The adult body contains 2.5-5 g of iron, approximately two-thirds of which is present in hemoglobin. Other essential iron-containing systems include muscle myoglobin (3%) and a variety of iron-containing enzymes (5-15%), including cytochromes. In addition to the role of iron in oxygen transfer via hemoglobin and myoglobin, iron is involved in energy metabolism and also affects neural myelina-tion and neurotransmitter metabolism. Iron stores vary considerably but may represent up to 30% of body iron, and iron that circulates with transferrin represents less than 1% of body iron. Men have a higher concentration of iron per kilogram body weight than women because they have larger erythrocyte mass and iron stores.
More than 90% of body iron is conserved through the recycling of iron through the reticuloendothelial system (Figure 1). Iron is transported through the body by the protein transferrin, which carries up to two iron atoms. The distribution of iron to body tissues is mediated by transferrin receptors (TfRs), which are upregulated in the face of increased tissue demand for iron. The transferrin/ TfR complex is internalized via cell invagination, iron is released into the cell cytosol, and transferrin is recycled back to the cell surface.
In hematopoietic cells, iron is used to produce hemoglobin through its combination with zinc protoporphyrin to form heme. Therefore, protopor-phyrin accumulates relative to hemoglobin in red blood cells during iron deficiency. Mature red blood cells circulate in the body for approximately 120 days before being destroyed. Macrophage cells of the liver and spleen phagocytize senescent red blood cells and the iron released in this process is recycled back to the circulation or, when iron is readily available, incorporated with ferritin or hemosiderin for storage. A typical ferritin molecule may contain 2000 iron atoms. Hemosiderin is a less soluble variant of ferritin that may contain even greater amounts of iron.
The production of transferrin receptors and ferritin is regulated by iron response proteins (IRPs) that 'sense' intracellular iron concentrations and interact with iron response elements (IREs) of protein mRNA. When cellular iron concentrations are low, the IRP-IRE interaction works to prevent translation of mRNA to ferritin or to stabilize mRNA to enhance the translation of transferrin receptors. Identifying other proteins regulated through the IRP-IRE interaction is an area of particular interest. Although body iron is highly conserved, daily basal losses of iron of mg/day do occur even in healthy individuals. These basal losses occur primarily through the gastrointestinal tract (in bile, sloughing of ferritin-containing enterocytes, and via blood loss), and sweat and urine are additional minor sources of iron loss (Figure 1). Iron losses are not strictly regulated; rather, iron balance is achieved through the regulation of dietary iron absorption.
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