Iron Absorption and Transport

Iron is predominantly absorbed in the duodenum. In its free form, iron is a potent pro-oxidant known to induce peroxidation of lipids, proteins and nucleic acids. Extracellular iron circulates in blood bound to transferrin. One transferrin molecule has two iron-binding sites. Under physiological conditions, cells take up iron from plasma by endocytosis, whereby one transferrin molecule binds to one transferrin receptor molecule, and the complex is transferred to the cytoplasm by invagination. As a result of low pH in the endosome, iron is released to the cytoplasm, where it is either used for various cellular functions or incorporated into ferritin. Upon loss of iron, the apotransferrin-transferrin-receptor complex is transported back to the cell membrane, where apotransferrin is released into the bloodstream, and the transferrin receptor is available for a new round of transferrin binding. The mechanism by which the apotransferrin-transferrin-receptor complex is transported to the cell membrane remains unclear. However, Sainte-Marie et al. (1997) suggest that changes in free cytoplasmic calcium concentrations might be involved in the recycling of transferrin receptor.

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