Exogenous lipoproteins pathway

Chylomicrons are the largest of the lipoproteins and the least dense because of their rich triacylglycerol content. They are synthesized from dietary lipids (the exogenous lipoprotein pathway) within the epithelial cells of the small intestine and then secreted into the lymphatic vessels draining the gut (see Fig. 32.13). They enter the bloodstream via the left subclavian vein. The major apoproteins of chylomicrons are apoB-48, apoCn, and apoE (see Table 34.3). The apoCn activates lipoprotein lipase... 

Figure 26-18 Exogenous lipoprotein metabolism pathway. TG, Triglyceride CE, cholesterol ester FC, free cholesterol Ft, phospholipids HDL, hIgh-density lipoproteins FA, fatty acid LPL, lipoprotein lipase 6, apolipoprotein B-48 A, apolipoprotein A-i C, apolipoprotein C-ll , apolipoprotein E. (From Rifai N. Lipoproteins and apolipoproteins Composition, metabolism, and association with coronary heart disease. Arch Pathol Lab Med 1986 10 694-701. Copyright 1986, American Medical Association.)...

Lipoprotein Metabolism. Figure 1 Exogenous pathway of lipoprotein metabolism. 

Lipoproteins are metabolised by two main pathways, according to the origin of the lipoprotein particle being handled. The exogenous... 

After its absorption into the intestinal mucosal cell, cholesterol, together with triglycerides, phospholipids, and a number of specific apoproteins, is assembled into a large lipoprotein called the chylomicron (see later section on lipoprotein metabolism, exogenous pathway). One apoprotein component known as apolipoprotein (apo) B-48 is vital to the formation of chylomicrons, and in people with a rare deficiency of apo B-48 synthesis, chylomicron formation, and consequently cholesterol and fat absorption, is severely impaired. Chylomicrons enter the lymphatics, which empty into the thoracic duct and eventually enter the systemic venous circulation at the junction of the left subclavian vein and left internal jugular vein. 

The pathways of lipoprotein metabolism are complex. They include exogenous and endogenous pathways based on whether they carry lipids of dietary or hepatic origin (Figures 26-18 and 26-19) and other pathways such as the... 

Fig. 1. Simplified schematic summary of the essential pathways for receptor-mediated human lipoprotein metabolism. The liver is the crossing point between the exogenous pathway (left-hand side), which deals with dietary lipids, and the endogenous pathway (right-hand side) that starts with the hepatic synthesis of VLDL. The endogenous metabolic branch starts with the production of chylomicrons (CM) in the intestine, which are converted to chylomicron remnants (CMR). Very-low-density lipoprotein particles (VLDL) are lipolyzed to LDL particles, which bind to the LDL receptor. IDL, intermediate-density lipoproteins LDL, low-density lipoproteins HDL, high-density lipoproteins LCAT, lecithinxholesterol acyltransferase CETP, cholesteryl ester transfer protein A, LDL receptor-related protein (LRPl) and W, LDL receptor. Lipolysis denotes lipoprotein lipase-catalyzed triacylglycerol lipolysis in the capillary bed.

In an exogenous pathway, dietary cholesterol and triglycerides are absorbed by the gut enterocytes in the form of free cholesterol, fatty acids, and monoacylglycer-ols. Subsequently, the cholesteryl esters and triglycerides are incorporated into the triglyceride-rich chylomicrons and these processes involve several of the apolipopro-teins. The chylomicrons pass into the intestinal lymph system and then into the vascular circulation via the thoracic duct. Lipoprotein lipase (LPL) enables the transfer of fatty acids and triglycerides to the cells. Chylomicron remnants are transported to hepatic receptor sites. 

Fig. 30.5. Endogenous and exogenous pathways for lipid transport and metabolism. FFA, free fatty acids LDLR, low-density lipoprotein receptor FC, free unesterified cholesterol LCAT, lecithin-cholesterol acyltransferase.

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