ABSORPTION

Iron is absorbed through epithelial cells of the small intestine. Particularly the enterocytes of the duodenum are involved in this process.

Figure
The intestinal wall contains structures termed villi. Each villus contain enterocytes that have microvilli in the cell wall facing the intestinal lumen.

.

Most of the non-haem iron in the diet consists of Fe³⁺ - ferric iron, which must be reduced to Fe²⁺ - ferrous iron by a ferric reductase (DcytB) before uptake in the enterocytes. This is how other oxidising/reducing agents of the diet can influence iron absorption (e.g. vitamin C which is is a reducing agent that enhances iron absorption).

Figure Normal absorption of haem iron and non-haem iron in duodenal enterocytes and effect of ligands in the diet.
Iron is transported via ferroportin, is oxidised to ferric iron by hephaestin (ceruloplasmin-like, Cu-containing) and is bound to transferrin in plasma.
Ferrireductase (duodenal cytochrome b; Dcytb); divalent metal ion transporter 1 (DMT1); haem-carrying protein 1 (HCP1); labile iron pool (LIP; low-molecular ferrocomponents)

.

The non-specific enzyme divalent metal transporter 1 (DMT1) transports Fe²⁺ into and through the the enterocytes. The transport of haem-bound iron is suggested to be carried out by a haem-carrier protein 1 (HCP1), and its expression is probably modulated by the organisms iron status. Inside the enterocytes an oxygenase (haem oxygenase) splits the Fe²⁺ from the haem molecule. From both sources, haem or non-haem, Fe²⁺ is now either; (I) becoming part of the enterocyte's synthesis of iron containing compounds, (II) transported to other tissues via the blood, or (III) stored.

Most of the iron intended for transport leaves the basolateral membrane of the enterocyte through specialised gaps controlled by the protein ferroportin. In immediate vicinity are also ferrooxidases (haephestin) converting the iron to ferric (Fe³⁺) form for binding to the transport molecule transferrin.

Hepcidin (produced in liver) binds to ferroportin on enterocytes and macrophages, thereby inactivating ferroportin. This causes reduced iron absorption in the enterocyte and reduced release of iron from the macrophage. The production of hepcidin is decreased in iron deficiency and increased in iron overload. Hepcidin is a promising candidate as the link between iron stores and iron uptake.