Iron Metabolism, Iron Deficiency and Disorders of Haem Synthesis

The ability of iron to exist in both ferric and ferrous states underlies its role in critical enzyme reactions concerned with oxygen and electron transport and the cellular production of energy. Because of the poor solubility of iron, living organisms developed a wide variety of proteins involved in its absorption, transport and storage. Intracellular iron homeostasis is achieved by coordinated regulation of transferrin receptor and ferritin at the level of RNA translation by two cytoplasmic iron-dependent proteins, IRP1 and IRP2 to maintain a constant intracellular iron content. Systemic iron homeostasis is controlled by hepcidin, a polypeptide made in the liver. Hepcidin regulates iron homeostasis by binding to cell-surface ferroportin, causing its degradation, thereby inhibiting iron absorption and iron release from macrophages. Hepcidin synthesis is itself regulated by iron stores, inflammatory cytokines and erythropoiesis. Despite these powerful homeostatic mechanisms, iron deficiency is one of the most common nutritional problems. It is the outcome of blood loss, increased physiologic demands and limited supply when the diet is of poor quality and mainly vegetarian. Despite some improvement in recent years, iron deficiency remains the top ranking cause of anaemia worldwide. Pathological alterations in haem synthesis include the porphyrias, and sideroblastic anaemia. Porphyrias are characterized by a defect in haem synthesis resulting in the accumulation of haem intermediates in the liver or the erythron. The sideroblastic anaemias comprise a group of refractory anaemias with an excess of iron appearing as "rings" in the developing erythroblasts.