Heterogeneity in tissue ferritins displayed by gel electrofocusing

Ferritin microheterogeneity, subunit composition, functional, and physiological implications

Ferritin is a ubiquitous intracellular iron storage protein that plays a crucial role in iron homeostasis. Animal tissue ferritins consist of multiple isoforms (or isoferritins) with different proportions of H and L subunits that contribute to their structural and compositional heterogeneity, and thus physiological functions. Using size exclusion and anion exchange chromatography, capillary isoelectric focusing (cIEF), and SDS-capillary gel electrophoresis (SDS-CGE), we reveal for the first time a significant variation in ferritin subunit composition and isoelectric points, in both recombinant and native ferritins extracted from animal organs. Our results indicate that subunits composition is the main determinant of the mean pI of recombinant ferritin heteropolymers, and that ferritin microheterogeneity is a common property of both natural and recombinant proteins and appears to be an intrinsic feature of the cellular machinery during ferritin expression, regulation, post-translational modifications, and post-subunits assembly. The functional significance and physiological implications of ferritin heterogeneity in terms of iron metabolism, response to oxidative stress, tissue-specific functions, and pathological processes are discussed.

The analysis of N-glycolylneuraminic acid(NeuGc) of hepatoma tissue and K562 cell ferritins using HPLC and mass spectrometry

Ferritin is an iron-storage protein and its serum level is known to increase in the patient of with inflammation and malignant tumor. To further elucidate the difference between ferritins from normal human liver tissue and that of cancer cells, their sialic acids were analyzed. The Western blot analysis and the cytochemical staining using anti-NeuGc antiserum indicated that ferritins from the human hepatocarcinoma tissue and malignant K562 cells contain NeuGc, but that from the normal liver does not. The result was also confirmed by HPLC analysis and MALDI-TOF/MS analysis of sialic acids which were derivatized by the DMB method. It was also shown that the sialic acid content in hepatocarcinoma ferritin was much higher than that in the normal liver ferritin. These results suggest that normal and cancerous liver ferritins are qualitatively and quantitatively different in sialylation. In addition, K562 cells were shown to express NeuGc even if the cells were cultured in serum-free media which lack NeuGc. This is of interest from the current concept that expression of NeuGc in human cells is due to uptake and utilization of exogenous NeuGc.

Measurements Using the Quartz Crystal Microbalance Technique of Ferritin Monolayers on Methyl-Thiolated Gold: Dependence of Energy Dissipation and Saturation Coverage on Salt Concentration

The adsorption kinetics of ferritin as a function of ionic strength has been studied with a new quartz crystal microbalance technique, allowing simultaneous measurement of the frequency shift (proportional to the mass uptake under certain conditions) and of changes in the energy dissipation caused by the adlayer. The measurements were performed with methyl-terminated (hydrophobic) thiol-covered gold surfaces, at pH 7.0 and ionic strengths in the range 1-200 mM KCl. The saturation uptake increases rapidly with increasing ionic strength in the range 20-50 mM and is then independent of ionic strength at >100 mM. The dissipation factor reveals, in the low coverage regime, distinct differences in the adlayer properties at low and high ionic strength, respectively. These results are briefly discussed in terms of the screening properties of the solvent and its influence on the protein-protein interaction in solution and on the surface. Copyright 1998 Academic Press.

Preparative isolation of distinct molecular forms of rabbit liver ferritin using high-performance liquid chromatography

Rabbit liver ferritin was separated to fractions of distinct molecular form using a chromatofocusing column coupled to high-performance liquid chromatography equipment. This purification method was fast, less than 1 h, and enabled preparation of fractions, highly enriched in particular subtypes of ferritin. Analytical isoelectrofocusing of these fractions demonstrated a gradual shift in the range of isoelectric points of these subtypes of ferritin. Gradient-pore polyacrylamide slab-gel electrophoresis showed a distinct shift in the subunit ratio of the ferritins, ranging from 87% low molecular weight (L) subunit in the first fraction eluting at a pH 5.4, to 28% L-subunit in the fraction eluting in the trailing edge of the protein peak at pH 4.0. The pI range of the fractions covered the complete range, from pH 4.9 to pH 5.4, of isoelectric points of the whole rabbit liver ferritin preparation.

Biosynthesis of ferritin subunits from different cell lines of HL-60 human promyelocytic leukaemia cells and the release of acidic isoferritin-inhibitory activity against normal granulocyte-macrophage progenitor cells

Biosynthesis of acidic isoferritins was investigated in human promyelocytic HL-60 cells, characterized by diploid (2C), tetraploid (4C) and mixed diploid--tetraploid (2C-4C) DNA cell lines. The three cell lines were studied for the biosynthesis of ferritin and its subunits and for the release of acidic isoferritin-inhibitory activity against normal CFU-GM before and after addition of DMSO. While the tetraploid and mixed diploid--tetraploid cell lines synthesized more H-(Mr = 21) than L-subunits (Mr = 19) after induction, the tetraploid line synthesized more H-subunit before and after induction, compared to the diploid line. The release of acidic isoferritin-inhibitory activity was greater before than after induction in both cell lines, but the tetraploid cell line released more acidic isoferritin-inhibitory activity consistent with its greater production of Mr = 21 subunit. However, after induction no inhibitory activity could be detected from the diploid cells and much less activity was detected with the tetraploid cells, suggesting that differentiation caused a decrease in production of acidic isoferritin-inhibitory activity.

Elevated serum ferritin in the altered ferrokinetics of toxemia of pregnancy

The serum concentration of ferritin in 11 patients with toxemia of pregnancy was compared to that of normal women in the early third trimester of pregnancy and in labor at term. The mean concentration of ferritin for toxemic patients was 91.8 ng/ml compared to 18.4 ng/ml and 19.4 ng/ml for the two control groups, respectively. The biochemical and physiologic features of ferritin are reviewed, and the relationship between iron and ferritin is discussed. Despite elevated serum levels of ferritin in toxemia, it is noted to be less reflective of the clinical course of the syndrome than serum ion.

A biochemical comparison of normal human liver and hepatocellular carcinoma ferritins

1. The iron contents, gel migration rates and isoelectric-focusing patterns of normal liver and hepatocellular carcinoma ferritins from the same patients were compared. 2. Sucrose-density-gradient centrifugation showed that the number of iron atoms per ferritin molecule was decreased to approximately half in carcinoma tissue when compared with normal liver. 3. On electrophoresis, hepatocellular carcinoma ferritin migrates faster and is therefore more negatively charged than normal liver ferritin, thus refuting the general view that the more negatively charged a ferritin molecule the greater its iron content. 4. Comparison of tumour and normal liver ferritin subunit compositions on acid/urea/polyacrylamide gels showed hepatocellular carcinoma ferritin to contain an additional, more negatively charged, subunit to normal liver ferritin. 5. Isoelectric focusing showed that hepatocellular carcinoma tissue contains isoferritins with isoelectric points intermediate between the ranges of normal liver and normal heart isoferritins.

Alteration in tryptic peptide patterns of ferritins purified from human colon carcinoma

Ferritin from malignant tissue differs electrophoretically from normal ferritin. The molecular basis of this difference has not yet been defined. Malignant tissue contains a mixture of ferritins from normal cells, inflammatory cells as well as cancer cells. GW-39 is a pure colon carcinoma cell system that synthesizes human carcinoembryonic antigen. Therefore, ferritin was isolated from normal colon mucosa and colon cancer tissues, as well as from the colon carcinoma cell line, to clarify the molecular relationship between normal and malignant ferritins. Colon carcinoma ferritin differs in primary structure from normal colon mucosal ferritin and contains at least six additional different tryptic peptides. These six peptides were also found in the ferritin from the colon carcinoma cell line. These data suggest that the alteration in ferritin structure occurs at the cellular level and is associated with the malignant state.

Inhibition of the growth of Neisseria meningitidis by reduced ferritin and other iron-binding agents

Serogroups of N. meningitidis were characterized as virulent or avirulent according to their capacity to establish meningococcal infection in mice. An agar plate diffusion technique demonstrated that iron had a definite growth-supporting role for both of these meningococcal types. The avirulent strains could use ionic or chelated iron as well as the virulent strains. Iron-reversible growth inhibition occurred to the same extent for both bacterial types in the presence of the synthetic iron-chelating agents Desferal and ethylenediamine-di-orthohydroxy phenylacetic acid. A difference in response was demonstrated for these bacterial types when grown in the presence of various iron-binding proteins from animal body fluids and tissues. The growth of the avirulent strain was inhibited to a greater degree by egg white conalbumin. The humoral iron-binding protein transferrin showed a significant inhibitory capacity only when used in conjunction with bicarbonate. Under conditions of increased iron saturation of this protein, the avirulent strain was inhibited to the furthest extent. In the presence of ferritin, the cellular iron-binding protein, which had been reduced, inhibition of the growth of either strain type did not occur on iron-poor media (less than 5 micrograms/100 ml). However, with the incorporation of iron into the media, the inhibitory effect of the protein became evident. As the concentration of iron increased, the inhibition increased to a certain level and subsequently declined. A substantial difference in the ability of the avirulent type to grow in the presence of reduced horse spleen ferritin was observed. For this microorganism, a correlation appears to exist between the capacity to grow by utilizing the available iron in the presence of reduced ferritin and the ability to establish infection. The host protein ferritin, in the reduced state, apart from simply being a storage protein for iron, can prevent the growth of a procaryotic organism. Our experiments suggest a role for ferritin in the prevention of emningococcal disease. A cehmotherapeutic potential for Desferal is also implied.

The kinetics of serum and tissue ferritins: relation to carbohydrate content

Significant differences were observed in the rate of disappearance from plasma of ferritins purified from rat serum and from different organs. Ferritin from all sources including purified serum ferritin was rapidly removed from plasma by the liver. No difference in biological half-life was observed between apoferritin prepared by ultracentrifugation of liver ferritin and whole liver ferritin and iron-loaded animals cleared injected serum ferritin from plasma at a comparable rate to normal rats. When amounts of 100 microgram of ferritin were injected into rats the half-life was significantly lengthened. The study confirmed the fact that ferritin iron and ferritin protein were removed from plasma at the same rate. No consistent effect of acidic or more basic isoferritin composition on biological half-life was apparent. After chromatography on concanavalin A-Sepharose 6B those ferritins which were predominantly bound to Con A-Sepharose had a half-life which was approximately twice that of ferritins which did not bind. It is concluded that the variation in plasma disappearance of ferritins of different tissue origin was explainable on the basis of carbohydrate content of the molecule.

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.

Properties of human tissue isoferritins

1. Human liver ferritin was separated by preparative isoelectric focusing into six fractions. 2. Except for the least acidic fraction the reactivity with antibody against spleen ferritin increased with rising pI, but with antibody against heart ferritin the reactivity decreased. 3. The highest iron content was found in the most acidic isoferritins and progressively decreased with rising pI. 4. Iron uptake was studied in apoferritin prepared from heart and liver ferritin fractions separated by ion-exchange chromatography. There was good correlation between the rate of iron uptake and pI. The most acidic fractions took up iron more rapidly than did the more basic ones. 5. Ferritin was prepared from heart, liver, spleen and kidney. There was little difference on isoelectric focusing between ferritin obtained from normal tissues and the corresponding iron-loaded tissues from patients who had received multiple blood transfusions. The iron-loaked heart ferritin invariably contained relatively more of the basic isoferritins. Normal and iron-overloaded heart ferritins were separated into isoferritin fractions by ion-exchange chromatography, and in each case there was a fall in iron content as the pI increased. The iron content of ferritin from the iron-overloaded heart was higher throughout than that from normal heart. 6. There is a relationship between the rate of iron uptake by apoferritin and pI, and this probably accounts for the variation in iron content of the isoferritins found in human liver and heart.

An enzyme-linked immunoassay for ferritin in human serum and rat plasma and the influence of the iron in serum ferritin on serum iron measurement, during acute hepatitis

To measure human serum ferritin and rat plasma ferritin a non-competitive enzyme-linked immunoassay has been developed using horseradish peroxidase as the enzyme. In this assay it proved necessary to use heated rat plasma to obtain reproducible ferritin values. The heating procedure caused a loss of 38% of the plasma ferritin. Rat plasma ferritin values have been corrected for this loss. The standard deviation, from duplicate normal human and rat samples is 10 ng ferritin/ml serum and 69 ng/ml plasma, respectively. (The mean ferritin concentrations are: in human sera, 82 ng/ml and in rat plasma 762 ng/ml.) Mean recovery of added liver ferritin in the human serum is 104% +/- 4% (+/-S.E.M') and in the rat plasma 101% +/- 3% (+/- S.E.M.). Normal ferritin concentrations varied in the human material between 30 ng/ml and 300 ng/ml serum, and in the rat plasma between 500 ng/ml and 1300 ng/ml. During increased body iron and acute hepatitis the ferritin concentrations, in patients as well as in rats, exceeded the upper limit of the normal values in most cases. During human hepatitis high serum ferritin levels combined with high serum iron levels were measured. The high serum iron concentrations could not be explained by the high serum ferritin concentrations, even if the iron content of the ferritin is supposed to be high.

Variation in the distribution of two human heart ferritin species. Isoferritin profile and subunit composition in normal and iron-overloaded subjects

On polyacrylamide slab electrophoresis two ferritin species, termed fast and slow, were present in three control and four iron-loaded human hearts. The ratio of the fast to slow species varied between hearts and correlated with the distribution of isoferritins, which had pI values ranging from 4.8 to 5.6. The acidic isoferritins were prominent in all hearts, but one control and the four iron-loaded hearts contained, in addition, basic isoferritins. All ferritins were composed of two subunits, an acidic H type and a more basic HL type, which was the main subunit in normal liver. Hearts with the highest proportions of acidic isoferritin contained the highest proportion of the H subunit.

Serum ferritin in haemochromatosis: changes in the isoferritin composition during venesection therapy

The isoferritin composition of serum ferritin in 13 patients with untreated idiopathic haemochromatosis (IHC) has been shown to differ from normal in exhibiting an increase in isoferritins in the pH range 5.54-5.62. A similar change was observed in four patients with gross iron overload secondary to haemolytic anaemia. During the course of venesection therapy there was a progressive rise in isoferritins of pI 5.02-5.06 relative to the more basic isoferritins. These observations are consistent with previous studies showing alterations in tissue isoferritins in untreated IHC before and after venesection therapy and they are compatible with the hypothesis that the more basic isoferritins correspond to a 'storage' ferritin and the more acidic to a 'secretory' ferritin. The studies also provide further evidence for a possible biological role of the individual isoferritins.

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.

Structural features of rat cardiac ferritins

Ferritin extracted from rat heart containes two species separable by gel electrophoresis. These were purified and examined for structural characteristics. As in gel electrophoresis, cardiac ferritin preparations yielded only two bands on isoelectric focusing in gels, with pI values of 4.6 and 4.8. After separation by preparative electrophoresis, the two species were found to have a different amino acid composition from each another and from liver ferritin. Similarly, peptide maps showed several components not found in liver ferritin. On dissociation and electrophoresis with sodium dodecyl sulfate, heart ferritins were found to contain subunits of the same sizes as in other rat ferritins but also some larger components. Since cardiac ferritins have apparent molecular weights greater than those of other ferritins, it is concluded they probably contain more subunits, and possibly some of larger size not present in ferritins of other tissues.

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.

Serum ferritin concentration in early breast cancer

The concentration of circulating ferritin was measured in 250 normal adult women and 229 women presenting with early breast cancer. Ferritin concentrations are higher in cancer patients than in normal women. Patients with an intial circulating ferritin concentration above 200 mug/1 have a higher tumour recurrence rate during the subsequent 4 years.

The purification and properties of ferritin from human serum

1. Ferritin has been isolated from the serum of four patients with iron overload by using two methods. 2. In method A, the serum was adjusted to pH 4.8 and heated to 70 degrees C. After removal of denatured protein, ferritin was concentrated and further purified by ion-exchange chromatography and gel filtration. In most cases, only a partial purification was achieved. 3. In method B, ferritin was extracted from the serum with a column of immuno-adsorbent [anti-(human ferritin)] and released from the column with 3M-KSCN. Further purification was achieved by anion-exchange chromatography followed by the removal of remaining contaminating serum proteins by means of a second immunoadsorbent. Purifications of up to 31 000-fold were achieved, and the homogeneity of the final preparations was demonstrated by polyacrylamide-gel electrophoresis. 4. Serum ferritin purified by either method has the same elution volume as human spleen ferritin on gel filtration on Sephadex G-200. Serum ferritin has a relatively low iron content and iron/protein ratios of 0.023 and 0.067 (mug of Fe/mug of protein) were found in two pure preparations. On anion-exchange chromatography serum ferritin has a low affinity for the column when compared with various tissue ferritins. Isoelectric focusing has demonstrated the presence of a high proportion of isoferritins of relatively high pI. 5. Possible mechanisms for the release of ferritin into the circulation are briefly discussed.

Immunological detection of isoferritins in normal human serum and tissue

A method is described which permits the detection of isoferritins in normal human serum and tissues. The technique makes use of 125I-labelled monospecific anti-human-liver-ferritin antibody to demonstrate the isoferritins after isoelectric focussing of the purified ferritin in polyacrylamide gels. The organ-specific variation in tissue isoferritin profile previously reported in normal subjects has been confirmed by this technique using only 50 ng of each ferritin sample. Serum ferritin from normal healthy subjects was also shown to exhibit a microheterogeneity on isoelectric focussing; six clearly defined isoferritin peaks were detected in the pH range of 5.04 to 5.62. This isoferritin profile of normal serum contained isoferritins over the whole range of the various tissue isoferritins suggesting that a number of organs may contribute to the normal serum ferritin pool.

Alterations in tissue ferritins in iron storage disorders

Purified tissue ferritins isolated from Bantu subjects with gross haemosiderosis, from a patient with idiopathic haemochromatosis (HC) treated by phlebotomy, and from rats with experimental iron overload were studied in order to determine the significance of the abnormality previously demonstrated in tissue isoferritins in patients with IHC. The isoferrin profile of the tissues from the Bantu subjects and the iron-loaded rats showed a similar abnormality to that previously found in patients with untreated IHC--that is, an abnormally uniform distribution of iron-containing isoferritins with an increase in the more basic isoferritins and an apparent absence of the more acidic ones. In contrast, tissues from the patient with treated IHC, who was iron depleted at the time of death, showed the normal organ-specific isoferritin distribution. These findings strongly suggest that the abnormal distribution of tissue isoferritins in IHC is an acquired phenomenon and unlikely to be related to an underlying genetic defect in ferritin or iron metabolism.

Human isoferritins: organ specific iron and apoferritin distribution

Ferritins from human liver, spleen, heart, pancreas and kidney were compared by electrophoresis and isoelectric focusing in polyacrylamide gels, by immunodiffusion against antisera to homologous and heterologous ferritin, and in some cases by their cyanogen bromide peptides. All ferritins appeared to consist of a single species on gel electrophoresis with the exception of heart ferritin which separated into two major components. Small differences in electrophoretic mobility were found in all tissue ferritins. By contrast, all tissue ferritins were found to consist of multiple forms when analysed by gel electrofocusing. At least five isoferritins were found in most tissues, several of which were common to most tissues. At least two were common to all tissues. Those ferritins which were most easily distinguishable electrophoretically, e.g. spleen and heart ferritin, showed the greatest differences on gel electrofocusing. The ferritin profile was characteristic of each organ and was reproducible both within individuals and between individual tissues. There were striking differences in the iron content of the various isoferritins within a tissue. Further, the iron content of isoferritins common to more than one tissue varied with the tissue of origin. Some isoferritins in several organs and all of the isoferritins in pancreas appeared to contain little, if any, iron. All five tissue ferritins contained antigenic determinants in common with liver ferritin. However, an additional antigenic determinant was found in liver ferritin which was not detectable in the ferritins from the other organs.