 intrinsic factor (if), also known as gastric intrinsic factor (gif), is a glycoprotein produced by the parietal cells of the stomach. it is necessary for the absorption of vitamin b12 (cobalamin) later on in the small intestine. in humans, the gastric intrinsic factor protein is encoded by the gif gene. haptocorrin (also known as hc, r protein, and transcobalamin i, tcn1), is a glycoprotein secreted by the salivary glands which binds to vitamin b12. vitamin b12 is acid sensitive and in binding to transcobalamin i it can safely pass through the acidic stomach to the duodenum. here in the less acidic environment of the small intestine, pancreatic enzymes digest the glycoprotein carrier and vitaminb12 can then bind to intrinsic factor. this new complex is then absorbed by the epithelial cells (enterocytes) of the ileum. inside the cells, b12 dissociates once again and binds to another protein,  transcobalamin ii (tcn2). the new complex can then exit the epithelial cells to be carried to the liver. == site of secretion == the intrinsic factor is 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 45&nbsp;Â°c. 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. in human beings it is present in the fundus and body of the stomach. == clinical significance == in pernicious anemia, which is usually an autoimmune disease, autoantibodies directed against intrinsic factor or parietal cells themselves lead to an intrinsic factor deficiency, malabsorption of vitamin b12, and subsequent megaloblastic anemia. atrophic gastritis can also cause intrinsic factor deficiency and anemia through damage to the parietal cells of the stomach wall. pancreatic exocrine insufficiency can interfere with normal dissociation of vitamin b12 from its binding proteins in the small intestine, preventing its absorption
Gastroenterology Genes_mutated_in_mice Hematology Human_proteins Stomach