Pancreatic Enzymes for Digestion – Types, Control and Action

What are pancreatic enzymes?

Pancreatic enzymes are the chemical catalysts secreted by the pancreas into the duodenum to digest food. This is known as the exocrine functions of the pancreas. Being a compound gland, the pancreas also has endocrine functions which is to release hormones like insulin and glucagon in response to changes in the blood sugar levels. These hormones also play a role in controlling the secretion of digestive enzymes. However, it is the enzymes itself that function in the small intestine to break down foods into simpler nutrients so that it can be easily absorbed into the blood stream. Along with water, bicarbonates ions and mucus, the total exocrine secretion from the pancreas is approximately 1 liter per day.

These digestive enzymes from the pancreas are secreted by the acini within the gland and this empties into the pancreatic duct. From here, it joins the common bile duct and empties into the duodenum of the small intestine. Apart from the enzymes for digestion, the pancreas also secretes bicarbonate ions and water. These compounds are secreted by the epithelial cells that line the pancreatic duct and help to neutralize the gastric acid from the stomach in addition to transporting the pancreatic enzymes out of the pancreas. The pancreatic enzymes do not act in isolation – digestive enzymes from other structures of the gastrointestinal tract play a role in the complete digestion of food.

Types of Pancreatic Enzymes

The pancreatic enzymes can be categorized according to their action on different types of food – carbohydrates, protein and fats.

Pancreatic Enzymes for Protein Digestion

There are three pancreatic enzymes for the digestion of proteins within the food in the small intestine. These type of enzymes are known as proteases.

1. Trypsin
2. Chymotrypsin
3. Carboxypolypeptidase
4. Elastase
   

Of these, trypsin is the most abundant. The pancreas do not secrete these enzymes in it active form as this may result in digestion of the pancreas itself. Instead each enzyme is secreted in an inactive form as follows :

• Trypsin is secreted as trypsinogen
• Chymotrpsin is secreted as chymotripsinogen
• Carboxypolypeptidase is secreted as procarboxypolypeptidase
• Proelastase is secreted as elastase

Once in the lumen of the duodenum, these enzymes are activated. Trypsinogen, which is the most abundant of the three pancreatic enzymes, is activated by the presence of enterokinase which is an enzyme secreted by the lining of the small intestine. Further trypsinogen is also activated by the presence of trypsin in the gut.

Chymotrypsin, procarboxypolypeptidase and proelastase are in turn activated by trypsin. The pancreas also has other protective mechanisms to ensure that these enzymes do not damage it. The cells of the pancreas secrete trypsin inhibitor which ensures that trypsin is not activated within the pancreas and it turn, neither are the other proteases.

Action of Trypsin, Chymotrypsin and Carboxypolypetidase

Trypsin and chymotrypsin breaks down protein into peptides. Carboxypolypetidase in turn breaks down some peptides into individual amino acids but it is the peptidases secreted by the enterocytes of the small intestine that digest the rest. Elastase is more specific and digests collagen, particularly the elastin that holds fibers in meat together.

Pancreatic Enzymes for Carbohydrate Digestion

Pancreatic amylase is the enzyme responsible for carbohydrate digestion.

Action of Pancreatic Amylase

Pancreatic amylase hydrolyzes most carbohydrates into disaccharides and trisaccharides. These in turn are digested into monosaccharides by the small intestine enzymes – lactase, sucrase, maltase and iso-maltase.

Pancreatic Enzymes for Lipid Digestion

There are three pancreatic enzymes responsible for lipid digestion, each of which act on the main types of lipids in the body.

• Pancreatic lipase
• Cholesterol esterase
• Phospholipase

The pancreatic enzymes for lipid digestion are dependent on the secretion of bile from the liver and gallbladder. Bile acids emulsify fats meaning that it breaks down the fat into smaller globules. This allows the lipid enzymes to act more effectively on fats and sterols in the food.

Action of Pancreatic Lipase, Cholesterol Esterase and Phospholipase

The most important and abundant of these pancreatic lipid enzymes is pancreatic lipase. It breaks down triglycerides (neutral fats) into fatty acids and glycerol. Cholesterol esterase hydrolyzes cholesterol esters in the food while phospholipase acts on phospholipids. The remaining digestive action on fats are handled by the small intestine lipase.

Control of Pancreatic Enzyme Secretion

The secretion of digestive enzymes is regulated  by nerve impulses and digestive hormones. When the stomach secretions are stimulated by the vagus nerves and enteric nervous system as discussed under stomach nerves, some of these impulses also trigger the release of acetylcholine in the pancreas. Acetylcholine along with cholecystokinin (CCK), which is a digestive hormone secreted in response to food in the duodenum, triggers the release of pancreatic enzymes into the acini.

Another digestive hormone known as secretin causes the secretion of large quantities of bicarbonate ions and water within the pancreas. This washes out the pancreatic enzymes in the acini into the duodenum. Secretin is released in response to acidic chyme in the duodenum and also neutralizes the gastric acid secretions that enter the duodenum.

Related Articles

1. Pancreatic Enzyme Deficiency
2. Pancreas Enzyme Tests
3. List of Digestive Enzymes
4. Digestive Hormones

Article reviewed by Dr. Greg. Last updated on August 21, 2010