ABSTRACT
The transport of free cholesterol, cholesteryl esters, triglycerides, phospholipids and other lipids in the circulation is achieved by the packing of the lipid moieties into water-soluble lipoproteins. The plasma lipoproteins are either spherical particles or discoidal particles. The spherical particle has a core of non-polar neutral lipid consisting of cholesteryl esters (CE) and triglycerides (TG), and coats of relatively polar materials such as phospholipids, cholesterol, and proteins (Figure 8.1a, Table 8.1).1 The discoidal particle consists of mostly polar lipids and proteins, and is in a bilayer conformation (Figure 8.1 a).1,2
Plasma lipoproteins have traditionally been grouped into five major classes and various subclasses, based on their buoyant density (Figure 8.1a): chylomicrons, very lowdensity lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Lipoprotein (a) (Lp(a)), which will be discussed later, floats in the LDL region.2,3
Lipoproteins are synthesized and catabolized in three distinct pathways: the chylomicron pathway, the VLDL/IDL/LDL pathway, and the HDL pathway, all of which are metabolically interrelated (Figure 8.1b-d). Several different proteins, including apolipoproteins, plasma enzymes, lipid transfer proteins, lipoprotein receptors, and lipid transporters, participate in these pathways and contribute to lipid homeostasis. The properties of the apolipoproteins, plasma enzymes and lipid transfer proteins, lipoprotein receptors, and receptors for modified lipoproteins (scavenger receptors) are shown in Tables 8.2, 8.3, 8.4, and 8.5, respectively.
