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Intrinsic factor

 

 
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Intrinsic factor
 
 
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Intrinsic factor is a glycoprotein
Glycoprotein

Not to be confused with peptidoglycan or proteoglycan.Glycoproteins are proteins that contain oligosaccharide chains covalently attached to their Peptide side-chains....
 produced by the parietal cell
Parietal cell

Parietal cells, or oxyntic cells, are the stomach epithelium cell s that secrete gastric acid and intrinsic factor....
s of the stomach
Stomach

In most mammals, the stomach is a hollow muscular organ of the gastrointestinal tract involved in the second phase of digestion, following mastication....
. It is necessary for the absorption of vitamin B
Vitamin B12

Vitamin B12 is a water soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood....
 later on in the terminal ileum
Terminal ileum

The terminal ileum is the most distal part of the small intestine. It connects to the cecum via the ileocecal valve.It is of importance in medicine as it can be affected in a number of infectious and inflammatory conditions, including:...
.

Upon entry into the stomach, vitamin B becomes bound to one of two B binding proteins present in gastric juice
Gastric juice

Gastric juice is a strong acidic liquid, pH 1 to 3 in humans, which is close to being colourless. The hormone gastrin is released into the bloodstream when peptides are detected in the stomach....
. In the less acidic environment of the small intestine, these proteins dissociate from the vitamin, enabling it to bind to intrinsic factor and enter the portal circulation
Portal venous system

In human anatomy, the hepatic portal system is the system of veins that comprises the hepatic portal vein and its tributaries. It is also called the portal venous system, although it is not the only example of a portal venous system, and splanchnic veins, which is not synonymous with hepatic portal system and imprecise...
 through a receptor in the ileal
Ileum

The ileum is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum....
 mucosa specific for the B-intrinsic factor complex.

intrinsic factor is an enzyme-like unidentified substance secreted by the stomach.






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Intrinsic factor is a glycoprotein
Glycoprotein

Not to be confused with peptidoglycan or proteoglycan.Glycoproteins are proteins that contain oligosaccharide chains covalently attached to their Peptide side-chains....
 produced by the parietal cell
Parietal cell

Parietal cells, or oxyntic cells, are the stomach epithelium cell s that secrete gastric acid and intrinsic factor....
s of the stomach
Stomach

In most mammals, the stomach is a hollow muscular organ of the gastrointestinal tract involved in the second phase of digestion, following mastication....
. It is necessary for the absorption of vitamin B
Vitamin B12

Vitamin B12 is a water soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood....
 later on in the terminal ileum
Terminal ileum

The terminal ileum is the most distal part of the small intestine. It connects to the cecum via the ileocecal valve.It is of importance in medicine as it can be affected in a number of infectious and inflammatory conditions, including:...
.

Upon entry into the stomach, vitamin B becomes bound to one of two B binding proteins present in gastric juice
Gastric juice

Gastric juice is a strong acidic liquid, pH 1 to 3 in humans, which is close to being colourless. The hormone gastrin is released into the bloodstream when peptides are detected in the stomach....
. In the less acidic environment of the small intestine, these proteins dissociate from the vitamin, enabling it to bind to intrinsic factor and enter the portal circulation
Portal venous system

In human anatomy, the hepatic portal system is the system of veins that comprises the hepatic portal vein and its tributaries. It is also called the portal venous system, although it is not the only example of a portal venous system, and splanchnic veins, which is not synonymous with hepatic portal system and imprecise...
 through a receptor in the ileal
Ileum

The ileum is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum....
 mucosa specific for the B-intrinsic factor complex.

Site of Secretion

The intrinsic factor is an enzyme-like unidentified substance secreted by the stomach. It is present in the gastric juice as well as in the gastric mucous membrane. The optimum pH for its action is 7 and it is inactivated at temperatures above 45oC. It does not necessarily run parallel with the amount of HCl or pepsin in the gastric juice. So in some cases, the intrinsic factor may be present even if there is no HCl or Pepsin or vice versa. The site of formation of the intrinsic factor varies in different species. In pigs it is obtained from the pylorus and beginning of the duodenum
Duodenum

The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms anterior intestine or proximal intestine may be used instead of duodenum....
. In human beings it is present in the fundus and body of the stomach
Stomach

In most mammals, the stomach is a hollow muscular organ of the gastrointestinal tract involved in the second phase of digestion, following mastication....
.

Clinical significance

In pernicious anemia
Pernicious anemia

Pernicious anemia is a form of megaloblastic anemia due to vitamin B-12 Avitaminosis, caused by impaired absorption of vitamin B-12 due to the absence of intrinsic factor in the setting of atrophic gastritis, and more specifically of loss of stomach parietal cells....
, an autoimmune disease
Autoimmune disease

Autoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. In other words, the body attacks its own cells....
, autoantibodies
Autoantibody

An autoantibody is an antibody manufactured by the immune system that is directed against one or more of the individual's own proteins.It is derived from the Greek "auto" which means "self", "anti" which means "against" and "body"....
 directed against intrinsic factor or parietal cells themselves lead to an intrinsic factor deficiency, malabsorption
Malabsorption

Malabsorption is a state arising from abnormality in digestion or absorption of Nutrient across the gastrointestinal tract.Impairment can be of single or multiple nutrients depending on the abnormality....
 of vitamin B, and subsequent megaloblastic anemia
Megaloblastic anemia

Megaloblastic anemia is an anemia which results from inhibition of DNA synthesis in red blood cell production. This is often due to deficiency of vitamin B12 and/or folic acid....
. Atrophic gastritis
Atrophic gastritis

Atrophic gastritis is a process of chronic inflammation of the stomach mucosa, leading to loss of gastric glandular cells and their eventual replacement by intestinal and fibrous tissues....
 can also cause intrinsic factor deficiency and anemia through damage to the parietal cells of the stomach wall. Pancreatic exocrine insufficiency
Exocrine pancreatic insufficiency

Exocrine pancreatic insufficiency is the inability to properly digest food due to a lack of digestive enzymes made by the pancreas. This disease is found frequently in dogs....
 can interfere with normal dissociation of vitamin B from its binding proteins in the small intestine, preventing its absorption via the intrinsic factor complex.

Bariatric surgery
Bariatric surgery

Bariatric surgery, also known as weight loss surgery, refers to the various surgical procedures performed to treat obesity by modification of the gastrointestinal tract to reduce nutrient intake and/or absorption....
 is a known risk factor in the development of pernicious anemia. Other risk factors contributing to this condition are stomach tumors, gastric ulcers, and excessive consumption of alcohol.

Treatment

Patients experiencing an insufficiency in their intrinsic factor levels cannot benefit from a low dose oral vitamin B-12 supplement, because it will not absorb through the wall of the small intestine. Historically, the disease was thought untreatable before the discovery that it could be managed with regular injections of vitamin B-12, thus bypassing the digestive tract. More recently, Swedish researchers discovered that sufficiently large doses of B-12 can also be absorbed sublingual
Sublingual

Sublingual, literally 'under the tongue', from Latin, refers to the pharmacological route of administration by which medications diffuse into the blood through tissues under the tongue....
ly, and it may be possible to avoid injectable B-12. However, as of yet, no standards have been set for treatment by the sublingual route and injections of B-12 are the only reliable method of treatment.

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