Page 279 - Color Atlas of Biochemistry

P. 279

270 Tissues and organs

Digestive processes stimulate pancreatic secretion and bile re-
lease.
Gastric juice is the product of several cell
types. The parietal cells produce hydrochloric
acid, chief cells release pepsinogen, and acces- B. Zymogen activation
sory cells form a mucin-containing mucus. To prevent self–digestion, the pancreas re-
leases most proteolytic enzymes into the du-
odenum in an inactive form as proenzymes
A. Formation of hydrochloric acid
(zymogens). Additional protection from the
+
–
The secretion of hydrochloric acid (H and Cl ) effects of premature activation of pancreatic
by the parietal cells is an active process that proteinasesisprovidedby proteinase inhibi-
uses up ATP and takes place against a concen- tors in the pancreatic tissue, which inactivate
tration gradient (in the gastric lumen, with a active enzymes by complex formation (right).
+
pH of 1, the H concentration is some 10 6 Trypsinogen plays a key role among the
times higher than in the parietal cells, which proenzymes released by the pancreas. In the
have a pH of 7). bowel, it is proteolytically converted into ac-
+
The precursors of the exported H ions are tive trypsin (see p. 176) by enteropeptidase,a
carbon dioxide (CO 2 )and water (H 2 O). CO 2 membrane enzyme on the surface of the en-
diffuses from the blood into the parietal cells, terocytes. Trypsin then autocatalytically acti-
and in a reaction catalyzed by carbonate de- vates additional trypsinogen molecules and
hydratase (carbonic anhydrase [2]), it reacts the other proenzymes (left).
+
with H 2 OtoformH and hydrogen carbonate
–
+
(HCO 3 ). The H ions are transported into the
+
gastric lumen in exchange for K by a mem- C. Fat digestion
+
+
brane-bound H /K -exchanging ATPase [1] (a Dueto the “hydrophobic effect” (seep. 28),
transport ATPase of the P type; see p. 220). water-insoluble neutral fats in the aqueous
The remaining hydrogen carbonate is re- environment of the bowel lumen would ag-
leased into the interstitium in electroneutral gregate into drops of fat in which most of the
–
antiport in exchange for chloride ions (Cl ), molecules would not be accessible to pancre-
–
andfrom thereinto the blood. TheCl ions atic lipase. The amphipathic substances in bile
follow the secreted protons through a chan- (bile acids, bile salts, phospholipids) create an
nel into the gastric lumen. emulsion in which they occupy the surface of
The hydrochloric acid in gastric juice is the droplets and thereby prevent them from
important for digestion. It activates pepsin- coalescing into large drops. In addition, the
ogen to form pepsin (see below) and creates bile salts, together with the auxiliary protein
an optimal pH level for it to take effect. It also colipase, mediate binding of triacylglycerol
denatures food proteins so that they are more lipase [1] to the emulsified fat droplets. Acti-
easily attacked by proteinases, and it kills vation of the lipase is triggered by a confor-
micro-organisms. mation change in the C-terminal domain of
Regulation. HCl secretion is stimulated by the enzyme, which uncovers the active center.
the peptide hormone gastrin, the mediator During passage through the intestines, the
histamine (see p. 380), and—via the neuro- active lipase breaks down the triacylglycerols
transmitter acetylcholine—by the autonomous in the interior of the droplets into free fatty
nervous system. The peptide somatostatin acids and amphipathic monoacylglycerols.
and certain prostaglandins (see p. 390) have Over time, smaller micelles develop (see
inhibitory effects. Together with cholecysto- p. 28), in the envelope of which monoacylgly-
kinin, secretin, and other peptides, gastrin cerols are present in addition to bile salts and
belongs to the group of gastrointestinal hor- phospholipids. Finally, the components of the
mones (see p. 370). All of these are formed in micelles are resorbed by the enterocytes in
the gastrointestinal tract and mainly act in the ways that have not yet been explained.
vicinity of thesitewhere they areformed— Monoacylglycerols and fatty acids are re-
i. e.,they are paracrinehormones (seep. 372). assembled into fats again (see p. 272), while
While gastrin primarily enhances HCl the bile acids return to the liver (enterohe-
secretion, cholecystokinin and secretin mainly patic circulation; see p. 314).

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