Digestive enzyme

Source: Wikipedia, the free encyclopedia.

Digestive enzymes are a group of

salivary glands, in the secretions of cells lining the stomach, in the pancreatic juice secreted by pancreatic exocrine cells, and in the secretions of cells lining the small and large intestines.[3]

Digestive enzymes are classified based on their target

substrates
:

Diagram of the digestive enzymes in the small intestine and pancreas

In the human digestive system, the main sites of digestion are the mouth, stomach, and small intestine. Digestive enzymes are secreted by different

exocrine
glands including:

  • Salivary glands
  • Gastric glands in the stomach
  • Secretory cells (islets) in the pancreas
  • Secretory glands in the small intestine

Mouth

Complex food substances that are taken by animals and humans must be broken down into simple, soluble, and diffusible substances before they can be absorbed. In the oral cavity, salivary glands secrete an array of enzymes and substances that aid in digestion and also disinfection. They include the following:[8]

  • Lingual lipase: Lipid digestion initiates in the mouth. Lingual lipase starts the digestion of the lipids/fats.
  • ptyalin
    .
  • Lysozyme: Considering that food contains more than just essential nutrients, e.g. bacteria or viruses, the lysozyme offers a limited and non-specific, yet beneficial antiseptic function in digestion.

Of note is the diversity of the salivary glands. There are two types of salivary glands:

  • parotid
    gland.
  • Mixed glands: These glands have both
    mucous cells, and include sublingual and submandibular glands. Their secretion is mucinous and high in viscosity
    .

Stomach

The enzymes that are secreted in the stomach are gastric enzymes. The stomach plays a major role in digestion, both in a mechanical sense by mixing and crushing the food, and also in an enzymatic sense, by digesting it. The following are enzymes produced by the stomach and their respective function:

Hormones or compounds produced by the stomach and their respective function:

  • hydrogen atoms
    (H+), or in lay-terms stomach acid, and is produced by the cells of the stomach called parietal cells. HCl mainly functions to denature the proteins ingested, to destroy any bacteria or virus that remains in the food, and also to activate pepsinogen into pepsin.
  • Intrinsic factor (IF): Intrinsic factor is produced by the parietal cells of the stomach. Vitamin B12 (Vit. B12) is an important vitamin that requires assistance for absorption in terminal ileum. Initially in the saliva, haptocorrin secreted by salivary glands binds Vit. B, creating a Vit. B12-Haptocorrin complex. The purpose of this complex is to protect Vitamin B12 from hydrochloric acid produced in the stomach. Once the stomach content exits the stomach into the duodenum, haptocorrin is cleaved with pancreatic enzymes, releasing the intact vitamin B12. Intrinsic factor (IF) produced by the parietal cells then binds Vitamin B12, creating a Vit. B12-IF complex. This complex is then absorbed at the terminal portion of the ileum.
  • Mucin: The stomach has a priority to destroy the bacteria and viruses using its highly acidic environment but also has a duty to protect its own lining from its acid. The way that the stomach achieves this is by secreting mucin and bicarbonate via its mucous cells, and also by having a rapid cell turn-over.
  • endocrine
    hormone and therefore enters the bloodstream and eventually returns to the stomach where it stimulates parietal cells to produce hydrochloric acid (HCl) and Intrinsic factor (IF).

Of note is the division of function between the cells covering the stomach. There are four types of cells in the stomach:

  • Parietal cells: Produce hydrochloric acid and intrinsic factor.
  • body of stomach
    , which is the middle or superior anatomic portion of the stomach.
  • Mucous neck and pit cells: Produce mucin and bicarbonate to create a "neutral zone" to protect the stomach lining from the acid or irritants in the stomach chyme.
  • antrum
    of the stomach, which is the most inferior region of the stomach.

Secretion by the previous cells is controlled by the

parasympathetic division of the autonomic nervous system) activates the ENS, in turn leading to the release of acetylcholine
. Once present, acetylcholine activates G cells and parietal cells.

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 the duodenum. Digestive or exocrine function of pancreas is as significant to the maintenance of health as its endocrine function.

Two of the 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 produce the hormone secretin and release 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. Secretin also inhibits production of gastrin by "G cells", and also stimulates acinar cells of the pancreas to produce their pancreatic enzyme.
  • 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 intestinal cells (I cells) in the duodenum. CCK stimulates production of the pancreatic zymogens.

Pancreatic juice, composed of the secretions of both ductal and acinar cells, contains the following digestive enzymes:[9]

Some of the preceding endogenous enzymes have pharmaceutical counterparts (pancreatic enzymes) that are administered to people with exocrine pancreatic insufficiency.

The pancreas's exocrine function owes part of its notable reliability to biofeedback mechanisms controlling secretion of the juice. The following significant pancreatic biofeedback mechanisms are essential to the maintenance of pancreatic juice balance/production:[11]

  • 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.
  • 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 production.

Small intestine

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) 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.[12] 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 ampulla 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 break down the chyme released from the stomach into absorbable particles. These enzymes are absorbed whilst peristalsis occurs. Some of these enzymes include:

  • Various
    endopeptidases including dipeptidase and aminopeptidases that convert peptones and polypeptides into amino acids.[13]
  • Maltase: converts maltose into glucose.
  • Lactase: This is a significant enzyme that converts lactose into glucose and galactose. A majority of Middle-Eastern and Asian populations lack this enzyme. This enzyme also decreases with age. As such lactose intolerance is often a common abdominal complaint in the Middle-Eastern, Asian, and older populations, manifesting with bloating, abdominal pain, and osmotic diarrhea.
  • Sucrase: converts sucrose into glucose and fructose.
  • Other disaccharidases

Plants

In carnivorous plants, digestive enzymes and acids break down

nitrates and phosphorus. The absorption of the needed nutrients are usually more efficient than in other plants. Digestive enzymes independently came about in carnivorous plants and animals.[14][15][16]

Some carnivorous plants like the Heliamphora do not use digestive enzymes, but use bacteria to break down the food. These plants do not have digestive juices, but use the rot of the prey.[17]

Some carnivorous plants digestive enzymes:[18]

See also

References

  1. ^ "22.10C: Digestive Processes of the Small Intestine". Medicine LibreTexts. 2018-07-22. Retrieved 2023-08-14.
  2. PMID 35604105
    .
  3. ^ "Enzymes: What Are Enzymes, Pancreas, Digestion & Liver Function". Cleveland Clinic. Retrieved 2023-08-14.
  4. ^ "Lipase | Fat-digesting, Pancreatic, Lipolytic | Britannica". www.britannica.com. Retrieved 2023-08-14.
  5. ^ "Proteolytic enzyme | Description, Types, & Functions | Britannica". www.britannica.com. Retrieved 2023-08-14.
  6. ^ "3.3: Digestion and Absorption of Carbohydrates". Medicine LibreTexts. 2017-06-14. Retrieved 2023-08-14.
  7. ^ "Pancreatic nucleases - Big Chemical Encyclopedia". chempedia.info. Retrieved 2023-08-14.
  8. ^ Brown, Thomas A. "Rapid Review Physiology." Mosby Elsevier, 1st Ed. p. 235
  9. ^ Bowen, R. [1] "Exocrine Secretion of the Pancreas"
  10. ^ Pandol SJ. The Exocrine Pancreas. San Rafael (CA): Morgan & Claypool Life Sciences; 2010
  11. ^ Brown, Thomas A. "Rapid Review Physiology." Mosby Elsevier, 1st Ed. p. 244
  12. S2CID 32097183
    .
  13. ^ "Small Intestinal Brush Border Enzymes".
  14. ^ carnivorousplants.org, digestion
  15. ^ The Uptake of Digestion Products by Drosera, by Chandler, Graeme, 1978
  16. ^ Carnivory of Byblis revisited - A simple method for enzyme testing on carnivorous plants, by Hartmeyer, Siegfried 1997
  17. ^ McPherson, S., A. Wistuba, A. Fleischmann & J. Nerz 2011. Sarraceniaceae of South America. Redfern Natural History Productions, Poole.
  18. PMID 29888132
    .