enzymes and their activities appears in
. 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
is embedded in the luminal plasma membranes of the intestinal
epithelial cells. It is a proteolytic enzyme that splits off a peptide
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
). 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 (
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 (
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-
Trypsin, chymotrypsin, elastase
Break peptide bonds in proteins to form peptide fragments
Splits off terminal amino acid from carboxyl end of protein
Splits off two fatty acids from triglycerides, forming free fatty
acids and monoglycerides
Splits polysaccharides into glucose and maltose
Split nucleic acids into free mononucleotides
Activation of pancreatic enzyme precursors in the small intestine.
Hormonal regulation of pancreatic bicarbonate secretion. Dashed
indicates that neutralization of intestinal acid (
turns off secretin secretion (negative feedback).
Neutralization of intestinal acid
Flow of bicarbonate into small intestine
Acid from stomach