Figure 2.

Absorption of vitamin B12. Pepsinogen is converted to pepsin by hydrochloric acid or by pepsin (autoactivation). Pepsin releases cobalamins (Cbl) bound to proteins from food. The released cobalamins bind in an acidic environment with haptocorin (HC), which are transported in this form to the small intestine. At the same time, intrinsic factor (IF) produced by the parietal cells of the stomach enters the intestine. In the duodenum, the Cbl-HC complex is broken down and a new Cbl-IF complex is formed. The cobalamins in this complex are recognized by cubam receptors located in the distal ileum and absorbed by endocytosis. In intestinal villus cells, the lysosomes break down the Cbl-IF complex and release Cbl via, inter alia, basolateral multidrug-resistance protein 1 (MRP1) into the bloodstream. In the blood, Cbl is bound to one of two proteins: HC or transcobalamin (TCL). Importantly, the main source of active Cbl for cells is the Cbl-TCL complex, which is much more easily taken up by cells than Cbl-HC. It should be remembered that, apart from the described mechanism of absorption, active Cbl is also absorbed by passive diffusion along the entire length of the small intestine [1,7,8,9].