Lipoproteins

In the bloodstream, all the cholesterol and triglycerides are carried in spherical macromolecular complexes called lipoproteins. The development of the lipoprotein system, from an evolutionary standpoint, was necessary because the major lipids in our blood, esterified cholesterol (cholesterol linked to a fatty acid) and triglyceride, are insoluble in plasma, which is an aqueous media. By covering the cholesteryl esters and triglyceride with a coating of phospholipid (which are both lipid-soluble and water-soluble molecules) and proteins, the lipoprotein system allows the water-insoluble core lipids to be transported through an aqueous circulatory system. The different lipoproteins have been defined by their physico-chemical characteristics, particularly by their flotation characteristics during very high-speed ultracentrifugation. Although lipoprotein particles actually form a continuum, varying in composition, size, density, and function, they have been separated into major groupings related to their overall composition and/or function (Table 1). Hundreds to thousands of triglyceride and cholesteryl ester molecules are carried in the core of different lipoproteins.

As noted above, the surface of the lipoproteins contains phospholipids and proteins, called apolipoproteins. The apolipoproteins not only help to solubilize the core lipids, but also play critical roles in the regulation of plasma lipid and lipoprotein transport. The major apolipoproteins are described in Table 2. Apo-lipoprotein (apo) B is a key protein on several of the lipoproteins. Apo B100 (so

Table 1 Physicochemical Characteristics of the Major Lipoprotein Classes
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