Digestive Enzymes

FIGURE 2.3 Schematic representation of the activation of proteolytic zymogens in the duodenal lumen.

The bile is synthesized by the hepatocytes and secreted in the bile canaliculi that are apposed to the hepatic cells. These canaliculi coalesce via the intralobular bile ducts to form the right and left hepatic ducts that join outside the liver to form the common hepatic duct. The common hepatic duct then unites with the cystic duct to form the bile duct. The cystic duct drains the gallbladder where the bile is stored and concentrated (because of water absorption) between meals. The bile duct enters the duodenum via the ampoule of Vater in which the bile mixes with the secretions of the exocrine pancreas. The sphincter of Oddi is usually closed but relaxes when food that enters the oral cavity surrounds the bile duct. Furthermore, the hormone CCK, secreted by the intestinal mucosa when the chyme enters the duodenum, causes the gallbladder to contract, thus increasing the bile flow in the cystic and bile ducts.

Bile (±0.5 l/d in human adults) is a solution of alkaline electrolytes (similar to the pancreatic juice) that contains biliary acids, bile pigments, and traces ( 0.1%) of cholesterol, phospholipids, fats, fatty acids, and proteins (some of which are with enzymatic activity, e.g., alkaline phosphatase). In a process known as enterohepatic circulation, some of the components of the bile are reabsorbed in the intestine to be transported into the liver via the portal vein, and then, to be excreted again into the small intestine via the bile.

The bile pigments (bilirubin and biliverdin) are the end products of the catabo-lism of hemoglobin. Bilirubin is present in bile as a water-soluble glucuronide. But in the intestine, the conjugate is hydrolyzed and the bilirubin is reabsorbed (entero-hepatic circulation), or it is further metabolized to biliverdin (mostly by the anaerobic bacteria in the large bowel, as discussed shortly) to a series of colorless compounds known as sterco- and urobilinogens.

The bile acids are synthesized in the liver cells from cholesterol and actively secreted via the bile into the small intestine as water-soluble Na+ and K+ salts of glycine and taurine conjugates (±20-30 g/24 h). They are effectively reabsorbed (via an active transport in the ileum and a passive transport in the large bowel) via the enterohepatic circulation (±6-7 cycles/24 h). Thus, fecal excretion remains limited (±0.6 g/24 h), but this still represents the major excretion route for cholesterol. The major bile acids synthesized in the liver are the primary bile acids: cholic and chenodeoxycholic acids. In the colon, the anaerobic bacteria metabolize them to produce the secondary bile acids (deoxycholic and lithocholic acids) and, eventually, the tertiary bile acid (ursodeoxycholic acid) (discussed later in Section 2.2.1.4). The bile acids are both hydrophilic (because of the polar peptide bond plus the carboxyl and hydroxyl groups on the surface) and hydrophobic (because of the cycloperhy-drophenanthrene nucleus). They are amphipathic, a characteristic that allows them to form micelles. When the critical micelle concentration is reached, lipids collect with cholesterol in the hydrophobic center, and the amphipathic lipids (phospholipids and glycerides) line up with their hydrophobic tails in the center and their hydrophilic heads on the outside. These micelles play an essential role in solubilizing the lipids and lipid-soluble vitamins so as to make them sensitive to enzymatic hydrolysis, as well as to facilitate their intestinal absorption. In addition, the bile acids also solu-bilize the lipids present in bile (including cholesterol, thus avoiding calculus formation); they stimulate bile secretion as well as colic motility. They are also an essential element of the homeostasis of cholesterol in the body.

The small intestine produces its own digestive enzymes that are synthesized in the enterocytes, the most abundant cells in the intestinal epithelium. These enzymes are either secreted in the intestinal lumen or remain anchored in the brush border of the enterocytes. The most important hydrolases synthesized in the enterocytes are disaccharidases (especially lactase, sucrase, and maltase), amylases, peptidases, nucleases, and alkaline phosphatase. On its luminal side, the brush border is lined by the glycocalix, an amorphous layer rich in neutral and amino sugars, which may serve a protective function.

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