552
Chapter 15
enzymes and their activities appears in
Table 15–6
. The pro-
teolytic enzymes are secreted in inactive forms (zymogens), as
described for pepsinogen in the stomach, and then activated in
the duodenum by other enzymes. Like pepsinogen, the secre-
tion of zymogens protects pancreatic cells from autodigestion.
A key step in this activation is mediated by
enterokinase,
which
is embedded in the luminal plasma membranes of the intestinal
epithelial cells. It is a proteolytic enzyme that splits off a peptide
from pancreatic
trypsinogen,
forming the active enzyme tryp-
sin. Trypsin is also a proteolytic enzyme, and once activated, it
activates the other pancreatic zymogens by splitting off peptide
fragments (
Figure 15–26
). This activating function is in addi-
tion to the role of trypsin in digesting ingested protein.
The nonproteolytic enzymes secreted by the pancreas
(e.g., amylase and lipase) are released in fully active form.
Pancreatic secretion increases during a meal, mainly as a
result of stimulation by the hormones secretin and CCK (see
Table 15–4). Secretin is the primary stimulant for bicarbonate
secretion, whereas CCK mainly stimulates enzyme secretion.
Because the function of pancreatic bicarbonate is to
neutralize acid entering the duodenum from the stomach,
it is appropriate that the major stimulus for secretin release
is increased acidity in the duodenum (
Figure 15–27
). In
analogous fashion, CCK stimulates the secretion of digestive
enzymes, including those for fat and protein digestion, so it is
appropriate that the stimuli for its release are fatty acids and
amino acids in the duodenum (
Figure 15–28
).
Luminal acid and fatty acids also act on afferent nerve
endings in the intestinal wall, initiating refl exes that act on
the pancreas to increase both enzyme and bicarbonate secre-
Table 15–6
Pancreatic Enzymes
Enzyme
Substrate
Action
Trypsin, chymotrypsin, elastase
Proteins
Break peptide bonds in proteins to form peptide fragments
Carboxypeptidase
Proteins
Splits off terminal amino acid from carboxyl end of protein
Lipase
Fats
Splits off two fatty acids from triglycerides, forming free fatty
acids and monoglycerides
Amylase
Polysaccharides
Splits polysaccharides into glucose and maltose
Ribonuclease, deoxyribonuclease
Nucleic acids
Split nucleic acids into free mononucleotides
Membrane-bound
enterokinase
Epithelial cell
Pancreas
Inactive enzymes
Trypsinogen
Trypsin
Active enzymes
Intestinal lumen
Figure 15–26
Activation of pancreatic enzyme precursors in the small intestine.
Figure 15–27
Hormonal regulation of pancreatic bicarbonate secretion. Dashed
line and
E
indicates that neutralization of intestinal acid (
pH)
turns off secretin secretion (negative feedback).
Small intestine
Neutralization of intestinal acid
Flow of bicarbonate into small intestine
Pancreas
Bicarbonate secretion
Small intestine
Secretin secretion
Acid from stomach
Plasma secretin
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