Apoferritin synthesis

In addition to the storage of iron in intestinal mucosal cells, iron is also stored, primarily as ferritin, in macrophages in the liver, spleen, and bone, and in parenchymal liver cells (Figure 33-1). Apoferritin synthesis is regulated by the levels of free iron. When these levels are low, apoferritin synthesis is inhibited and the balance of iron binding shifts toward transferrin. When free iron levels are high, more apoferritin is produced to sequester more iron and protect organs from the toxic effects of excess free iron. 

The synthesis of apoferritin by reticulocytes has been studied (231) and it has been established that administration of iron stimulates ferritin synthesis two fold over controls. The control of ferritin induction again seems to be at the level of translation. An analysis of apoferritin synthesis in human erythroid cells from normal and thalassemic subjects showed that thalassemic cells synthesised 37 times as much apoferritin... 

Internal exchange of iron is accomplished by the plasma protein transferrin. This 76 kDa /Ij-glycoprotein has 2 binding sites for ferric iron. Iron is delivered from transferrin to intracellular sites by means of specific transferrin receptors in the plasma membrane. The iron-transferrin complex binds to the receptor, and the ternary complex is taken up by receptor-mediated endocytosis. Iron subsequently dissociates in the acidic, intracellular vesicular compartment (the endosomes), and the receptor returns the apotransferrin to the cell surface, where it is released into the extracellular environment. Cells regulate their expression of transferrin receptors and intracellular ferritin in response to the iron supply. Apoferritin synthesis is regulated post-transcriptionally by 2 cytoplasmic binding proteins (IRP-1 and lRP-2) and an iron-regulating element on its mRNA (IRE). 

The early work of Fineberg and Greenberg 128) as well as that of Drysdale and Monroe 129) indicate that oral or intramuscular administration of iron enhances the rate of ferritin synthesis in the intestinal mucosa as well as other tissue. Drysdale and Monroe foimd little effect of actinomycin, an inhibitor of RNA synthesis, on iron-induced ferritin synthesis and suggested that the apoferritin was stabilized by being bound to the iron core. Recently Yoshino et al. 130) using higher concentrations of actinomycin were able to inhibit markedly the induced synthesis of ferritin. The mechanism for iron induction of ferritin is certainly not clear at this time. 

Absorption, transport, and storage of iron. Intestinal epithelial cells actively absorb inorganic iron and heme iron (H). Ferrous iron that is absorbed or released from absorbed heme iron in the intestine (1) is actively transported into the blood or complexed with apoferritin (AF) and stored as ferritin (F). In the blood, iron is transported by transferrin (Tf) to erythroid precursors in the bone marrow for synthesis of hemoglobin (Hgb) (2) or to hepatocytes for storage as ferritin (3). The transferrin-iron complexes bind to transferrin receptors (TfR) in erythroid precursors and hepatocytes and are internalized. After release of the iron, the TfR-Tf complex is recycled to the plasma membrane and Tf is released. Macrophages that phagocytize senescent erythrocytes (RBC) reclaim the iron from the RBC hemoglobin and either export it or store it as ferritin (4). Hepatocytes use several mechanisms to... 

This model has a number of attractive features. Not only is it compatible with the fact that Fe3+ is the preferred form of iron under physiological conditions, where the rate of autoxidation of Fea+ to insoluble hydroxide under mildly oxidising conditions is extremely rapid but it could also account for the stimulation of ferritin synthesis by iron at the level of translation of a stable wRNA (Section X), since it seems reasonable to assume that, whereas the apoferritin monomer shows no tendency to dissociate under physiological conditions (Section VIB), the molecule is presumably synthesised as apoferritin subunits, which could then associate around performed micellar cores. 

A different effect of iron-induced ferritin synthesis is obtained by excess dietary Zn2+ (187). This results in formation of iron-poor ferritin the treated animals have an iron protein ratio in liver ferritin that is only one-third of control animals. However, the Zn toxicity does not appear to interfere with the incorporation of radioactive acids into apoferritin and the Zn-treated animals can still synthesise apoferritin de novo in response to iron administration. The turnover rate of ferritin iron and ferritin protein from Zn-fed rats seem to be more rapid than the controls, which would be consistent with the view of Drysdale and Munro (184) that ferritin of low iron content is more rapidly degraded than ferritin of high iron content. The effect of Zn on the catalytic activity of ferritin has been discussed earlier. 

Already in 1955, when studies on the induction of protein synthesis in mammalian tissue were in their infancy, Fineberg and Greenberg demonstrated that iron administration induced apoferritin formation [10]. Later, Loftfield and Harris [11] demonstrated that the amino acids valine, isoleucine, and leucine were incorporated in the newly synthesized protein. These experiments are in fact of considerable historical significance to modern knowledge of protein synthesis. 

A delicate control mechanism for the biosynthesis of apoferritin has been proposed. The cell does not contain free apoferritin—iron induces the synthesis of apoferritin. Apoferritin exerts a feedback inhibition on its own biosynthesis, which is relieved when apoferritin is converted to ferritin. Of course, the mechanism of induction at the nucleic acid level remains to be studied. 

Inasmuch as almost no iron is excreted, the mechanism controlling the levels of the iron reserves in the body must operate at the level of absorption [27, 28]. For many years the most popular theory invoked to explain the control of iron uptake was the theory of the mucosal block mechanism. The theory postulated the existence of a single, short-loop feedback mechanism within the cell of the intestinal mucosa. The absorbed iron would induce the de novo synthesis of apoferritin, and the apoferritin would then chelate iron to yield... 

Only under one condition can something resembling a synthesis be accomplished (4). It has been mentioned that after tiie completion of crystallization of ferritin, there remains a brown mother liquor from which no further crystals can be obtained. When apoferritin is dissolved in this brown mother liquor and CdSO added, brown crystals of ferritin are formed. 

Later studies by Granick and Hahn (11), although not conveying a clear-cut picture, seem to suggest that the synthesis of ferritin from iron and apoferritin, is due to a specific, probably enzymatic process. 

The formation of apoferritin in the body appears to be remarkable in that the presence of apoferritin itself has not been demonstrated in any of the cells of tissues associated with ferritin production, viz., the liver, the spleen, bone marrow, and intestinal mucosa. The explanation may be either that synthesis and breakdown of apoferritin is a continuous process in which breakdown is prevented only by the presence of iron, or that synthesis of apoferritin is stimulated by the presence of iron with breakdown occurring immediately upon removal of iron. ... 

For this synthesis, a deaerated solution of iron [(NH4)2Fe(S04)-6H20], cobalt [Co(N03)2-61120] and H2O2 was added to apoferritin in NaCl (pH 8.5, 65 C, under... 

Hiroko, F., Hideyuki, Y. and Mamoru, A. (2008) In vitro synthesis of calcium nanoparticles using the protein cage of apoferritin. Key Engineering Materials, 361-363,183-6. 

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