the ring and the olefinic carbons, suggesting that strong interactions occur between cholesterol ester molecules in this region of the FA chain. These results also suggest that the FA chains are approximately extended in the isotropic liquid. Steroid ring methine C-6 and C-3 nu 1/2's increased differentially on cooling to the liquid leads to liquid crystal transition temperature (Tm) of each ester, indicative of increasingly anisotropic ring rotations. The rotational anisotropy was quantitated by using a prolate ellipsoid model for the cholesterol ester molecule for which two correlation times (corresponding to rotations about the long and short molecular axes) were calculated from the C-3 and C-6 nu 1/2 values. The C-3/C-6 nu 1/2 ratio was directly proportional to the anisotropy of the ring motions as measured by the ratio of the two correlation times. At any given temperature relative to Tm, the C-3 and C-6 nu 1/2's and the C-3/C-6 nu 1/2 ratios were larger for cholesterol esters which have a cholesteric phase than for esters which have no cholesteric phase, showing that steroid ring motions were more restricted and more anisotropic prior to the formation of a cholesteric phase. Cholesteryl erucate and cholesteryl nervonate have longer regions of FA chain interactions which result in greater chain cooperativity, apparently preventing the preordering of steroid rings to the degree necessary for formation of a cholesteric phase. Thus, these esters form the smectic phase directly from the isotropic liquid. These results are applied to the cholesterol ester transition in plasma low-density lipoproteins.

Physical properties of cholesteryl esters having 20 carbons or more

By polarizing microscopy and differential scanning calorimetry we observed that the relative stability of the smectic and cholesteric mesophases of cholesteryl esters of acyl chain length of 20 carbons or more depends on the length of the acyl chain and its degree of unsaturation. Significantly, the addition of a single double bond to the acyl chain of a fully saturated cholesteryl ester which exhibits no mesophases (e.g., cholesteryl behenate (C22:0) and cholesteryl lignocerate (C24:0) yields an ester which displays an unusually stable smectic mesophase, bot no cholesteric mesophase. In fact, increasing unsaturation was found to have a destabilizing effect on the cholesteric phase. Similarly, a decrease in thermal stability of the cholesteric mesophase was observed with increasing thermal stability of the smectic mesophase increased in the same series. X-ray scattering data are presented on the smectic mesophase of cholesteryl erucate (C22:1) and cholesteryl nervonate (C24:1). Significant differences in molecular packing of these two monounsaturated omega = 9 cholesteryl esters in the crystalline state are demonstrated by preliminary X-ray scattering experiments.