Human Physiology, 14th edition (2016)

644 Chapter 18

The activation of trypsin, therefore, is the triggering event

for the activation of other pancreatic enzymes. Actually, the

pancreas does produce small amounts of active trypsin, but

the other enzymes are not activated until pancreatic juice has

entered the duodenum. This is because pancreatic juice also

contains a small protein called pancreatic trypsin inhibitor that

attaches to trypsin and inhibits its activity in the pancreas.

is minimized. The inactive form of trypsin, called trypsinogen, is

activated within the small intestine by the catalytic action of the

brush border enzyme enterokinase (also called enteropeptidase ).

Enterokinase converts trypsinogen to active trypsin. Trypsin, in

turn, activates the other zymogens of pancreatic juice ( fig. 18.29 )

by cleaving off polypeptide sequences that inhibit the activity of

these enzymes.

Figure 18.28 Secretion of bicarbonate into

pancreatic juice. Cells of the pancreatic duct take in CO 2

from the blood and use it to generate carbonic acid (H 2 CO 3 ).

This dissociates into bicarbonate (HCO 32 ) and H^1. The HCO 32 is

secreted into the lumen of the duct by a carrier that exchanges it

for Cl^2. The Cl^2 then leaks passively back into the lumen through

a different, CFTR chloride channel (see text for details).

Lumen

HCO 3

HCO 3

H 2 CO 3

CI–

CI– CI–

H+

K+

K+

K+

Na+

Na+

AT P

CO 2

H+

K+

Na+

Na+

Blood

+

COCOCOCOCOCOCOCOCOCOCOCOC 222222222

NaNaNaNaNaNaNaNaNa++++++++++

NaNaNaNaNaNaNaNaNNNaNaNaNaNaNa++++++++++++

HCO– 3

Figure 18.29 The activation of pancreatic juice

enzymes. The pancreatic protein-digesting enzyme trypsin is

secreted in an inactive form known as trypsinogen. This inactive

enzyme (zymogen) is activated by a brush border enzyme,

enterokinase (EN), located in the cell membrane of microvilli.

Active trypsin in turn activates other zymogens in pancreatic

juice.

Epithelial cell of duodenum

Microvilli

Trypsinogen

Other inactive

enzymes

(zymogens)

Pancreatic

juice

Trypsin

Active

enzymes

En En

En

En En

Table 18.4 | Enzymes Contained in Pancreatic Juice

Enzyme Zymogen Activator Action

Trypsin Trypsinogen Enterokinase Cleaves internal peptide bonds

Chymotrypsin Chymotrypsinogen Trypsin Cleaves internal peptide bonds

Elastase Proelastase Trypsin Cleaves internal peptide bonds

Carboxypeptidase Procarboxypeptidase Trypsin Cleaves last amino acid from carboxyl-terminal end of polypeptide

Phospholipase Prophospholipase Trypsin Cleaves fatty acids from phospholipids such as lecithin

Lipase None None Cleaves fatty acids from glycerol

Amylase None None Digests starch to maltose and short chains of glucose molecules

Cholesterolesterase None None Releases cholesterol from its bonds with other molecules

Ribonuclease None None Cleaves RNA to form short chains

Deoxyribonuclease None None Cleaves DNA to form short chains

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