++
LIPOPROTEIN METABOLISM AND ATHEROSCLEROSIS
++
The major lipids of plasma, cholesterol and triglyceride, are transported in molecular complexes called lipoproteins. The basic structure of all lipoproteins is similar. They contain a core of neutral lipid (triglyceride and cholesterol ester) that is surrounded by a polar coat containing nonesterified cholesterol, phospholipid, and proteins (called apolipoproteins) (Fig. 29–1). The major categories of lipoproteins consist of low-density lipoprotein (LDL), very-low-density lipoprotein (VLDL), high-density lipoprotein (HDL), and chylomicrons. These lipoproteins vary in size and density (Fig. 29–2). Because lipoproteins can be separated by electrophoresis, they also have been named according to their migration relative to serum proteins. LDL is called beta lipoprotein, VLDL is pre-beta lipoprotein, and HDL is alpha lipoprotein.
++
++
++
The primary function of lipoproteins is to transport triglycerides and cholesterol.1,2,3,4,5 The major forms of triglyceride-rich lipoproteins (TGRLs) are chylomicrons (Fig. 29–3)6,7 and VLDLs (Fig. 29–4). Chylomicrons are synthesized in the liver from dietary fat. They are secreted into the circulation for transport of lipids to tissues. In the circulation, chylomicron triglyceride undergoes hydrolysis by an enzyme, lipoprotein lipase (LPL), located on the surface of capillary endothelial cells; fatty acids released during lipolysis are taken up by adipose tissue and muscle. In addition, the liver synthesizes triglyceride from fatty acids and incorporates it into VLDL particles. VLDL triglyceride likewise undergoes lipolysis, releasing fatty acids for adipose tissue and muscle. Following triglyceride lipolysis, an intermediate lipoprotein, called a VLDL remnant, is formed. A portion of VLDL remnants is removed by the low-density lipoprotein receptor (LDLR), but most VLDL is converted through intermediate lipoproteins to LDL. The latter is removed from the circulation by LDLRs that are located on the surface of liver cells.8,9
++