Ferritin source

Ferritin source Fe atoms/ molecule Fe atoms/ phosphate" Fe atoms/ P atoms Crystallinity n 7 ord Ref"... 

Figure 6.1 Schematic representation of human isoferritins of different subunit composition. Each ferritin subunit is represented as a sausage and subunits are packed in a symmetrical shell. Twelve of the 24 subunits are visible, with H-chain subunits stippled and L-chain subunits plain. Homopolymers of H-chain and L-chain subunits are at the top and bottom of the figure respectively. The sources of various ferritins are listed in the right hand column. Reprinted from Harrison and Arosio, 1996. Copyright (1996), with permission from Elsevier Science.

Ferrihydrites from ferritin also show a range of TbS, depending on the source organism (human > limpet > bacterial) (Webb St.Pierre, 1989) (see Fig. 17.3). The Tb of a 2-line ferrihydrite dropped from 50 to 25K when citrate was added at a level of ci-trate/Fe -0.08 (Zhao et al. 1994). Particle interactions in aggregates also affect the Mossbauer parameters. 

Formation of ferritin involves assemblage of the protein subunits to form the apo-ferritin shell which is then filled with the phosphated ferrihydrite core. The mechanism by which ferritin is filled and the iron core built up, has been investigated intensively in vitro. The experiments usually involved incubating apoferritin (from horse spleen) with Fe salts in the presence of an oxidant such as molecular oxygen. They showed that ferritin could be reconstituted from apoferritin and a source of Fe both the iron and the oxygen enter the protein shell, whereupon oxidation of Fe is catalysed by the interior surface of the protein shell (Macara et al., 1972). 

To exclude possible superoxide-independent contribution of xanthine oxidase-stimulated iron release from ferritin, Paul [12] applied a new chemical source of superoxide, di-(4-carboxybenzyl) hyponitrite (SOTS-1). It was found that prolonged superoxide flux on ferritin stimulated the release of as many as 130 iron atoms from the ferritin molecule. 

SOURCE J. K. Grudy, J. Zang, T. M. Lane, P. Arosio, and N. D. Chasteen, Characterization of the H- and L-Subunit Ratios in Ferritins by Sodium Dodecyl Sulfate-Capillary Gel Electrophoresis," Anal. Biochem. 2002, 302, 263. 

The richest dietary sources of total iron are organ meats (liver and kidney), egg yolk, dried legumes, com, molasses and parsley. Liver is particularly valuable because of the high absorbability of its iron. However, only about 10% of dietary iron is absorbed. Iron deficiency anemia can be treated with soluble iron(II) compounds providing 200 mg in three or four daily divided doses. Oral iron(II) sulfate is the least expensive and is in wide use. Ascorbic acid increases the absorption efficiency of iron(II) sulfate. Parenteral administration of iron is used when oral iron is ineffective. Iron-dextran, a colloid formed from iron(III) chloride and an alkali-modified dextran, is one of several preparations available which has found extensive clinical use. It contains up to 28% Fe by weight and has a structural similarity to ferritin. Transfusion therapy may also be used in severe chronic anemia or acute hemorrhage. 

The ability to synthesise ascorbic acid from glucose is absent in a small group of animal species that include man, primates, the guinea pig and the fruit-bat this is due to the absence of the gene that codes for one of the enzymes required for ascorbate synthesis. These species are therefore dependent on an external source of the vitamin in their diet and it is needed as a cofactor for several hydroxylase enzymes, notably the iron-dependent proline and lysine hydroxylases and the copper-dependent dopamine-(3-hydroxylase the function of ascorbate in these enzymes is likely to be its ability to keep the metal in the reduced form which is necessary for hydroxylation. The ability of ascorbate to reduce Fe3+ to Fe2+ is important in promoting the gastrointestinal uptake of iron and for its release from the iron store ferritin. 

Reconstitution experiments with apoferritins from animal and bacterial sources, whose native iron-loaded ferritins had crystalline and amorphous cores respectively, have been informative in showing that the core morphology is not determined by the protein shell. For example, Baaghil et al and Mann etalP were able to form crystalline cores in bacterioferritins, and Rohrer formed cores of iron-... 

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