Structure of human plasma low-density lipoproteins: molecular organization of the central core.

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Abstract
Human plasma low density lipoprotein (LDL) exhibits a thermal transition over the temperature range 20-40.degree. C. This transition is associated with a structural change within the lipoprotein particle and is reflected in the small-angle X-ray scattering profiles from LDL. The scattering profile of the quasispherical LDL particle at 10.degree. C shows a relatively intense maximum at 1/36 .ANG.-1 which is absent from the scattering of LDL at 45.degree. C. Theoretical calculations, using model electron density distributions, were carried out to describe the packing of cholesterol esters in the core of LDL. At 10.degree. C, a radially oriented arrangement of the cholesterol esters, based on perturbations of the molecular packing or crystalline cholesteryl myristate, adequately reproduces the high relative intensity of the X-ray scattering maximum at 1/36 .ANG.-1. The perturbations of the packing in the crystal structure of cholesteryl myristate involve melting of the hydrocarbon chains of the esters together with translations of pairs of molecules parallel to the molecular long axis. The interaction of opposing steroid moieties, with C18 and C19 angular methyl groups interlocked, exhibited in the crystal structure is retained in the perturbed arrangement. At 45.degree. C, thermally induced disorder of this arrangement averages the electron density of the central core. The X-ray scattering profiles of particles with a homogeneous electron density in the core region do not show a high relative intensity of the subsidiary maxima in the 1/36 .ANG.-1 region, in agreement with experimental observation. The thermal transition observed for LDL may be due to a smectic .fwdarw. disordered transition of the cholesterol esters in the core of the LDL particle.