The distribution of iron in ferritin

Maximum iron loading of ferritin: half a century of sustained citation distortion

Analysis of citation networks in biomedical research has indicated that belief in a specific scientific claim can gain unfounded authority through citation bias (systematic ignoring of papers that contain content conflicting with a claim), amplification (citation to papers that don't contain primary data), and invention (citing content but claiming it has a different meaning). There is no a priori reason to expect that citation distortion is limited to particular fields of science. This Pespective presents a case study of the literature on maximum iron loading of the ferritin protein to illustrate that the field of metallomics is no exception to the rule that citation distortion is a widespread phenomenon.

Purification and characterization of liver ferritins from different animal species

The ferritins were purified from liver homogenates of buffalo, camel, cattle, sheep and shark by thermal denaturation, ammonium sulphate fractionation, Sephacryl S-300 gel filtration and DEAE-blue gel affinity chromatography. The yield and iron content of affinity-purified liver ferritins ranged from 0.008 to 0.052 mg/g and 3.17% to 11.4% respectively. As they are glycoproteins, the ferritins contained variable amounts of neutral carbohydrates. Except for shark ferritin, the ferritins all exhibited immunological cross-reactivity with anti-buffalo liver ferritin and anti-horse spleen ferritin by immunodiffusion and immunoelectrophoresis. Gel electrophoresis, gel filtration and ultracentrifugal analysis indicated the presence of a monomeric ferritin in all cases. SDS-gel electrophoresis of shark ferritin gave a protein band of 18 kDa. Ovine, buffalo and bovine ferritin comprised two protein subunits, the H (20 and 21 kDa) and the L types (18 and 19 kDa). Oligomeric ferritin subunits with molecular weights of 27, 37 and 55 kDa were also found for bovine and buffalo ferritin. SDS-PAGE of camel ferritin revealed a complex pattern with four prominent bands of 61, 51, 44 and 39 kDa. Two fast-migrating components of 15 and 16 kDa were also found in the purified liver ferritins, including reference preparations. The PO4(3-)/Fe ratios of purified shark (0.10) and bovine ferritin (0.12) were similar to that of standard equine spleen ferritin (0.11). However, the ratio was higher in ovine (0.17), camel (0.22) and bovine (0.26) ferritins. The amino acid compositions, molecular weights and sedimentation coefficients of the different liver ferritins were similar.

Effect of hepatic isoferritins from iron overloaded rats on lymphocyte proliferative response: role of ferritin iron content

Iron and ferritin impair a variety of immunological functions. To evaluate the effect of ferritin iron content on rat lymphocyte proliferative response, isoferritins that differ in their iron content and isoelectric point (pI) were isolated from iron overload rat livers by ultracentrifugation (isoferritins with high iron content and low pI) or crystallization (isoferritins with low iron content and high pI) methods. Additionally, commercial horse splenic ferritin (with a lower pI and higher iron content than rat isoferritins) was also tested. Proliferative response to Con A was decreased in a dose-dependent manner in all assays in which spleen cells were incubated with rat and horse isoferritins. However, isoferritins with higher iron contents (rat isoferritin obtained by ultracentrifugation and horse ferritin) caused a greater decrease of proliferative response at 5 and 25 micrograms/ml than the others. Rat and horse apoferritins showed no inhibitory effect on lymphocyte proliferative response, suggesting that the effect is due to iron probably through the damaging effect of reactive oxygen species generated by iron released by the isoferritins on lymphocyte functions. Additionally, the role of serum ferritin level on proliferative response was studied in an experimental model of iron overload in rats. An inverse relationship between the proliferative response and serum ferritin levels was observed. Our results suggest that the inhibitory effect of the isoferritins on lymphocyte proliferative response is due, at least partially, to the iron content of this protein and not exclusively to variation in pI as suggested by other authors. These results are in agreement with the possible immunosuppressor role of ferritin in vivo.

Interleukin-6 enhances hepatic transferrin uptake and ferritin expression in rats

To explore a mechanism of interleukin (IL)-6-induced hypoferremia in rats, iron metabolism was investigated both in vivo and in vitro. Recombinant IL-6 was intraperitoneally administered to male Wistar rats and the serial change of parameters related to iron metabolism was examined. After administration of IL-6, plasma IL-6 concentration increased rapidly, reached its maximum in 1 hr and thereafter decreased quickly. Plasma IL-6 3 hr after IL-6 injection (50 micrograms/kg) was 3 units/ml, which is a concentration capable of inducing hepatic 125I-labeled transferrin uptake in vitro using isolated hepatocytes. Plasma iron concentration and transferrin saturation had decreased to approximately one third of the initial level within 3 hr and then recovered. Total iron binding capacity remained unchanged for 6 hr, then began to decrease. Red blood cell count and hemoglobin concentration showed no remarkable changes during this period. By ferrokinetic study with plasma that contained iron 59-labeled transferrin, the plasma iron disappearance half time, calculated from the disappearance curve, was significantly shortened from 55 min to 22 min by IL-6 treatment (p < 0.01). The ferritin concentration in the liver was increased significantly after the administration of IL-6 (p < 0.001), but transiently decreased in the spleen. The plasma ferritin showed a gradual increase during the 6-hr period after IL-6 injection. The uptake of 125I-labeled diferric transferrin by isolated hepatocytes was increased by IL-6 treatment and this increment was inhibited by addition of 100-fold excess unlabeled transferrin. On the other hand, no significant increment of 125I-labeled diferric transferrin uptake was observed in Kupffer cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation, purification and characterization of bovine spleen ferritin

Ferritin was isolated from bovine spleen and used to prepare apoferritin and reconstituted ferritin. The mol. wt of bovine ferritin was 464,000 with monomer subunits about 18,000-19,500. Gel electrophoresis showed three bands each for ferritin, apoferritin and reconstituted ferritin; all stained for protein and carbohydrate. Only apoferritin failed to stain for iron. Bovine ferritin had higher concentrations of proline, threonine, and valine than equine or human ferritin. The iron:protein ratio of bovine ferritin was 0.161 and of equine ferritin was 0.192. After iron uptake by the apoferritins the iron:protein ratios were 0.186 and 0.278 for the bovine and equine ferritins, respectively.

Characterization of ferritin from human placenta. Implications for analysis of tissue specificity and microheterogeneity of ferritins

Mammalian ferritins can be resolved into multiple components by isoelectric focusing, and each tissue contains a characteristic subset of isoferritins. Ferritin isolated from human liver was compared to acidic ferritin isolated from mid-gestational human placenta to define a structural basis for ferritin heterogeneity. Placenta ferritin contained several major bands with isoelectric points in the range of pI = 4.7-5.0 which were more acidic than the predominant isoferritins of human liver. Ferritin from each tissue was resistant to denaturation by 10 M urea and appeared to be identical by electron microscopy. Circular dichroism measurements revealed that placenta ferritin had substantially less ordered secondary structure than liver ferritin. Both types of ferritin contained only two subunits when analyzed by electrophoresis in sodium dodecyl sulfate gels, but isoelectric focusing of dissociated subunits in urea revealed 6-7 different components. In this system, placenta ferritin was enriched in the more acidic subunits and it completely lacked the most basic subunits noted in liver ferritin; placental ferritin had no unique components. Differences in isoelectric points among assembled ferritins from these two tissues appear to result from different proportions of these acidic and basic subunits.

Quantitation of antibody binding to cell surface antigens by x-ray fluorescence spectrometry

An X-ray spectrometric method has been developed to quantitate antibody bindng to whole cell surfaces in order to obtain a distribution of binding within a population of cells. The method involves incubation of target cells with ferritin-labeled antibody. Analysis of prepared samples in a modified transmission electron microscope with an X-ray detector and data analysis equipment, yields quantitative results on the binding of labeled antibody to individual cells. The binding of anti-2,4-dinitrophenol serum to Chinese hamster ovary cells with attached 2,4-dinitrophenol haptens was measured by X-ray spectrometry. Measurements of attached hapten by a radioisotopic marker correlated with the X-ray spectrometric determination of bound antibody. The use of synchronized cells in metaphase and G1 phases of the cell cycle permitted investigations into the binding per unit surface area. The distribution of antibody binding among a given population of cells was related to the surface area of the cells.

Correlations between subunit distribution, microheterogeneity, and iron content of human liver ferritin

Subunit heterogeneity of human liver ferritin was investigated by two-dimensional electrophoretic methods. The protein which ordinarily remains assembled in 10 M urea solution was dissociated into subunits in acid-urea or sodium dodecyl sulfate solutions. In agreement with earlier studies, the subunits migrated as two bands in sodium dodecyl sulfate or acid-urea gel electrophoresis systems or in two-dimensional combinations of these systems. Isoelectric focusing methods, however, resolved four major subunit bands and three to five minor bands. Each of these components migrated as either a 22 000 or a 19 000 molecular weight component in sodium dodecyl sulfate gel electrophoresis in the second dimension. The multiple subunit model, which is contrary to currently accepted representations of ferritin structure, is compatible with certain inherent properties of the protein. Thus, ferritin was fractionated on the basis of iron content to show that the relative amounts of individual subunit types were directly dependent upon the iron composition of the protein. Iron-loaded molecules were deficient in the most basic subunit types, and apoferritin was enriched in these components. Aspects of microheterogeneity of assembled ferritin molecules were correlated to subunit heterogeneity, and discrete differences in subunit populations among purified isoferritin components were demonstrated.

Characterization of serum ferritin in iron overload: possible identity to natural apoferritin

Serum ferritins from a patient with haemochromatosis and from a patient with transfusional siderosis were compared with tissue isoferritins on the basis of their iron content, isoferritin spectrum and immunological properties. Both serum ferritins had a low iron content and corresponded to only the most basic isoferritins in liver. The serum ferritins were very similar to the natural apoferritin from liver in all respects.

Ferritin phenotypes: structure and metabolism

Recent advances in our understanding of the biochemistry of ferritin have provided new insights into its role in iron metabolism. Findings of multiple structural forms in many tissues may have important consequences for ferritin's function and metabolism. This article reviews the molecular basis of apoferritin heterogeneity and discusses mechanisms operating in the phenotypic expression of ferritin in normal and malignant cells.