Digestive Enzyme - Small Intestine

Small Intestine

The small intestine is traditionally divided into three anatomic sections defined from their distance from the pyloric sphincter:

• duodenum: It is the first portion of the small bowel that is itself divided into four distinct anatomic positions called first, second, third, and fourth sections of duodenum. Duodenum is the only portion of the small bowel that is partially retroperitoneal, and peritoneal. Duodenum is a secretary portion of the small intestine.
• jejunum: This is the section of the small intestine that begins immediately from the insertion point of the ligament of Treitz, and is the longest of the three sections. Jejunum is an absorptive surface.
• ileum: This is the terminal portion of the small intestine and as such has a limited but vital role in absorption. Vitamin B12 and bile are absorbed at this portion.

The following enzymes/hormones are produced in the duodenum:

• secretin: This is an endocrine hormone produced by the duodenal "S cells" in response to the acidity of the gastric chyme.
• Cholecystokinin (CCK): This is not by definition a hormone; there is new evidence suggesting that CCK works by a very complex neuronal bi-directional pathway. Regardless of its pathway, its eventual role is to increase secretion of acinar cells and increased production of pancreatic juice. CCK also increases gallbladder contraction, causing release of pre-stored bile into the cystic duct, and eventually into the common bile duct and via the ampulla of Vater into the second anatomic position of the duodenum. CCK also decreases the tone of the sphincter of Oddi, which is the sphincter that regulates flow through the ampula of Vater. CCK also decreases gastric activity and decreases gastric emptying, thereby giving more time to the pancreatic juices to neutralize the acidity of the gastric chyme.
• Gastric inhibitory peptide (GIP): This peptide decreases gastric motility and is produced by duodenal mucosal cells.
• motilin: This substance increases gastro-intestinal motility via specialized receptors called "motilin receptors."
• somatostatin: This hormone is produced by duodenal mucosa and also by the delta cells of the pancreas. Its main function is to inhibit a variety of secretory mechanisms.

Throughout the lining of the small intestine there are numerous "brush border" enzymes whose function is to further cleave the already-broken-down products of digestion into absorbable particles. Some of these enzymes include:

• Sucrase
• Lactase: This is a significant brush border enzyme in that a majority of Middleastern and Asian population lack this enzyme and also this enzyme decreases with age, and as such lactose intolerance is often a common abdominal complaint in the Middleastern, Asian, and older population, manifesting with bloating, abdominal pain, and osmotic diarrhea.
• Maltase
• Other disaccharidases