The structure of plasma low density lipoproteins: experimental facts and interpretations--a minireview

From data on size and chemical composition, low density lipoprotein (LDL) can be described as a spherical particle having cholesteryl esters and triglycerides contained in a spherical core covered by the closely packed hydrophobic ends of phospholipids and unesterified cholesterol, while the head groups of the phospholipids, together with protein, occupy the surface. Such a model is compatible with early small angle X-ray and neutron scattering studies which, by prostulating spherical symmetry, assigned the LDL constituents to locations predicted from the radial electron density distribution. However, the concept of spherical symmetry, as applied to LDL structure, was recently challenged by results obtained from freeze-etching electron microscopy and small angle X-ray scattering experiments. Novel interpretations of these data suggest that the surface of LDL contains 4 electron-dense globules, located at tetrahedral positions, which have a capacity for structural remodeling at least as a function of the 2 temperatures studied (21C and 41C). It is reasonable to presume that the LDL protein (apo LDL) plays a role in the organization of the surface and overall LDL structure. However, until the chemical properties of apoLDL, and its behavior in solution and at the water-lipid interface are better understood, the validity of the proposed models cannot be assessed.