Structure and interactions of lipids in human plasma low density lipoproteins.

R J Deckelbaum,G G Shipley,D M Small

doi:10.1016/s0021-9258(17)32781-3

Abstract

Temperature-dependent techniques (differential scanning calorimetry, polarizing microscopy, and x-ray scattering and diffraction techniques) were used to compare the properties of human plasma low density lipoproteins (LDL) with its extracted lipid classes. Three types of thermal transitions were characterized: (a) a reversible transition in intact LDL near body temperature associated with a liquid crystalline order-disorder phase change of cholesterol esters within the particles; (b) an irreversible high temperature transition (approximately 70-90 degrees) associated with LDL denaturation and release of cholesterol esters from the disrupted particles; and (c) low temperature transitions related to liquid crystalline and crystalline phase changes in these released esters. The temperature of the reversible transition in intact LDL varies among individual donors. Correlation analysis shows that the temperature of this transition negatively correlates with the amount of triglyceride relative to cholesterol ester in LDL. Studies on mixtures of cholesterol esters and triglycerides isolated from LDL show a similar effect, increasing amounts of triglycerides decreasing the temperature of the liquid leads to smectic liquid crystalline transition of the isolated esters. Thus, the amount of triglyceride in LDL influences the fluidity of the cholesterol esters in LDL. The enthalpy of the reversible transition in intact LDL is 0.69 cal/g of LDL cholesterol ester. This compares with 0.89 cal/g for the liquid leads to liquid crystalline transition of the cholesterol esters released from denatured LDL and 1.01 cal/g for the same transition in the extracted esters. Unlike the cholesterol esters released from denatured LDL, or isolated LDL esters, cholesterol ester in the intact LDL particle does not crystallize. These findings suggest that the behavior of cholesterol esters in intact LDL is constrained relative to their behavior when freed from the restrictions of the particle. These results together with experiments on partitioning of the individual lipid classes of LDL allow us to define the distribution and interaction of lipids in the intact LDL particle.

Highlights

Temperature-dependent techniques were used to compare the properties of human plasma low density lipoproteins (LI)L) with its extracted lipid classes
There were no significant differences in either the contribution of each lipid class to the total lipid weight or the fatty acyl cholesterol ester composition between the 14 low density lipoprotein (LDL) samples obtained from normal donors and the 7 obtained from hypercholesterolemic donors
We have defined three types of thermal transitions in human plasma low density lipoproteins: (a) a reversible transition in intact LDL encompassing body temperature which is associated with an order-disorder change of cholesterol esters within liquid crystalline the LDL particle; phase (h) an irreversible high temperature transition associated with LDL

Summary

AND METHODS

Isolation of Lipoproteins -Blood was collected in vacuum packs containing 1 mg/ml of d&odium EDTA from 14 fasting male normolipemic subjects and 7 fasting patients with type IIa hypercholesterolemia (five males and two females). Lipid composition of each LDL preparation was analyzed by quantitative thin layer chromatography using the method of Downing [19] as modified by Katz et al [20]. Individual lipid classes were isolated by preparative thin layer chromatography on glass plates (20 x 26 cm) coated with 1 mm of Silica Gel G. Partitioning ofLipoprotein Lipids -Total lipoprotein lipid extracts were equilibrated with excess d = 1.006 saline by repeated low speed centrifugation at 55” through a narrow central constriction in a glass tube sealed under nitrogen. The composition of each isolated zone was analyzed as described above and each layer was studied using DSC, polarizing light microscopy, and x-ray diffraction techniques. Light Microscopy-Samples of concentrated LDL and isolated lipid extracts were examined using a Zeiss standard NL polarizing light microscope on a controlled heating-cooling stage at rates of 1-3”imin. When two variables showed a significant correlation

RESULTS

Transition of LDL denaturation

DISCUSSION