Box 164

Functional Considerations: Digestive and Absorptive Functions of Enterocytes

The plasma membrane of the microvilli of the enterocyte plays a role in digestion as well as absorption. Digestive enzymes are anchored in the plasma membrane, and their functional groups extend outward to become part of the glycocalyx. This arrangement brings the end products of digestion close to their site of absorption. Included among the enzymes are peptidases and disacchari-dases. The plasma membrane of the apical microvilli also contains the enzyme enteropepticlase (enterokinase), which is particularly important in the duodenum, where it converts trypsinogen into trypsin. Trypsin can then continue to convert additional trypsinogen into trypsin, and trypsin converts several other pancreatic zymogens into active enzymes (Fig. 16.29). A summary of digestion and absorption of the three major nutrients is outlined in the following paragraphs.

Triglycerides are broken down into glycerol, monoglycerides, and long- and short-chain fatty acids. These substances are emulsified by bile salts and pass into the apical portion of the enterocyte. Here, the glycerol and long-chain fatty acids are resynthesized into triglycerides. The resynthesized triglycerides appear first in apical vesicles of the sER (see Fig. 16.21), then in the Golgi (where they are converted into chylomicrons, small droplets of neutral fat), and finally in vesicles that discharge the chylomicrons into the intercellular space. The chylomicrons are conveyed away from the intestine via both venous capillaries and lacteals. Short-chain fatty acids and glycerol leave the intes tine exclusively via capillaries that lead to the portal vein and the liver.

Carbohydrate final digestion is brought about by enzymes bound to the microvilli of the enterocytes (Fig. 16.30). Galactose, glucose, and fructose are conveyed to the liver by the vessels of the hepatic portal system. Some infants and a larger percentage of adults cannot tolerate milk and unfermented milk products because of the absence of lactase, the disaccharidase that splits lactose into galactose and glucose. If given milk, these individuals become bloated because of the gas produced by bacterial digestion of the unprocessed lactose and suffer from diarrhea. The condition is completely alleviated if lactose (milk sugar) is eliminated from the diet. For some individuals, milk intolerance may be also partially or completely alleviated by using lactose-reduced milk products or tablets of lactase (enzyme that digests lactose), which are available as over-the-counter drugs.

Protein digestion and absorption is shown in Figure 16.31. The major end products of protein digestion are amino acids, which are absorbed by enterocytes. However, some peptides are also absorbed and are evidently broken down intracellular^. In one disorder of amino acid absorption (Hartnup's disease), free amino acids appear in the blood when dipeptides are fed to patients but not when free amino acids are fed. This supports the conclusion that dipeptides of certain amino acids are absorbed via a pathway different from that of the free amino acids.

pancreatic zymogens (inactive proenzymes)

chymotrypsinogen proelastase procarboxypeptidase A procarboxypeptidase B prophospholipase A2

pancreatic zymogen chymotrypsinogen proelastase procarboxypeptidase A procarboxypeptidase B prophospholipase A2

chymotrypsin elastase carboxypeptidase A carboxypeptidase B phospholipase A2

active enzymes chymotrypsin elastase carboxypeptidase A carboxypeptidase B phospholipase A2

active enzyme trypsinogen

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