Transport of Lipids and Lipoprotein Dynamics

The chemical-physical properties of lipoproteins are listed in Table 19.1. It is seen that the density of lipoproteins is lowest for chylomicrons, followed by VLDL, IDL, LDL, and HDL. As density increases, the amount of lipid de- 

Source Largely from Stein E. Lipids, lipoproteins, and apolipoproteins. In Tietz NW, ed. Textbook of Clinical Chemistry. Philadelphia Saunders, 1986, p. 849. 

Several types of proteins are associated with lipoproteins. These are termed apolipoproteins, or simply apoproteins. Table 19.2 shows the various apolipo-proteins (Apos), their chemical properties and occurrence, and their function, which is discussed later. Note that the A apoproteins are found largely in HDL, the B-100 is found largely in LDL, VLDL, and IDL, and C apoproteins are largely seen in chylomicrons. Nevertheless, there is a large degree of apoprotein overlap among the various lipoprotein classes. 

Chylomicrons are triglyceride rich and contain apolipoprotein B-48 and the A types. The latter are synthesized in the intestinal tract cells. Additional apoproteins are transferred to the chylomicrons from HDL in circulation the apoE and apoC types. Their site of synthesis is the liver. The chylomicrons are subject to degradation by lipoprotein lipase in the peripheral tissue, especially adipose tissue. Lipoprotein lipase activity is increased by increased blood insulin levels. This enzyme is extracellular, attached to the capillary endothelial cells, and activated by ApoC-II, which is present in the chylomicrons. Lipoprotein lipase causes the hydrolysis of triglycerides, thus decreasing chylomicron size 

Apolipo- protein Lipoprotein designation class Molecular weight Mean plasma concentration (mg/dL) Function 

---