Plasma Lipoproteins

The plasma lipoproteins are spherical macromolecular complexes of lipids and proteins (apolipoproteins).

The lipoprotein particles include chylomicrons, very low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). They differ in lipid and protein composition, size, density (Fig. 1), and site of origin. [Note: Because lipoprotein particles constantly interchange lipids and apolipoproteins, the actual apo-lipoprotein and lipid content of each class of particles is somewhat variable.]

 Lipoproteins function both to keep their component lipids soluble as they transport them in the plasma and to provide an efficient mechanism for transporting their lipid contents to (and from) the tissues. In humans, the transport system is less perfect than in other animals and, as a result, humans experience a gradual deposition of lipid (especially cholesterol) in tissues.

Figure 1: Plasma lipoprotein particles exhibit a range of sizes and densities,  and typical values are shown. Ring widths approximate the amount of each component. [Note: Although cholesterol and its esters are shown as one component in the center of each particle, physically, cholesterol is on the surface, whereas cholesteryl esters are in the interior.]