Transport of Triglycerides and Other Lipids from the Gastrointestinal Tract by Lymph—The Chylomicrons
As explained in Chapter 65, almost all the fats in the diet, with the principal exception of a few short-chain fatty acids, are absorbed from the intestines into the intestinal lymph. During digestion, most triglycerides are split into monoglycerides and fatty acids. Then, while passing through the intestinal epithelial cells, the mono-glycerides and fatty acids are resynthesized into new molecules of triglycerides that enter the lymph as minute, dispersed droplets called chylomicrons, whose diameters are between 0.08 and 0.6 micron. A small amount of apoprotein B is adsorbed to the outer surfaces of the chylomicrons. This leaves the remainder of the protein molecules projecting into the surrounding water and thereby increases the suspension stability of the chylomicrons in the lymph fluid and prevents their adherence to the lymphatic vessel walls.
Most of the cholesterol and phospholipids absorbed from the gastrointestinal tract enter the chylomicrons. Thus, although the chylomicrons are composed principally of triglycerides, they also contain about 9 per cent phospholipids, 3 per cent cholesterol, and 1 per cent apoprotein B. The chylomicrons are then transported upward through the thoracic duct and emptied into the circulating venous blood at the juncture of the jugular and subclavian veins.
Removal of the Chylomicrons from the Blood
About 1 hour after a meal that contains large quantities of fat, the chylomicron concentration in the plasma may rise to 1 to 2 per cent of the total plasma, and because of the large size of the chylomicrons, the plasma appears turbid and sometimes yellow. However, the chylomicrons have a half-life of less than 1 hour, so the plasma becomes clear again within a few hours. The fat of the chylomicrons is removed mainly in the following way.
Chylomicron Triglycerides Are Hydrolyzed by Lipoprotein Lipase, and Fat Is Stored in Adipose Tissue and Liver Cells. Most of the chylomicrons are removed from the circulating blood as they pass through the capillaries of adipose tissue or the liver. Both adipose tissue and the liver contain large quantities of the enzyme lipoprotein lipase. This enzyme is especially active in the capillary endothelium, where it hydrolyzes the triglycerides of chylomicrons as they come in contact with the endothelial wall, thus releasing fatty acids and glycerol.
The fatty acids, being highly miscible with the membranes of the cells, immediately diffuse into the fat cells of the adipose tissue and into the liver cells. Once inside these cells, the fatty acids are again synthesized into triglycerides, with new glycerol being supplied by the metabolic processes of the storage cells, as discussed later in the chapter. The lipase also causes hydrolysis of phospholipids; this, too, releases fatty acids to be stored in the cells in the same way.
"Free Fatty Acids" Are Transported in the Blood in Combination with Albumin
When fat that has been stored in the adipose tissue is to be used elsewhere in the body to provide energy, it must first be transported from the adipose tissue to the other tissue. It is transported mainly in the form of free fatty acids. This is achieved by hydrolysis of the triglycerides back into fatty acids and glycerol.
At least two classes of stimuli play important roles in promoting this hydrolysis. First, when the amount of glucose available to the fat cell is inadequate, one of the glucose breakdown products, a-glycerophosphate, is also available in insufficient quantities. Because this substance is required to maintain the glycerol portion of triglycerides, the result is hydrolysis of triglycerides. Second, a hormone-sensitive cellular lipase can be activated by several hormones from the endocrine glands, and this also promotes rapid hydrolysis of triglycerides. This is discussed later in the chapter.
On leaving fat cells, fatty acids ionize strongly in the plasma, and the ionic portion combines immediately with albumin molecules of the plasma proteins. Fatty acids bound in this manner are called free fatty acids or nonesterified fatty acids, to distinguish them from other fatty acids in the plasma that exist in the form of (1) esters of glycerol, (2) cholesterol, or (3) other substances.
The concentration of free fatty acids in the plasma under resting conditions is about 15 mg/dl, which is a total of only 0.45 gram of fatty acids in the entire circulatory system. Strangely enough, even this small amount accounts for almost all the transport of fatty acids from one part of the body to another for the following reasons:
1. Despite the minute amount of free fatty acid in the blood, its rate of "turnover" is extremely rapid: half the plasma fatty acid is replaced by new fatty acid every 2 to 3 minutes. One can calculate that at this rate, almost all the normal energy requirements of the body can be provided by the oxidation of transported free fatty acids, without using any carbohydrates or proteins for energy.
2. Conditions that increase the rate of utilization of fat for cellular energy also increase the free fatty acid concentration in the blood; in fact, the concentration sometimes increases fivefold to eightfold. Such a large increase occurs especially in cases of starvation and in diabetes; in both these conditions, the person derives little or no metabolic energy from carbohydrates.
Under normal conditions, only about 3 molecules of fatty acid combine with each molecule of albumin, but as many as 30 fatty acid molecules can combine with a single albumin molecule when the need for fatty acid transport is extreme. This shows how variable the rate of lipid transport can be under different physiologic conditions.
Lipoproteins—Their Special Function in Transporting Cholesterol and Phospholipids
In the postabsorptive state, after all the chylomicrons have been removed from the blood, more than 95 per cent of all the lipids in the plasma are in the form of lipoprotein. These are small particles—much smaller than chylomicrons, but qualitatively similar in composition—containing triglycerides, cholesterol, phospholipids, and protein. The total concentration of lipoproteins in the plasma averages about 700 mg per 100 ml of plasma—that is, 700 mg/dl. This can be broken down into the following individual lipoprotein constituents:
mg/dl of plasma
Types of Lipoproteins. Aside from the chylomicrons, which are themselves very large lipoproteins, there are four major types of lipoproteins, classified by their densities as measured in the ultracentrifuge: (1) very low density lipoproteins, which contain high concentrations of triglycerides and moderate concentrations of both cholesterol and phospholipids; (2) intermediate-density lipoproteins, which are very low density lipoproteins from which a share of the triglycerides has been removed, so that the concentrations of cholesterol and phospholipids are increased; (3) low-density lipopro-teins, which are derived from intermediate-density lipoproteins by the removal of almost all the triglycerides, leaving an especially high concentration of cholesterol and a moderately high concentration of phospholipids; and (4) high-density lipoproteins, which contain a high concentration of protein (about 50 per cent) but much smaller concentrations of cholesterol and phospholipids.
Formation and Function of Lipoproteins. Almost all the lipoproteins are formed in the liver, which is also where most of the plasma cholesterol, phospholipids, and triglycerides are synthesized. In addition, small quantities of high-density lipoproteins are synthesized in the intestinal epithelium during the absorption of fatty acids from the intestines.
The primary function of the lipoproteins is to transport their lipid components in the blood. The very low density lipoproteins transport triglycerides synthesized in the liver mainly to the adipose tissue, whereas the other lipoproteins are especially important in the different stages of phospholipid and cholesterol transport from the liver to the peripheral tissues or from the periphery back to the liver. Later in the chapter, we discuss in more detail special problems of cholesterol transport in relation to the disease atherosclerosis, which is associated with the development of fatty lesions on the insides of arterial walls.
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