life11e_ch50

The liver produces lipoproteins: the good, the bad, and the ugly

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The liver is the major controller of fat metabolism through its production of lipoproteins. A lipoprotein is a particle made up of a core of hydrophobic fat and cholesterol with a covering of hydrophilic protein that allows it to be suspended in water. Lipoproteins are the most abundant fuel reserve in the body, and they move fats from sites of absorption or synthesis to sites of storage, and from sites of storage to sites of use. You saw in Key Concept 50.3 how bile in the intestine solves the problem of processing hydrophobic fats in an aqueous medium. Transport of fats in the circulatory system presents the same problem, and lipoproteins provide the solution.

The chylomicrons produced by the mucosal cells of the intestine are the largest lipoprotein particles in the blood. As the circulation carries chylomicrons through muscle, adipose tissue, and liver, lipoprotein lipases attached to the endothelial cells of capillaries break down the chylomicrons’ triglycerides so that the component fatty acids, monoglycerides, and glycerol can be taken up by the cells. The chylomicron remnants have lipid cores with a lot of cholesterol, and they are taken up by liver cells.

Lipoproteins different from chylomicrons are synthesized in the liver. These lipoproteins can be classified according to their density. Fat has a low density (it floats on water) and protein has a high density, so the greater the fat-to-protein ratio in the lipoprotein, the lower its density.

High-density lipoproteins (HDLs) transport lipids and cholesterol from tissues to the liver where the cholesterol can be used to synthesize bile. HDL consists of about 50 percent protein, 35 percent lipids, and 15 percent cholesterol. These are the “good” lipoproteins, and their levels are higher in people who exercise and are fit.
Low-density lipoproteins (LDLs) transport cholesterol around the body for use in biosynthesis and for storage. LDL consists of about 25 percent protein, 25 percent lipids, and 50 percent cholesterol. These are the “bad” lipoproteins associated with a high risk for cardiovascular disease.
Very low-density lipoproteins (VLDLs) contain mostly triglycerides, which are taken up by fat cells in adipose tissues around the body. VLDL consists of about 2 percent protein, 94 percent lipids, and 3 percent cholesterol. These are the “ugly” lipoproteins, as they are associated with obesity and diabetes as well as a high risk for cardiovascular disease.