Neutral Neutral, aromatic, aliphatic
PHE Phenylalanine, Methionine
Acidic Aspartate, glutamate
Imino Proline, hydroxyproline y + Basic
L Hydrophobic neutral
Yes Yes Yes Yes No No
Yes Yes Yes Yes No No
Yes Yes No No
Brush border membrane
Decreases in the amounts (or absence) of proteolytic enzymes occur in cases of pancreatic insufficiency as might develop in patients with pancreatitis or cystic fibrosis. Such decreases in the amounts of active enzymes may impair protein digestion to the point at which absorption is decreased and nitrogen is lost in the stool. The congenital absence of trypsin alone can also result in protein malabsorption. Trypsin is important for adequate protein assimilation not only because it digests dietary protein but because it is also necessary for the conversion of chymotrypsinogen, proelastase, and procarboxypeptidases to the active enzymes. Thus, patients lacking trypsin also lack active forms of the other proteolytic enzymes.
Several conditions have been described in which the uptake of amino acids is impaired because of the genetic lack of one of the transport systems for amino acids. These carrier proteins appear to be identical in the gut and kidney, because the same amino acids that cannot be absorbed in the gut are also lost in the urine. Cystinuria is a condition in which the dibasic amino acids cystine, lysine, arginine, and ornithine are not absorbed by the gut or reabsorbed by the proximal renal tubule. Another similar condition, called Hartnup's disease, affects the uptake of neutral amino acids. These patients, however, do not become amino acid deficient because the same amino acids can be absorbed as di- and tripeptides. This is excellent evidence that the carriers for amino acids are distinct from those for peptides. There is also a certain amount of overlap in the affinities of the transport carriers, so that a small amount of amino acid may be absorbed in the absence of its primary carrier.
The major component of dietary fat consists of triglycerides, three fatty acids esterified to glycerol. The principal saturated fatty acids are palmitic (C16) and stearic (C18). The unsaturated ones are oleic (CIS, one double bond) and linoleic (C18, two double bonds). All fatty acids contain an even number of carbon atoms. Phospholipids are present in the diet in small amounts and consist of glycerol esterified to two fatty acids and to either choline or inositol at the 3 position of glycerol. Cholesterol is ingested in the form of cholesterol esters with the fatty acid esterified to the 3-OH group of the sterol nucleus. These three types of compounds are shown in Fig. 8.
The churning and mixing that occur in the distal portion of the stomach break lipids into small droplets, greatly increasing the total surface area available to digestive enzymes. These droplets are kept apart by emulsifying agents. The latter are amphipathic compounds (see Chapter 34) such as bile salts, phos-pholipids, fatty acids, monoglycerides, and proteins. The major emulsifying agents in the stomach are some of the dietary proteins. However, the overall process of emulsification is inhibited by low pH. Approximately 10% of dietary triglycerides are hydrolyzed to glycerol and free fatty acids within the stomach by gastric lipase secreted by cells in the fundus. In humans, lingual lipase is much less important. These enzymes have pH optima between 3 and 6. The resulting lipase breakdown products aid in the emulsification process.
The rate of gastric emptying is carefully controlled to deliver amounts of fat into the duodenum that can be digested easily and absorbed without overloading the system. Duodenal receptors respond to fat by releasing CCK and enterogastrones and by triggering neural reflexes that inhibit the rate of gastric emptying.
As chyme enters the duodenum, the fat is emulsified further by the constituents of bile. Bile acids themselves are poor emulsifying agents, but bile also contains lecithin. The combination of lecithin with bile salts and the polar lipids (lysolecithin and monoglycerides) produced by digestion results in a mixture with a greatly enhanced emulsifying power. As a result, fats are totally dispersed into a fine suspension of droplets ranging from 0.5-1.0 mm in diameter. This is called an emulsion, and each droplet is covered with negatively charged emulsifying agents that prevent them from coalescing. The final result is a significant increase in the surface area available for enzyme action. The pancreas secretes three enzymes and one additional protein that play major roles in luminal fat digestion. The most important
of these is pancreatic lipase, also called glycerol ester hydrolase. It is secreted as an active enzyme with a pH optimum of 8. It functions at the oil-water interface to hydrolyze the ester linkages at the 1 and 3 positions of glycerol. This results in the production of fatty acids and 2-monoglycerides (see Fig. 8). In the presence of bile salts, the pH optimum of pancreatic lipase is reduced to 6.0, which is the approximate pH of the duodenal contents. The exact mechanism for this shift is unknown. Bile salt molecules, however, occupy the oil-water interface and inhibit the activity of pancreatic lipase by displacing the enzyme molecules from the surface of the emulsion droplets. This inactivation is prevented by a protein called colipase, which is secreted by the pancreas as inactive procolipase. Procolipase is activated by proteolytic enzymes (mainly trypsin), which cleave a peptide fragment, reducing it from 105 amino acids to 94 amino acids. Colipase has no lipolytic activity itself, but it binds to lipase in a 1:1 ratio and displaces a bile salt molecule from the interface. It thus serves as an anchor for lipase, allowing it to digest triglycerides. One molecule of colipase also binds one bile salt micelle, thus keeping the micelle near the site of hydrolysis and facilitating the removal and solubiliza-tion of the by-products of lipolysis. Pancreatic lipase is secreted in great excess, and the amount of active enzyme must be reduced by approximately 80% before dietary fat appears in the stool (steatorrhea). Duodenal pH of less than 3 inactivates lipase, but this does not occur in individuals with normal rates of gastric and pancreatic bicarbonate secretion.
The second noteworthy pancreatic lipolytic enzyme is cholesterol ester hydrolase, which is probably identical to nonspecific esterase. Approximately 15% of dietary cholesterol consists of cholesterol esterified to fatty acids. This linkage is cleaved by cholesterol ester hydrolase giving rise to free cholesterol and fatty acids (see Fig. 8). Cholesterol ester hydrolase lacks the specificity of pancreatic lipase and also hydrolyzes ester linkages at all 3 positions of triglycerides as well as those of vitamins A and D. The action of this enzyme, therefore, results in the production of free glycerol.
The most important enzyme for the digestion of phospholipids is pancreatic phospholipase A2, which is secreted as the proenzyme. The approximately 2 g of daily dietary phospholipids are augmented by the 12 g contained in biliary secretions and sloughed cells. As shown in Fig. 8, phospholipase A2 releases fatty acids from the 2 position to yield lysophospholipids (1-acylglycerophosphatides). Pancreatic lipase can also hydrolyze phospholipids at the 1 position, but only at a low rate. Therefore, almost all phospholipids are absorbed as lysophosphatides. Phospholipase A2 requires bile salts for its activity and has a pH optimum of 7.5.
Some foods also contain enzymes that digest dietary lipids. One of these of importance to humans is milk lipase found in breast milk. The activity of this enzyme is increased by low concentrations of bile salts. It is sometimes referred to as bile salt-sensitive lipase and is not active until it reaches the duodenum. This enzyme is important to newborns for the digestion of milk and aids pancreatic lipase in the hydrolysis of triglycerides, for it hydrolyzes all three ester linkages.
The products of lipid digestion undergo an interesting journey before they reach the bloodstream. First, lipolytic by-products are solubilized in micelles within the intestinal lumen. (See Chapter 34 for a discussion of micelle formation.) They then diffuse from the micelle across the enterocyte brush border membrane and enter the cytoplasm of the cell. Within the cell, triglycerides are resynthesized. The resynthe-sized lipids are combined with P-lipoprotein to form particles called chylomicrons. The chylomicrons leave the cell by exocytosis and enter the lymph via the lacteals present in the villi. Finally, they reach the bloodstream via the thoracic duct.
Lipolytic products are solubilized in mixed micelles within the intestinal lumen. Mixed micelles are cylindrical disks 30-70 A in diameter consisting of an outer layer of bile salts surrounding the hydrophobic products of fat digestion (Fig. 9; also see Chapter 34, Fig. 28). The bile salts are oriented with their hydrophilic portions outward in the aqueous phase and hydropho-bic sides toward the center. In this fashion, the hydrophobic products of lipid digestion are made water soluble. In addition to bile salts, mixed micelles contain fatty acids, monoglycerides, phospholipids, cholesterol, and fat-soluble vitamins. Glycerol is water soluble and is not a constituent of micelles.
All fat digestion products were believed, until recently, to be absorbed by simple diffusion. However, evidence now indicates that some carrier-dependent mechanisms are involved. In some cases transport shows a specificity that cannot be explained by simple diffusion. Transport maxima or saturation has been shown to occur for some products of digestion such as linoleate.
Micelles diffuse through the unstirred water layer, which is a major diffusion barrier to the absorption of lipids, and bring the products of lipolysis into contact with the brush border membrane. An equilibrium is established between lipids in the micellar and aqueous phases, with lipids moving in and out of micelles and striking the brush border membrane. When they contact the brush border membrane, because of their lipid solubilities, they are able to diffuse through the membrane and enter the cells (see Fig. 9). Thus, the lipolytic products diffuse into the cells in direct proportion to their concentration in solution. The concentrations of lipolytic products in solution are, in turn, dependent on their concentration in the micelles. A low pH microclimate exists within the unstirred layer at the surface of the intestine, and fatty acids become proto-nated, reducing their micellar solubility. This facilitates absorption by shifting the equilibrium so that more lipolytic products leave the micelles and are free to enter the cell membranes. Short-chain and medium-chain triglycerides are sufficiently water soluble in that they can diffuse through the unstirred layer without the aid of micelles. Therefore, their uptake, like that of glycerol, is independent of micelle formation.
The evidence for the absorption of lipids from micelles is several-fold. First, lipid digestion products
form mixed micelles with bile salts within the lumen of the intestine. Second, long-chain fatty acids and monoglycerides are absorbed more rapidly from micel-lar solution than from emulsions. Third, lipolytic products are absorbed at different rates, with cholesterol absorbed more slowly than the other constituents of micelles. As micelles move distally through the gut, their concentration of cholesterol increases. These findings also provide evidence that the micelles themselves are not absorbed. Finally, most ingested fat is absorbed by midjejunum, but the bile salts themselves are not actively reabsorbed until the ileum. Thus, bile salts are present in sufficient concentrations to ensure that lipid absorption is complete. The separate mechanism for active bile salt absorption is another strong indication that lipids are not absorbed as part of intact micelles.
Within the enterocytes the products of lipolysis are reesterified (see Fig. 9). The resynthesized triglycerides and phospholipids, together with cholesterol, are combined with apoprotein to form chylomicrons, which are taken up by the lymph vessels in the villi.
Lipolytic products partition into the lipid matrix of the cell membrane rather than the cytosol. The cytosol, however, contains specific proteins with high affinities for fatty acids. These proteins are called fatty acid-binding proteins (FABPs) (see Fig. 9). They have a high specificity for long-chain fatty acids and their mono-glycerides, and low affinity for medium- and short-chain fatty acids. FABPs preferentially bind unsaturated fatty acids but transport all long-chain fatty acids from the membrane to the smooth endoplasmic reticulum, where resynthesis takes place. This explains the findings that resynthesized triglycerides contain only long-chain fatty acids and that medium-chain fatty acids are absorbed directly into the bloodstream without resynthesis into triglyceride.
The most important resynthesis pathway makes use of 1- and 2-monoglycerides, although 2-monoglycerides are preferred and the most abundant. Fatty acids are activated to acyl coenzyme A (CoA), a reaction catalyzed by acyl CoA synthetase and making use of coenzyme A, ATP, and Mg2 + . In the presence of mono- and diglyceride transferases, the reaction then proceeds to form triglycerides (see Fig. 9):
1. Acyl CoA + monoglyceride ! diglyceride.
2. Acyl CoA + diglyceride ! triglyceride.
All of these reactions take place within the smooth endoplasmic reticulum, which becomes engorged with lipid after a meal containing fat.
In the presence of monoglyceride, the phosphatidic acid pathway is the minor route of triglyceride resynth-esis. During fasting, however, it becomes the major mechanism. The overall reaction combines three molecules of acyl CoA with a-glycerophosphate, derived from hexose metabolism, to form one molecule of triglyceride. Less than 4% of absorbed glycerol is used for triglyceride resynthesis. The intermediate product of this reaction is phosphatidic acid, which can be used for triglyceride or phospholipid synthesis (see Fig. 9):
2Acyl CoA + a-glycerophosphate
! phosphatidic acid 
In the production of triglycerides, phosphatidic acid is dephosphorylated to yield a 1,2-diglyceride, but this pool of diglyceride remains separate from that produced by the monoglyceride acylation pathway. Thus, the two pathways function independently.
Phospholipids can be produced independently of the phosphatidic acid pathway by direct acylation of absorbed lysophospholipids. Thus, the acylation of lysoleci-thin produces lecithin (phosphatidylcholine), which is an important constituent of chylomicrons.
Cholesterol is absorbed in free form, but a significant portion is re-esterified with fatty acids within the enterocytes. The ratio of free to esterified cholesterol leaving the enterocyte depends on the amount of dietary cholesterol. As dietary cholesterol decreases, the proportion of free cholesterol in the lymph increases.
Within the enterocytes, resynthesized triglycerides, cholesterol and cholesterol esters, phospholipids, and apoproteins form chylomicrons, lipid-carrying particles. Chylomicrons are the largest of a variety of lipid-carrying lipoproteins found in lymph. Chylomicrons
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