(2) Chylomicron Metabolism Chylomicrons are metabolized in adipose tissue and muscle. 6. Rate-limiting enzymes limit the rate at which a metabolic pathway proceeds. Triglycerides and cholesterol combine to form chylomicrons. 1. The Pathways of Lipid Transport. Chylomicron remnants then travel to the liver and are removed by the binding of apoE to their remnant receptor. The reactions in this initial stage are the same as in ketogenesis (see slide 10.4.1).However, while ketogenesis occurs in the mitochondria, HMG-CoA destined for … After unloading their fats, chylomicrons are smaller and referred to as chylomicron remnants. Abnormalities in the metabolism of VLDL, IDL, and LDL [apolipoprotein B-100 (apoB-100)-containing lipoproteins] and HDL (apolipoprotein A-containing lipoproteins) are associated with risk of atherosclerosis. The triglycerides carried in VLDL are metabolized in muscle and adipose tissue by lipoprotein lipase releasing free fatty acids and IDL are formed. The pharmaceutical industry has taken advantage of this knowledge to lower people's LDL levels with drugs known as statins. The intestine secretes chylomicron particles into the lymphatics. Triglycerides are formed in the intestinal cell from free fatty acids and glycerol and cholesterol is esterified. Chylomicron remnants are then taken up by the liver. Cholesterol synthesis starts with acetyl-CoA, which is used to synthesize hydroxymethylglutaryl-CoA (HMG-CoA). Exogenous pathway for lipid metabolism: Dietary cholesterol and fatty acids are absorbed. These surface constituents are transferred from the chylomicron remnant for the formation of high density lipoprotein. Exogenous cholesterol metabolism pathway Endogenous Pathway. Chylomicrons enter the circulation and travel to peripheral sites. The endogenous lipoprotein pathway begins in the liver with the formation of VLDL. Chylomicrons are the ‘orphans’ of the lipoprotein family. The endogenous pathway of lipid metabolism begins with Abnormalities in chylomicron (apolipoprotein B-48-containing lipoproteins) metabolism are associated with risk of pancreatitis. Although the proportion of the apoE-dependent/non-LDLR pathway in the overall metabolism of chylomicron remnants is only (null)/1;6, chylomicron remnants do not accumulate in the plasma of the LDLR-deficient humans and animals fed a normal chow (14, 15, 25). Chylomicron Metabolism Since the intestine is primarily an absorptive organ, it must have the means of exporting newly absorbed lipids. Difficulty of measurement has impeded understanding of their metabolism. The enterocyte re-esterifies fatty acids and monoglycerides to form triglycerides and phospholipids. The chylomicron remnants contain a smaller core of lipids that is enveloped by excess surface components. The enzyme, lipoprotein lipase, located on capillary walls, is activated by apoC-II and hydrolyses triglyceride to glycerol and fatty acids. 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