Lipoproteins diameters

Fig. 1. Major lipoprotein classes (CM, VLDL, LDL, HDL) based on their density. Lipoprotein diameters range from about 6000 A for CM to 70 A for HDL. The outer shell ( 20 A) of all lipoproteins consists of apolipoproteins, unesterified cholesterol, and phospholipids the spherical core contains triacylglycerols and cholesteryl esters. CM and VLDL have the highest contents of triacylglycerols, and 1-10% of apolipoproteins by weight LDL and HDL contain mostly cholesteryl esters in their cores and 20-50% of apolipoproteins. The major apolipoprotein components of the various classes of lipoproteins are indicated with the solid lines secondary or minor apolipoprotein components are indicated with the dashed lines. In this figure, cholesterol refers to both esterified and unesterified cholesterol triglycerides = triacylglycerols.

Fig. 2. Negative-staining electron micrographs of human plasma lipoproteins (diameters 10-1(KX) nm). The laigest particles [chylomicrons (Chylo) and VLDLs] contain a higher ratio of lipid to protein, and are therefore less dense, than low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs) that contain relatively more protein. Photograph courtesy of Dr. R. Hamilton, University of California at San Francisco, with permission.

Lipoprotein Class Density (g/mL) Diameter (nm) Composition % dry weight) ... 

The main transport form of lipids in the cir culation. They are spherical macromolecules of 10-1200 nm diameter-composed of a core of neutral lipids (mostly cholesterol ester and triglycerides) surrounded by an amphipathic shell of polar phospholipids and cholesterol. Embedded in the shell of lipoproteins are apolipoproteins that are essential for assembly of theparticles in tissues that secrete lipoproteins, and for their recognition by target cells. 

Lipoprotein Source Diameter (nm) Density (g/mL) Composition Main Lipid Components ApolipoprotGins... 

Reaction with lipoprotein lipase results in the loss of approximately 90% of the triacylglycerol of chylomicrons and in the loss of apo C (which remrns to HDL) but not apo E, which is retained. The resulting chy-lotnicron remnant is about half the diameter of the parent chylomicron and is relatively enriched in cholesterol and cholesteryl esters because of the loss of triacylglycerol (Figure 25-3). Similar changes occur to VLDL, with the formation of VLDL remnants or IDL (intermediate-density lipoprotein) (Figure 25-4). 

Low-density lipoproteins in plasma and arterial wall are susceptible to oxidation to form oxidized LDL, which are thought to promote the development of atherosclerosis. LDL particles have a density of about 1.05, a molecular weight of about 2.5 x 106, and a diameter of about 20 nm [119]. LDL composition from different donors varies widely an average LDL particle contains about 1200 molecules of unsaturated acids and antioxidants about six molecules of a-tocopherol, about 0.53 molecule of 7-tocopherol, about 0.33 molecule of (3-carotene, and about 0.18 molecule of lycopene [120], Rapid oxidation of LDL is started only after the depletion of tocopherols and carotenoids [121]. 

The rationale for this type of contrast agent is to use the endogenous metabolic pathway of lipid metabolism in the liver for the transport of iodinated substances. Chylomicron remnants are naturally occurring lipoproteins in the blood that are responsible for the transport of lipids into the liver. Three different mechanisms for this transport are discussed direct uptake by the low-density lipoprotein receptor transport to the low-density lipoprotein receptor-related protein (LRP) mediated by heparan sulfate proteoglycan (HSPG) or direct HSPG-LRP uptake and direct HSPG uptake. One of the prerequisites for particles to be transported by these mechanisms is a mean diameter of less than 100-300 run. 

Triglycerides are removed in extrahepatic tissues through a pathway shared with VLDL that involves hydrolysis by the lipoprotein lipase (LPL) system. Decrease in particle diameter occurs as triglycerides are depleted. Surface lipids and small apoproteins are transferred to HDL. The resultant chylomicron remnants are taken up by receptor-mediated endocytosis into hepatocytes. 

Generalized structure of human lipoproteins. The lipoproteins are spherical and vary in diameter from 10 nm to as much as 1,000 nm, depending on the particular proteins and lipids. Each of the lipoprotein classes contains a neutral lipid core composed of triacylglycerol or cholesteryl ester or both. Around the core is a layer of protein, phospholipid, and cholesterol that is oriented with the polar portions exposed to the surface of the lipoprotein. Note micellar-like construction. 

Due to dynamic remodeling within different HDL subfractions and between HDL and other lipoproteins, HDL is a heterogeneous class of lipoproteins that can be generalized by a high density (>1.063 g/mL) and a small size (5- to 17-nm diameter). Differences are characterized by shape, density, size, charge, and antigenicity,... 

In the intestinal mucosal cells, the triacylglycerols are resynthesized from fatty acids and monoacylglycerols and then packaged into lipoprotein transport particles called chylomicrons, stable particles ranging from approximately 180 to 500 nm in diameter (Figure 22.5). These particles are composed mainly of triacylglycerols, with apoprotein B-48 as the main protein component. Protein constituents of lipoprotein particles are called apolipoproteins. Chylomicrons also function in the transport of fat-soluble vitamins and cholesterol. 

Figure 26.16. Schematic Model of Low-Density Lipoprotein. The LDL particle is approximately 22 nm (220 A) in diameter.

Apo C-II activates lipoprotein lipase. A diagram of a chylomicron particle and a VLDL particle about to pass through a capillary is shown below. Chylomicrons are relatively large, having a diameter of 500 nm, while VLDLs are smaller, with a diameter of 100 nm. Lipoprotein lipase is shown as small dots adhering to the lumen of the capillary, and the chylomicron is shown contacting one of the lipases (Goldberg, 1996), The released free fatty aeids (FFA) are taken up and used by cells that reside near the capillary ... 

Stampfer, M. J., Krauss, R. M., Ma, J., Blanche, P, Holl, L. G., Sacks, F., and Hennekens, C. li (1996). A prospective study of triglyceride level, low-den.sily lipoprotein particle diameter, and risk of myocardial infarction. JAMA 276, 882-888-... 

It has been proposed that the vitamin E occurring in low-density lipoprotein (LDL) particles in the bloodstream, along with other antioxidants, normally acts to reduce or minimize the trend to atherosclerosis that occurs in most or all people. The natural rate of progression of atherosclerosis, as measured during its mature phase, involves the reduchon in diameter of arteries by about 1.5% per year (Superko and Krauss, 1994). 

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