Lipoprotein metabolism and its relationship to atherosclerosis

LlPOPROTEINSLipoproteins are spherical particles made up of hundreds of lipid and protein molecules.They are much smaller than red blood cells and are visible only with an electron microscope.When the larger, triglyceride-rich lipoproteins are present in very high concentrations, however, plasma can appear to be turbid, or even milky, to the naked eye.The major lipids of the lipoproteins are cholesterol, triglycerides, and phospholipids.Triglycerides and the esterified form of cholesterol (cholesteryl esters) are insoluble in aqueous environments (hydrophobic) and constitute the core of the lipoproteins.Phospholipids and free (unesterified) cholesterol, which are soluble in both lipid and aqueous environments (amphipathic), cover the surface of the particles, where they act as the interface between the plasma and core components.Finally, there is a family of proteins, called apoproteins (or apolipoproteins), which also occupy the surface of the lipoproteins.These proteins, while serving as an interface between lipid and aqueous environments, also play crucial roles in the regulation of lipid transport and lipoprotein metabolism (Fig. 1).Although all of the lipoprotein classes to be discussed here are actually composed of a continuous spectrum of lipoprotein particles varying in composition, size, density, and function, several major classes of lipoproteins have been defined by their physical-chemical characteris-