Digestive Enzyme - Pancreas

Pancreas

Pancreas is both an endocrine and an exocrine gland, in that it functions to produce endocrinic hormones released into the circulatory system (such as insulin, and glucagon), to control glucose metabolism, and also to secrete digestive/exocrinic pancreatic juice, which is secreted eventually via the pancreatic duct into duodenum. Digestive or exocrine function of pancreas is as significant to the maintenance of health as its endocrine function.

Two population of cells in the pancreatic parenchyma make up its digestive enzymes:

• Ductal cells: Mainly responsible for production of bicarbonate (HCO3), which acts to neutralize the acidity of the stomach chyme entering duodenum through the pylorus. Ductal cells of the pancreas are stimulated by the hormone secretin to produce their bicarbonate-rich secretions, in what is in essence a bio-feedback mechanism; highly acidic stomach chyme entering the duodenum stimulates duodenal cells called "S cells" to prouduce the hormone secretin and release it to the bloodstream. Secretin having entered the blood eventually comes into contact with the pancreatic ductal cells, stimulating them to produce their bicarbonate-rich juice. It is interesting to note that secretin also inhibits production of gastrin by "G cells", and also stimulates acinar cells of the pancreas to produce their pancreatic enzyme.
• Acinar cells: Mainly responsible for production of the inactivate pancreatic enzymes (zymogens) that, once present in the small bowel, become activated and perform their major digestive functions by breaking down proteins, fat, and DNA/RNA. Acinar cells are stimulated by cholecystokinin(CCK), which is a hormone/neurotransmitter produced by the duodenal cells called the "I cells." CCK stimulates production of the pancreatic zymogens.

Pancreatic juice, composed of the secretions of both ductal and acinar cells, is made up of the following digestive enzymes:

• Trypsinogen, which is an inactive(zymogenic) protease that, once activated in the duodenum, into trypsin, breaks down proteins at the basic amino acids. Trypsinogen is activated via the duodenal enzyme enterokinase into its active form trypsin.
• Chymotrypsinogen, which is an inactive (zymogenic) protease that, once activated by duodenal enterokinase, breaks down proteins at their aromatic amino acids. Chymotrypsinogen can also be activated by trypsin.
• Carboxypeptidase, which is a protease that takes off the terminal amino acid group from a protein
• Several elastases that degrade the protein elastin and some other proteins.
• Pancreatic lipase that degrades triglycerides into fatty acids and glycerol.
• Cholesterol esterase
• Phospholipase
• Several nucleases that degrade nucleic acids, like DNAase and RNAase
• Pancreatic amylase that breaks down, besides starch and glycogen, most other carbohydrates. Humans lack the enzyme to digest the carbohydrate cellulose, mainly due to its special hydrogen-bonding structure.

Pancreas's exocrine function owes part of its immaculate function to bio-feedback mechanisms controlling secretion of its juice. The following significant pancreatic bio-feedback mechanisms are essential to the maintenance of pancreatic juice balance/production:

• Secretin, a hormone produced by the duodenal "S cells" in response to the stomach chyme containing high hydrogen atom concentration (high acidicity), is released into the blood stream; upon return to the digestive tract, secretion decreases gastric emptying, increases secretion of the pancreatic ductal cells, as well as stimulating pancreatic acinar cells to release their zymogenic juice.
• Cholecystokinin (CCK) is a unique peptide released by the duodenal "I cells" in response to chyme containing high fat or protein content. Unlike secretin, which is an endocrine hormone, CCK actually works via stimulation of a neuronal circuit, the end-result of which is stimulation of the acinar cells to release their content. CCK also increases gallbladder contraction, resulting in bile squeezed into the cystic duct, common bile duct and eventually the duodenum. Bile of course helps absorption of the fat by emulsifying it, increasing its absorptive surface. Bile is made by the liver, but is stored in the gallbladder.
• Gastric inhibitory peptide (GIP) is produced by the mucosal duodenal cells in response to chyme containing high amounts of carbohydrate, proteins, and fatty acids. Main function of GIP is to decrease gastric emptying.
• Somatostatin is a hormone produced by the mucosal cells of the duodenum and also the "delta cells" of the pancreas. Somatostatin has a major inhibitory effect, including on pancreatic juice production.