Intrinsic factor

Intrinsic factor (IF), also known as cobalamin binding intrinsic factor, or gastric intrinsic factor (GIF), is a glycoprotein produced by specialized cells of the stomach, either by parietal cells (in humans) or chief cells (in rodents). It is necessary for the absorption of vitamin B12 later on in the distal ileum of the small intestine. In humans, the gastric intrinsic factor protein is encoded by the CBLIF gene. Haptocorrin (transcobalamin I) is another glycoprotein secreted by the salivary glands which binds to vitamin B12. Vitamin B12 is acid-sensitive and in binding to haptocorrin 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 B12 can then bind to intrinsic factor. This new complex is then absorbed by the epithelial cells (enterocytes) of the ileum. Inside the cells, vitamin B12 dissociates once again and binds to another protein, transcobalamin II; the new complex can then exit the epithelial cells to be carried to the liver.