Figure 3
From: Inherited cobalamin malabsorption. Mutations in three genes reveal functional and ethnic patterns
Release of cobalamin from the food and intestinal uptake. Cobalamin (Cbl) uptake and transport pathway from food intake to the portal system. Cbl is released from food proteins (P) by salivary and gastric enzymes and then binds to haptocorrin (H). In the proximal intestine, pancreatic enzymes degrade haptocorrin and Cbl binds to intrinsic factor (IF). In the ileum, the Cbl-IF complex binds to the cubam receptor (CUBN-AMN) and enters the enterocyte (ileal epithelia). Inside the cell, IF is degraded and Cbl is moved to the portal system by an unknown process. The cubam receptor is recycled back to the membrane. In the blood, transcobalamin 2 (TC2) transports Cbl to the tissues. The role of haptocorrin carrying 80% of the Cbl serum fraction is unknown.