Pancreatic Digestive Enzymes
Pancreatic secretion contains multiple enzymes for digesting all of
the three major types of food: proteins, carbohydrates, and fats. It also
contains large quantities of bicarbonate ions, which play an important role in
neutralizing the acidity of the chyme emptied from the stomach into the
The most important of the pancreatic enzymes for digesting proteins
are trypsin, chymotrypsin, and car-boxypolypeptidase.
By far the most abundant of theseis trypsin.
Trypsin and chymotrypsin split whole and partially digested
proteins into peptides of various sizes but do not cause release of individual
amino acids. However, carboxypolypeptidase does split some peptides into individual
amino acids, thus completing digestion of some proteins all the way to the
amino acid state.
The pancreatic enzyme for digesting carbohydrates is pancreatic amylase, which hydrolyzes
starches, glyco-gen, and most other carbohydrates (except cellulose) to form
mostly disaccharides and a few trisaccharides.
The main enzymes for fat digestion are (1) pancre-atic lipase, which is capable of hydrolyzing neutral fatinto
fatty acids and monoglycerides; (2) cholesterolesterase,
which causes hydrolysis of cholesterol esters;and (3) phospholipase, which splits fatty acids from phospholipids.
When first synthesized in the pancreatic cells, the proteolytic
digestive enzymes are in the inactive forms trypsinogen,
chymotrypsinogen, and procar-boxypolypeptidase, which are all
inactive enzymati-cally. They become activated only after they are secreted
into the intestinal tract. Trypsinogen is activated by an enzyme called enterokinase, which is secreted by the
intestinal mucosa when chyme comes in contact with the mucosa. Also,
trypsinogen can be autocatalytically activated by trypsin that has already been
formed from previously secreted trypsinogen. Chymotrypsinogen is activated by
trypsin to form chy-motrypsin, and procarboxypolypeptidase is activated in a
of Trypsin Inhibitor Prevents Digestion of the Pancreas Itself. It is important that the
proteolyticenzymes of the pancreatic juice not become activated until after
they have been secreted into the intestine because the trypsin and the other
enzymes would digest the pancreas itself. Fortunately, the same cells that
secrete proteolytic enzymes into the acini of the pancreas secrete
simultaneously another substance called trypsin
inhibitor. This substance is formed in the cytoplasm of the glandular
cells, and it prevents acti-vation of trypsin both inside the secretory cells
and in the acini and ducts of the pancreas. And, because it is trypsin that
activates the other pancreatic proteolytic enzymes, trypsin inhibitor prevents
activation of the others as well.
When the pancreas becomes severely damaged or when a duct becomes
blocked, large quantities of pan-creatic secretion sometimes become pooled in
the damaged areas of the pancreas. Under these condi-tions, the effect of
trypsin inhibitor is often over-whelmed, in which case the pancreatic
secretions rapidly become activated and can literally digest the entire
pancreas within a few hours, giving rise to the condition called acute pancreatitis. This sometimes is
lethal because of accompanying circulatory shock; even if not lethal, it
usually leads to a subsequent life-time of pancreatic insufficiency.