Receptor for vitamin B12-intrinsic factor complex is located in:
Ref: W F Canong. Digestion. Absorption, & Nutritional Principles. In: Review of Medical Physiology. 24th ed. California: McGraw-Hill Companies: 2012, p485.Explanation:"Most vitamins are absorbed in the upper small intestine, hut Vitamin B12 is absorbed in the terminal ileum " (Ref: Ganong)Vitamin B12 AbsorptionProtein-bound vitamin B12 must be released from the proteins by the action of digestive proteases in both the stomach and small intestine.Gastric acid & Pepsin releases the vitamin from food particle.R-proteins (such as haptocorrins [aka transcobalamin 1) and cobalaphilin) are B12 binding proteins that are produced in the salivary glands.V itamin B12 then hinds to the R-Proteins to avoid degradation in acidic stomach environmentThe next binding protein is intrinsic factor (IF), a protein synthesized by gastric parietal cellsIn the duodenum, proteases digest R-proteins and release B12. which then binds to IF. to form a complex (IF/B12).Vitamin B12 must be attached to IF for it to be absorbed, as receptors on the enterocytes in the terminal ileum of the small bowel only recognize the B12-IF complexIF also protects the vitamin from catabolism by intestinal bacteria.Once the IF/B 12 complex is recognized by- specialized ileal receptors, it is transported into the portal circulation.The vitamin is then transferred to transcob- alaniin II (TC-II/BI2). which serves as the plasma transporter.
Ileum
Colon
Jejunum
Duodenum
Digestion and Absorption