CLINICAL CASES
173
Endosaccharidase: An enzyme that randomly hydrolyzes glycosidic bonds
within polysaccharides.
Lipase: An enzyme that hydrolyzes the ester linkage between a fatty acid
and glycerol in a triglyceride.
Pancreas: A major endocrine and exocrine organ located behind the stom-
ach. It secretes pancreatic juice into the duodenum to neutralize the
effluent from the stomach and supply digestive enzymes. It also synthe-
sizes and secretes the hormones insulin, glucagon, and somatostatin into
the bloodstream from cells within the islets of Langerhans.
Zymogen: A proenzyme; an inactive precursor of an enzyme stored in
secretory granules. After secretion, the zymogen is activated by cleavage
of certain peptide bonds either by low pH or by other enzymes.
D ISC U SSIO N
The pancreas is a major exocrine organ that synthesizes and secretes diges-
tive enzymes. It also produces and secretes NaHCO3 to neutralize the acidic
effluent from the stomach. The pancreas also has an important endocrine role
because it synthesizes and secretes the hormones insulin, glucagon, and
somatostatin into the bloodstream from cells within its islets of Langerhans.
The exocrine gland is divided into small globules that are drained by an
intralobular duct. The intralobular ducts feed into the interlobular duct, which
is joined to the main pancreatic duct. The pancreatic duct joins with the com-
mon bile duct (usually) in the hepatopancreatic ampulla, which exits into the
duodenum. The secretory unit of the pancreas consists of the acinus and the
intercalated duct. The epithelial cells of the intercalated duct have high con-
centrations of the enzyme carbonic anhydrase, which generates HCO3- from
CO2 and H2O for neutralization of stomach acid entering the duodenum. The
acinus is a cluster of acinar cells that are grouped around the intercalated duct.
The acinar cells are specialized epithelial cells that synthesize and secrete the
20 or so enzymes that will be used to digest the macromolecules in the lumen
of the intestine. Most of the digestive enzymes, particularly those used to degrade
proteins, are synthesized as zymogens or proenzymes that must be activated.
These proenzymes are synthesized on ribosomes on the rough endoplasmic
reticulum. They are then transported to the Golgi apparatus and are sequestered
in zymogen granules until they are secreted. Storing these inactive enzymes in
zymogen granules protects the acinar cell from digesting itself. Secretion of
these zymogens is regulated by cholecystokinin receptors and muscarinic acetyl-
choline receptors. The proenzymes are activated in the intestine, usually by the
action of trypsin. There are some enzymes that are synthesized and stored as the
active enzymes in the zymogen granules. These include a-amylase, carboxyl
ester lipase, lipase, colipase, RNase, and DNase.
Acute pancreatitis is a result of anatomical changes that arise from two
events. The first is the autodigestion of the acinar cells by inappropriate activation
of the pancreatic enzymes (especially trypsinogen) within the cell. The second is
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