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Function and regulation of transferrin and ferritin.

Iron represents a paradox for living systems by being essential for a wide variety of metabolic processes (oxygen transport, electron transport, DNA synthesis, etc) but also having the potential to cause deleterious effects. Because of Iron's virtual insolubility and potential toxicity under physiological conditions, specialized molecules for the acquisition, transport, and storage of iron in a soluble, nontoxic form have evolved to meet cellular and organismal iron requirements. Physiologically, the majority of cells in the organism acquire iron from a well-characterized plasma glycoprotein, transferrin. Iron uptake from transferrin is reasonably well understood, and involves the binding of transferrin to the transferrin receptor, internalization of transferrin within an endocytic vesicle by receptor-mediated endocytosis, and the release of iron from the protein by a decrease in endosomal pH. Most of the transferrin-bound iron is used for the synthesis of hemoglobin by developing erythroid cells. Senescent erythrocytes are internalized by the macrophages that liberate hemoglobin iron and release it back to plasma transferrin at a rate that normally matches the rate of iron transport for erythropoiesis. Unfortunately, the mechanisms and controls involved in the release of iron from macrophages have not been defined. After iron release from transferrin within endosomes, iron passes through the endosomal membrane by ill-understood mechanisms and then enters the poorly characterized intracellular labile pool. Iron in the labile pool that exceeds requirement for the synthesis of functional heme and nonheme iron-containing proteins is stored within the iron-storage protein, ferritin. Evidence in vitro indicates that relatively soluble ferrous iron can enter or be released from ferritin. However, we know virtually nothing about the exchange of iron with ferritin in intact cells, and some evidence indicates that the degradation of the ferritin protein may be an important mechanism for the release of iron within the cell. Cellular iron uptake and storage are coordinately regulated through a feedback control mechanism mediated at the post-transcriptional level by cytoplasmic factors know as iron-regulatory proteins 1 and 2. These proteins "sense" levels of iron in the transit pool and, when iron in this pool is scarce, they bind to stem-loop structures known as iron-responsive elements on the 5' untranslated region of the ferritin mRNA and 3' untranslated region of the transferrin mRNA. Such a binding inhibits translation of ferritin mRNA and stabilizes the mRNA for transferrin receptors. The opposite scenario develops when iron in the transit pool is plentiful. This remarkable regulatory mechanism prevents the expansion of a catalytically active intracellular iron pool, while maintaining sufficient concentrations of the metal for metabolic needs.

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