<P> 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 later on in the ileum of the small intestine . In humans, the gastric intrinsic factor protein is encoded by the GIF gene . </P> <P> Haptocorrin (also known as HC, R protein, and transcobalamin I, TCN1) is another glycoprotein secreted by the salivary glands which binds to vitamin B. Vitamin B is acid sensitive and in binding to transcobalamin I it can safely pass through the acidic stomach to the duodenum . In the less acidic environment of the small intestine, pancreatic enzymes digest the glycoprotein carrier and vitamin B can then bind to intrinsic factor . This new complex is then absorbed by the epithelial cells (enterocytes) of the ileum . Inside the cells, B 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 . </P> <P> The intrinsic factor is secreted by the stomach, and so is present in the gastric juice as well as in the gastric mucous membrane . The optimum pH for its action is approximately 7 . Its concentration does not correlate with the amount of HCl or pepsin in the gastric juice, e.g., intrinsic factor may be present even when pepsin is largely absent . 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 . </P> <P> The limited amount of normal human gastric intrinsic factor limits normal efficient absorption of B to about 2 μg per meal, a nominally adequate intake of B . </P>

Where is intrinsic factor secreted what is its function
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