Gastric Intrinsic Factor (GIF)

Intrinsic factor is a glycoprotein produced by the parietal cells of the stomach. It is necessary for the absorption of vitamin B₁₂ later on in the terminal ileum.

Upon entry into the stomach, vitamin B₁₂ becomes bound to haptocorrin (R factor), a glycoprotein. The resulting complex enters the duodenum, where pancreatic enzymes digest haptocorrin. In the less acidic environment of the small intestine, B₁₂ can then bind to intrinsic factor. This new complex travels to the ileum, where special epithelial cells endocytose them. Inside the cell, B₁₂ dissociate once again and bind to another protein, transcobalamin II. The new complex can exit the epithelial cells to enter the liver.