Chapter 25

After studying this chapter, you should be able to:

• Identify the four major groups of plasma lipoproteins and the four major lipid classes they carry.
• Illustrate the structure of a lipoprotein particle.
• Indicate the major types of apolipoprotein found in the different lipoprotein classes.
• Explain that triacylglycerol is carried from the intestine (after intake from the diet) to the liver in chylomicrons and from the liver to extrahepatic tissues in very low density lipoprotein (VLDL), and these particles are synthesized in intestinal and liver cells, respectively, by similar processes.
• Illustrate the processes by which chylomicrons are metabolized by lipases to form chylomicron remnants, which are then removed from the circulation by the liver.
• Explain how VLDL is metabolized by lipases to VLDL remnants (also called intermediate-density lipoprotein (IDL)) which may be cleared by the liver or converted to low-density lipoprotein (LDL), which functions to deliver cholesterol from the liver to extrahepatic tissues and is taken up via the LDL (apoB100,E) receptor.
• Explain how high-density lipoprotein (HDL), which returns cholesterol from extrahepatic tissues to the liver in reverse cholesterol transport, is synthesized, indicate the mechanisms by which it accepts cholesterol from tissues, and show how it is metabolized in the HDL cycle.
• Understand how the liver plays a central role in lipid transport and metabolism and how hepatic VLDL secretion is regulated by the diet and hormones.
• Be aware of the roles of LDL and HDL in promoting and retarding, respectively, the development of atherosclerosis.
• Indicate the causes of alcoholic and nonalcoholic fatty liver disease.
• Appreciate that adipose tissue is the main store of triacylglycerol in the body and explain the processes by which fatty acids are released and how they are regulated.
• Understand the role of brown adipose tissue in the generation of body heat.

Fat absorbed from the diet and lipids synthesized by the liver and adipose tissue must be transported between the various tissues and organs for utilization and storage. Since lipids are insoluble in water, the problem of how to transport them in the aqueous blood plasma is solved by associating nonpolar lipids (triacylglycerol and cholesteryl esters) with amphipathic lipids (phospholipids and cholesterol) and proteins to make water-miscible lipoproteins.

In a meal-eating omnivore such as the human, excess calories are ingested in the anabolic phase of the feeding cycle, followed by a period of negative caloric balance when the organism draws upon its carbohydrate and fat stores. Lipoproteins mediate this cycle by transporting lipids from the intestines as chylomicrons—and from the liver as very low density lipoproteins (VLDL)—to most tissues for oxidation and to adipose tissue for storage. Lipid is mobilized from adipose tissue as free fatty acids (FFA) bound to serum albumin. Abnormalities of lipoprotein metabolism cause various hypo- or hyperlipoproteinemias. The most common of these is in diabetes mellitus, where insulin deficiency causes excessive mobilization of FFA and underutilization of chylomicrons and VLDL, leading to hypertriacylglycerolemia. Most other pathologic conditions affecting lipid transport ...

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