The intrinsic factor (IF) is a glycoprotein produced by the parietal cells (oxyntic cells) located at the gastric body and fundus. It plays a crucial role in the transportation and absorption of the vital micronutrient vitamin B12 (cobalamin, Cbl) by the terminal ileum. Insufficiency of intrinsic factor could lead to devastating consequences on body homeostasis. The effect ranges from hematological to neurological disorders and, in unfortunate cases, fatal cardiovascular disease.
Cbl binds the first Cbl-binding protein haptocorrin (HC) in the stomach, following Cbl separation from the protein of food, to form the Cbl-HC complex. In the duodenum, pancreatic proteases break the HC-Cbl complex. Therefore, Cbl binds IF to form the IF-Cbl complex the ligand of the cubam receptor localized in the distal ileum. Subsequently, receptor-mediated endocytosis into enterocyte accomplished. Finally, lysosomal enzymes liberate the Cbl molecule from the complex to travel directly to the target tissue through the plasma riding transcobalamin II (TC II).
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