Intrinsic factor (IF) is essential for vitamin B12 absorption. IF is secreted by gastric parietal cells and binds B12 in the stomach. The IF-B12 complex is absorbed in the terminal ileum by specific receptors. What are these receptors and what happens to IF after endocytosis?

• A Cubilin-amnionless receptor complex on ileal enterocytes; IF is degraded intracellularly while B12 is transported to portal blood via transcobalamin II ✓
• B Megalin receptors on ileal brush border; IF-B12 complex enters intact and IF recycles to the intestinal lumen
• C FcRn receptors on ileal enterocytes; the complex undergoes transcytosis to the basolateral surface intact
• D PCFT transporters (proton-coupled folate transporter) on ileal enterocytes; IF acts as a cofactor for PCFT-mediated B12 uptake

Explanation

The IF-B12 complex binds to the cubilin-amnionless (CUBAM) receptor complex on the brush border of terminal ileal enterocytes. After receptor-mediated endocytosis, the complex is degraded in lysosomes; intrinsic factor is broken down while vitamin B12 is released and transferred across the basolateral membrane bound to transcobalamin II, which is the transport protein that delivers B12 to portal blood and then to body tissues. This explains why patients with autoimmune gastritis lacking IF, or with terminal ileal disease (Crohn's), develop B12 deficiency and megaloblastic anemia.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.