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 duodenum.
The most important of the pancreatic enzymes for digesting proteins are trypsin, chymotrypsin, and car boxypolypeptidase. By far the most abundant of these is 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, car boxypolypeptidase splits 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, glycogen, and most other carbohydrates (except cellulose) to form mostly disaccharides and a few trisaccharides.
The main enzymes for fat digestion are (1) pancreatic lipase, which is capable of hydrolyzing neutral fat into fatty acids and monoglycerides; (2) cholesterol esterase, 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 their enzymatically inactive forms trypsinogen, chymotrypsinogen, and pro carboxypolypeptidase. 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. Trypsinogen also can be autocatalytically activated by trypsin that has already been formed from previously secreted trypsinogen. Chymotrypsinogen is activated by trypsin to form chymotrypsin, and procarboxypolypeptidase is activated in a similar manner.
Secretion of Trypsin Inhibitor Prevents Digestion of the Pancreas. It is important that the proteolytic enzymes 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. Fortunately, the same cells that secrete proteolytic enzymes into the acini of the pancreas simultaneously secrete another substance called trypsin inhibitor. This substance, which is formed in the cytoplasm of the glandular cells, prevents activation of trypsin both inside the secretory cells and in the acini and ducts of the pancreas. In addition, because it is trypsin that activates the other pancreatic proteolytic enzymes, trypsin inhibitor prevents activation of the other enzymes as well.
When the pancreas becomes severely damaged or when a duct becomes blocked, large quantities of pancreatic secretion sometimes become pooled in the damaged areas of the pancreas. Under these conditions, the effect of trypsin inhibitor is often overwhelmed, 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 condition is sometimes lethal because of accompanying circulatory shock; even if it is not lethal, it usually leads to a subsequent lifetime of pancreatic insufficiency.
