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- Gastric mucosa (secretes gastrin)

(b) Exocrine cells of pancreas

Zymogens active proteases

Small intestine

Stomach Pancreas

-Parietal cells (secrete HCl)

- Gastric mucosa (secretes gastrin)

(b) Exocrine cells of pancreas

Chief cells (secrete pepsinogen)

-Rough ER

(c) Villi of small intestine

Villus

- Intestinal mucosa (absorbs amino acids)

Chief cells (secrete pepsinogen)

-Rough ER

Zymogen granules

Collecting duct

(c) Villi of small intestine

Villus

- Intestinal mucosa (absorbs amino acids)

cholecystokinin, which stimulates secretion of several pancreatic enzymes with activity optima at pH 7 to 8. Trypsinogen, chymotrypsinogen, and procarboxypeptidases A and B, the zymogens of trypsin, chymo-trypsin, and carboxypeptidases A and B, are synthesized and secreted by the exocrine cells of the pancreas (Fig. 18-3b). Trypsinogen is converted to its active form, trypsin, by enteropeptidase, a proteolytic enzyme secreted by intestinal cells. Free trypsin then catalyzes the conversion of additional trypsinogen to trypsin (see Fig. 6-33). Trypsin also activates chymotrypsinogen, the pro-carboxypeptidases, and proelastase.

Why this elaborate mechanism for getting active digestive enzymes into the gastrointestinal tract? Synthesis of the enzymes as inactive precursors protects the exocrine cells from destructive proteolytic attack. The pancreas further protects itself against self-digestion by making a specific inhibitor, a protein called pancreatic trypsin inhibitor (p. 231), that effectively prevents premature production of active proteolytic enzymes within the pancreatic cells.

Trypsin and chymotrypsin further hydrolyze the peptides that were produced by pepsin in the stomach. This stage of protein digestion is accomplished very efficiently, because pepsin, trypsin, and chymo-trypsin have different amino acid specificities (see Table 3-7). Degradation of the short peptides in the small intestine is then completed by other intestinal peptidases. These include carboxypeptidases A and B (both of which are zinc-containing enzymes), which remove successive carboxyl-terminal residues from peptides, and an aminopeptidase that hydrolyzes successive amino-terminal residues from short pep-tides. The resulting mixture of free amino acids is transported into the epithelial cells lining the small intestine (Fig. 18-3c), through which the amino acids enter the blood capillaries in the villi and travel to the liver. In humans, most globular proteins from animal sources are almost completely hydrolyzed to amino acids in the gastrointestinal tract, but some fibrous proteins, such as keratin, are only partly digested. In addition, the protein content of some plant foods is protected against breakdown by indigestible cellulose husks.

Acute pancreatitis is a disease caused by obstruction of the normal pathway by which pancreatic secretions enter the intestine. The zymogens of the proteolytic enzymes are converted to their catalyt-ically active forms prematurely, inside the pancreatic cells, and attack the pancreatic tissue itself. This causes excruciating pain and damage to the organ that can prove fatal. ■

Pyridoxal Phosphate Participates in the Transfer of a-Amino Groups to a-Ketoglutarate

The first step in the catabolism of most l-amino acids, once they have reached the liver, is removal of the a-amino groups, promoted by enzymes called amino-transferases or transaminases. In these transamination reactions, the a-amino group is transferred to the a-carbon atom of a-ketoglutarate, leaving behind the corresponding a-keto acid analog of the amino acid (Fig. 18-4). There is no net deamination (loss of amino groups) in these reactions, because the a-ketoglutarate becomes aminated as the a-amino acid is deaminated. The effect of transamination reactions is to collect the amino groups from many different amino acids in the form of l-glutamate. The glutamate then functions as an amino group donor for biosynthetic pathways or for

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