Intrinsic factor

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 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, 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.