In vitro loading of apoferritin.

This study compared the effect of loading apoferritin either with ferrous ammonium sulfate in various buffers or with ceruloplasmin and chelated ferrous iron. It was shown that loading of apoferritin with ferrous ammonium sulfate was dependent on buffer and pH, and was directly related to the rate of iron autoxidation. The ceruloplasmin-dependent loading of apoferritin, however, was unaffected by these factors. Isoelectric focusing and amino acid analysis of the differently loaded ferritins showed that ferrous ammonium sulfate loading of apoferritin resulted in the depletion of the basic amino acids, lysine and histidine, probably as a result of protein oxidation. No significant differences in amino acid composition was noted for ceruloplasmin-loaded ferritin. Furthermore, ferritin loaded with ferrous ammonium sulfate released more iron than either native or ceruloplasmin-loaded ferritin when either paraquat or EDTA was used as an iron mobilizing agent. We suggest that the loading of apoferritin with ferrous ammonium sulfate occurred as a result of iron autoxidation and may result in oxidation of amino acids and loss of integrity of the protein, and that ceruloplasmin may act as a catalyst for the incorporation of iron into apoferritin in a manner more closely related to that occurring in vivo.     

Uptake of iron by apoferritin from a ferric dihydrolipoate complex

A study was made on the uptake of iron by horse spleen apoferritin, by using as an iron source the same ferric dihydrolipoate complex which represents the major product in the anaerobic removal of ferritin-bound iron by dihydrolipoate at neutral pH. The ferric dihydrolipoate complex was chemically synthesized and used as an iron donor to apoferritin. Iron uptake was studied, at slightly alkaline pH and in anaerobic conditions, as a function of the concentration of both the iron donor and apoferritin. Isolation of ferritin from mixtures of ferric dihydrolipoate and apoferritin, and subsequent identification of the oxidation state of ferritin-bound iron, showed that the first metal atoms were taken up in the ferrous form and that this early step was accompanied by accumulation of ferric iron. Total iron uptake increased with the molar ratio of complex to apoprotein and ranged over 25-40% of the iron being supplied. The amount of ferrous iron found inside the protein did not exceed 50-60 mol iron/mol ferritin after a 48-h incubation. At this time, ferric iron represented a significant fraction of the iron found in the isolated ferritin. Analytical and spectroscopic data indicated that fractional rates and equilibria for disassembly of the ferric complex in the presence of apoferritin were independent of the concentration of the protein and of the complex itself.     

The formation of ferritin from apoferritin. Catalytic action of apoferritin

The iron-storage protein ferritin consists of a protein shell and has an iron content of up to 4500 iron atoms as a microcrystalline ferric oxide hydrate. A study was made of the uptake of ferrous iron by apoferritin in the presence of an oxidizing agent at very low iron:protein ratios. At ratios of less than about 150 iron atoms per apoferritin molecule hyperbolic progress curves were obtained, whereas at higher ratios the curves became sigmoidal under the conditions used. A computer model, developed previously (Macara et al., 1972), was shown to account for this result. The experimental evidence indicates that apoferritin binds ferrous iron and catalyses the initial stage in the formation of the ferric oxide hydrate inside the protein shell. This stage involves the oxidation of sufficient iron within the protein molecule to form a stable nucleus on which the growth of the microcrystalline iron-core particles can proceed. A possible schematic mechanism for the action of apoferritin is suggested.     

Modification of ferritin during iron loading

Recombinant human ferritin loaded with iron via its own ferroxidase activity did not sediment through a sucrose-density gradient as a function of iron content. Analysis of the recombinant ferritin by native PAGE demonstrated an increase in altered migration pattern of the ferritins with increasing sedimentation, indicating an alteration of the overall charge of ferritin. Additionally, analysis of the ferritin by SDS-PAGE under nonreducing conditions demonstrated that the ferritin had formed large aggregates, which suggests disulfide bonds are involved in the aggregation. The hydroxyl radical was detected by electron spin resonance spectroscopy during iron loading into recombinant ferritin by its own ferroxidase activity. However, recombinant human ferritin loaded with iron in the presence of ceruloplasmin sedimented through a sucrose-density gradient similar to native ferritin. This ferritin was shown to sediment as a function of iron content. The addition of ceruloplasmin to the iron loading assay eliminated the detection of the DMPO-*OH adduct observed during loading using the ferroxidase activity of ferritin. The elimination of the DMPO-*OH adduct was determined to be due to the ability of ceruloplasmin to completely reduce oxygen to water during the oxidation of the ferrous iron. The implications of these data for the present models for iron uptake into ferritin are discussed.     

Stability and iron oxidation properties of a novel homopolymeric plant ferritin from adzuki bean seeds: A comparative analysis with recombinant soybean seed H-1 chain ferritin

Background

All reported plant ferritins are heteropolymers comprising two different H-type subunits. Whether or not homopolymeric plant ferritin occurs in nature is an open question.

Methods

A homopolymeric phytoferritin from adzuki bean seeds (ASF) was obtained by various protein purification techniques for the first time, which shares the highest identity (89.6%) with soybean seed H-1 ferritin (rH-1). Therefore, we compared iron oxidation activity and protein stability of ASF with those of rH-1 by stopped-flow combined with light scattering or UV/Vis spectrophotography, SDS- and native- PAGE analyses. Additionally, a new rH-1 variant (S68E) was prepared by site-directed mutagenesis approach to elucidate their difference in protein stability.

Results

At high iron loading of protein, the extension peptide (EP) of plant ferritin was involved in iron oxidation, and the EP of ASF exhibited a much stronger iron oxidative activity than that of rH-1. Besides, ASF is more stable than rH-1 during storage, which is ascribed to one amino acid residue, Ser68.

Conclusions

ASF exhibits a different mechanism in iron oxidation from rH-1 at high iron loading of protein, and a higher stability than rH-1. These differences are mainly stemmed from their different EP sequences.

General significance

This work demonstrates that plant cells have evolved the EP of phytoferritin to control iron chemistry and protein stability by exerting a fine tuning of its amino acid sequence.     

Stoichiometry of Fe(II) oxidation during ceruloplasmin-catalyzed loading of ferritin.

Ceruloplasmin catalyzed the incorporation of iron into apoferritin with a stoichiometry of 3.8 Fe(II)/O2. This value remained the same when ferritin containing varying amounts of iron was used. Contrary to the "crystal growth" model for ferritin formation, no iron incorporation into holoferritin was observed in the absence of ceruloplasmin. Fe(II)/O2 ratios close to 2 were obtained for iron incorporation into apo- and holoferritin in Hepes buffer, in the absence of ceruloplasmin, indicating the formation of reduced oxygen species. Sequential loading of ferritin in this buffer resulted in increasing oxidation of the protein as measured by carbonyl formation. Sequential loading of ferritin using ceruloplasmin did not result in protein oxidation and a maximum of about 2300 atoms of iron were incorporated into rat liver ferritin. This corresponded to the maximum amount of iron found in rat liver ferritin in vivo after injection with iron. These results provide evidence for ceruloplasmin as an effective catalyst for the incorporation of iron into both apo- and holoferritin. The possibility that these findings may have physiological significance is discussed.     

Luminol peroxidation catalyzed by human isoferritins

The role of ferritin in catalyzing the oxidation of luminol with the production of chemiluminescence was investigated. The effect of pH was compared to its effect on K3Fe(CN)6-catalyzed oxidation and different pH optima were recorded for the two catalysts. The ferrous iron chelator, bipyridyl, enhanced the production of chemiluminescence catalyzed by FeSO4 and ferritin but had little effect on the K3Fe(CN)6-catalyzed reaction. Desferal reduced the level of chemiluminescence in the presence of FeSO4 and ferritin but was a much more effective inhibitor of chemiluminescence catalyzed by K3Fe(CN)6. The hydroxyl radical scavenger, mannitol, had little effect upon light production whereas superoxide dismutase inhibited light production. The addition of antihuman spleen ferritin completely inhibited activity. The catalytic activity of both H and L rich ferritins was affected by iron content. Activity increased until the Fe/protein ratio reached 0.04 micrograms Fe/micrograms protein and then decreased with increasing iron content. Thus activity is controlled by the iron content of the molecule and influenced by its subunit composition as is the uptake of iron into ferritin. These findings suggest that ferroxidation by ferritin is associated with the ability to generate radicals of the nitrogenous base luminol with the production of chemiluminescence. Although activity is greatest at alkaline pH there is significant activity at pH 7.4. Ferritin therefore may be able to generate free radical reactions in vivo with the acidic isoferritin being most active.     

Changes in the characteristics and distribution of ferritin in iron-loaded cell cultures.

When Chang liver cells are grown in an iron-rich medium for up to 20 weeks, iron loading up to 50 times the normal cellular iron content may be obtained, although ferritin increases only to about 10 times normal. Ferritin has been isolated from such cells, and the isoferritin pattern found on elution from DEAE-Sephadex A-50 by increasing chloride concentrations has been used as a basis for studying changes in the properties of ferritin under conditions of cellular loading. A consistent shift of peak ferritin-elution position to higher chloride concentrations (lower pI) occurs when cells are loaded with ferric nitrilotriacetate for increasing lengths of time. A change in immunoreactivity also takes place on loading, the ratio of ferritin reacting with heart and spleen ferritin antibodies increasing at any particular value of pI. Cells were pulse-labelled with [59Fe]ferric nitrilotriacetate and [3H]leucine followed by non-radioactive iron in the same form. During the 72 h after the synthesis of new protein and its incorporation of iron, there is a slight acid shift in its isoelectric point. This effect is seen in both normal and loaded cells, with the whole spectrum being shifted towards lower pI in the loaded state. These findings suggest that the shift to more acidic ferritins on iron loading and the associated changes in antigenicity may be unrelated to subunit composition.     

Transferrin iron interactions with cultured hepatocellular carcinoma cells (PLC/PRF/5)

Hepatocellular carcinoma cells of the PLC/PRF/5 cell line had 1.9 x 10(5) transferrin receptors per tumor cell with a Kd of 1.5 x 10(-8) M. At high concentrations of transferrin the binding was not saturable. Transferrin internalization by hepatoma cells was shown by time and temperature-dependent binding studies and by pronase experiments. Transferrin recycling was confirmed by the demonstration of a progressive increase in the cellular molar ratios of iron to transferrin and by chase experiments. Ammonium chloride interfered with iron unloading. The vinca alkaloid vincristine inhibited iron and transferrin uptake. The hepatocarcinoma cells appeared to lack asialoglycoprotein receptors and therefore internalized partially desialated transferrin by the regular route. Iron uptake from transferrin was markedly inhibited by the hydrophobic ferrous chelator 2,2' bipyridine but was relatively unaffected by the hydrophilic ferric chelator desferroxamine. The implication that ferrous iron was involved in postendocytic transvesicular membrane iron transport was supported by a study in which hepatoma cells were shown to take up large amounts of ferrous iron suspended in 270 mM sucrose at pH 5.5. The interaction at this pH between surface labeled hepatoma cell extracts and ferrous iron on a Sephacryl S-300 column suggested that the postendocytic transvesicular transport of iron through the membrane was in part protein mediated. The endocytosed iron in hepatoma cells was found in association with ferritin (33%), transferrin (31%) and a low molecular weight fraction (21%).     

Uptake of ionic 55Fe by mouse peritoneal macrophages in vitro.

Mouse peritoneal macrophages were maintained in vitro up to 3 days and exposed to radiolabelled ⁵⁵Fe in the form of ferrous citrate, ferrous sulfate, and ferric chloride in concentrations of 3–5 γ Fe/ml. The divalent iron compounds were taken up 10–40 times more extensively per weight of iron than the trivalent iron compounds. The net uptake of ferrous citrate was linear during the first day and thereafter increased at a slower rate. Macrophages in culture for 1 week showed one-third the average uptake of freshly cultured cells during comparable periods of exposure to ferrous citrate. The iron taken up was used in the synthesis of mouse ferritin. Uptake of ferrous citrate was influenced by serum concentration in the tissue culture medium, temperature, pinocytosis and phagocytosis of both latex particles and heated rat erythrocytes. Uptake of ferrous citrate was enhanced by exposure to either sodium fluoride (5×10^?3 M), or 2,4-dinitrophenol (1×10^?5 M), but was not affected by cyanide, azide, or cycloheximide. The effect of sodium fluoride was not demonstrated when ferrous sulfate was substituted for ferrous citrate. The results reported here suggest that the ability of macrophages to take up ferrous citrate is good in freshly explanted cultures, is a temperature-dependent process, is suppressed by pinocytosis and phagocytosis, and paradoxically enhanced by certain metabolic inhibitors.     

High-rate ferrous iron oxidation by immobilized Acidithiobacillus ferrooxidans with complex of PVA and sodium alginate

By four different methods, Acidithiobacillus ferrooxidans cells were immobilized by the complex of PVA and sodium alginate. The beads formed by these different methods were evaluated in terms of relative mechanical strength, biological activity, dilatability, and so on. The results indicate that the technique utilizing the complex of PVA and sodium alginate crosslinked with Ca(NO(3))(2) is more appropriate for the immobilization of A. ferrooxidans than any others. So the PVA-calcium nitrate beads were used in batch and continuous culture. A maximum ferrous iron oxidation rate of 4.6 g/l/h was achieved in batch culture. Long-time performance of packed-bed bioreactor was evaluated systematically over 40 days, depending on the conversion ratio of ferrous iron and the residence time. At a residence time of 2.5 h, 96% of the initial ferrous iron was oxidized. This study shows this new immobilization technique will be a feasible and economical method for A. ferrooxidans.     

Influence of some physicochemical parameters on bacterial activity of biofilm: Ferrous iron oxidation by Thiobacillus ferrooxidans

The influence of temperature, pH, and substrate and product concentrations on the oxidation rate of ferrous iron by biofilm of Thiobacillus ferrooxidans was determined. The experiments were performed in an inverse fluidized-bed biofilm reactor in which the biofilm thickness was kept constant at 80 mum. Oxygen concentration and diffusion through the biofilm did not limit the oxidation rate. The oxidation rate was almost unaffected by temperature between 13 and 38 degrees C, pH between 1.3 and 2.2, ferric iron concentration up to 14 g/L, or ferrous iron concentration from 4 to 13 g/L. The kinetics of the process was described by the Monod equation with respect to the mass of the biofilm and with ferrous ions as the limiting substrate.     

The ferroxidase center is essential for ferritin iron loading in the presence of phosphate and minimizes side reactions that form Fe(III)-phosphate colloids

Ferritin iron loading was studied in the presence of physiological serum phosphate concentrations (1 mM), elevated serum concentrations (2–5 mM), and intracellular phosphate concentrations (10 mM). Experiments compared iron loading into homopolymers of H and L ferritin with horse spleen ferritin. Prior to studying the reactions with ferritin, a series of control reactions were performed to study the solution chemistry of Fe²⁺ and phosphate. In the absence of ferritin, phosphate catalyzed Fe²⁺ oxidation and formed soluble polymeric Fe(III)-phosphate complexes. The Fe(III)-phosphate complexes were characterized by electron microscopy and atomic force microscopy, which revealed spherical nanoparticles with diameters of 10–20 nm. The soluble Fe(III)-phosphate complexes also formed as competing reactions during iron loading into ferritin. Elemental analysis on ferritin samples separated from the Fe(III)-phosphate complexes showed that as the phosphate concentration increased, the iron loading into horse ferritin decreased. The composition of the mineral that does form inside horse ferritin has a higher iron/phosphate ratio (~1:1) than ferritin purified from tissue (~10:1). Phosphate significantly inhibited iron loading into L ferritin, due to the lack of the ferroxidase center in this homopolymer. Spectrophotometric assays of iron loading into H ferritin showed identical iron loading curves in the presence of phosphate, indicating that the ferroxidase center of H ferritin efficiently competes with phosphate for the binding and oxidation of Fe²⁺. Additional studies demonstrated that H ferritin ferroxidase activity could be used to oxidize Fe²⁺ and facilitate the transfer of the Fe³⁺ into apo transferrin in the presence of phosphate.     

Uptake of ionic Fe by mouse peritoneal macrophages in vitro

Mouse peritoneal macrophages were maintained in vitro up to 3 days and exposed to radiolabelled ⁵⁵Fe in the form of ferrous citrate, ferrous sulfate, and ferric chloride in concentrations of 3–5 γ Fe/ml. The divalent iron compounds were taken up 10–40 times more extensively per weight of iron than the trivalent iron compounds. The net uptake of ferrous citrate was linear during the first day and thereafter increased at a slower rate. Macrophages in culture for 1 week showed one-third the average uptake of freshly cultured cells during comparable periods of exposure to ferrous citrate. The iron taken up was used in the synthesis of mouse ferritin. Uptake of ferrous citrate was influenced by serum concentration in the tissue culture medium, temperature, pinocytosis and phagocytosis of both latex particles and heated rat erythrocytes. Uptake of ferrous citrate was enhanced by exposure to either sodium fluoride (5×10⁻³ M), or 2,4-dinitrophenol (1×10⁻⁵ M), but was not affected by cyanide, azide, or cycloheximide. The effect of sodium fluoride was not demonstrated when ferrous sulfate was substituted for ferrous citrate. The results reported here suggest that the ability of macrophages to take up ferrous citrate is good in freshly explanted cultures, is a temperature-dependent process, is suppressed by pinocytosis and phagocytosis, and paradoxically enhanced by certain metabolic inhibitors.     

In Vitro Studies of Ferritin Iron Release and Neurotoxicity

Abstract: The increase in brain iron associated with several neurodegenerative diseases may lead to an increased production of free radicals via the Fenton reaction. Intracellular iron is usually tightly regulated, being bound by ferritin in an insoluble ferrihydrite core. The neurotoxin 6-hydroxydopamine (6-OHDA) releases iron from the ferritin core by reducing it to the ferrous form. Iron release induced by 6-OHDA and structurally related compounds and two other dopaminergic neurotoxins, 1-methyl-4-phenylpyridinium iodide (MPP⁺) and 1-trichloromethyl-1,2,3,4-tetrahydro-β-carboline (TaClo), were compared, to identify the structural characteristics important for such release. 1,2,4-Trihydroxybenzene (THB) was most effective in releasing ferritin-bound iron, followed by 6-OHDA, dopamine, catechol, and hydroquinone. Resorcinol, MPP⁺, and TaClo were ineffective. The ability to release iron was associated with a low oxidation potential. It is proposed that a low oxidation potential and an ortho -dihydroxyphenyl structure are important in the mechanism by which ferritin iron is mobilized. In the presence of ferritin, both 6-OHDA and THB strongly stimulated lipid peroxidation, an effect abolished by the addition of the iron chelator deferoxamine. These results suggest that ferritin iron release contributes to free radical-induced cell damage in vivo.     

A comparison of iron availability from commercial iron preparations using an in vitro digestion/Caco-2 cell culture model

The objectives of this study were to compare iron availability from commercial preparations of FeSO(4), ferrous gluconate, ferrous fumarate, and a polysaccharide-iron complex using an in vitro digestion/Caco-2 cell culture model. In addition, we sought to determine if calcium carbonate and calcium acetate (common phosphate binding agents) inhibited iron availability from an oral iron supplement when digested simultaneously. Caco-2 cell ferritin formation following exposure to simulated gastric and intestinal digests of the iron supplements was used as a measure of iron uptake and availability. Plates without cell monolayers were included in each replication of the experiment to measure the total amount of soluble iron that resulted from the in vitro digestion. Significantly more iron was taken up from the FeSO(4), ferrous gluconate, and ferrous fumarate than the polysaccharide-iron complex. Similar results comparing FeSO(4) and the polysaccharide-iron complex have been observed in humans. In addition, less iron was taken up from digests with calcium carbonate relative to calcium acetate even though similar amounts of soluble iron were observed in these experiments. The results indicate that when iron supplements and phosphate binders are consumed simultaneously, calcium acetate may be the preferred phosphate binder to maximize iron availability.     

Relationships between the copper and iron systems in hemodialysis patients and variables affecting these systems

The copper-binding protein ceruloplasmin oxidizes ferrous iron to ferric iron, an action that is critical for the binding of iron to transferrin in plasma. Ceruloplasmin, in common with ferritin and transferrin, is an acute-phase protein that is altered by inflammation. We sought to identify interrelationships between the copper and iron systems by measuring copper, ceruloplasmin, ferroxidase, ferritin, transferrin, iron, and iron-binding capacity in a group of hemodialysis patients. We looked for evidence of inflammation and free-radical injury by assaying for protein carbonyl groups, protein pyrrolation, di-tyrosine, and advanced oxidation protein products. Our findings were compatible with an active inflammatory state that affected both iron and copper metabolism. Transferrin levels were low, whereas ceruloplasmin levels were elevated compared to normal. Copper concentration was increased proportional to ceruloplasmin. Several variables including ceruloplasmin and transferrin were observed to correlate significantly with the level of pyrrolated protein. The data suggest that posttranslational modification of circulating proteins may affect their structural, enzymatic, and ligand-binding properties. Abnormalities in copper metabolism and their influence on iron handling in renal failure are complex and will require additional study before their importance can be defined.     

Pathways in the binding and uptake of ferritin by hepatocytes

The binding and uptake of rat liver ferritin by primary cultures of rat liver hepatocytes was studied in order to assess the relative importance of saturable, high-affinity pathways and nonspecific processes in the incorporation of the protein by the cells. To minimize artifacts, ferritin not subjected to heat treatment and labeled in vivo with 59Fe was used. Binding to cell membranes was estimated from incubations performed at 4 degrees C. After 2 h, when a steady state in cell-associated ferritin had been achieved, approx. 4-10(4) binding sites per cell were observed, with an affinity constant for ferritin of 1 x 10(9) M-1. At 37 degrees C, the maximal uptake from these sites was 1.3 x 10(5) ferritin molecules/cell per h. For ferritin molecules bearing an average of 2400 iron atoms, this uptake amounts to 5 x 10(6) iron atoms/cell per min. Half-maximal uptake was achieved at a ferritin concentration, or KM1, of 3 x 10(-9) M. Although uptake rates at least a thousand times greater could be achieved by binding to the much larger number of low-affinity sites, the apparent KM2 for such 'nonspecific' uptake was 4 x 10(-7) M. At ferritin concentrations up to 2 nM, at least 90% of ferritin bound and taken up by hepatocytes involves saturable, high-affinity sites, presumably true ferritin receptors.     

Balance of macroergic compounds during the growth of Thiobacillus ferrooxidans

The balance of energy-rich compounds (ERC) was drawn up for the growth of Thiobacillus ferrooxidans in a medium with ferrous ions as an energy source. The balance items and the phosphorylating efficiency of oxidation (P/2e-) were calculated basing on the experimental yield values using the ERC balance equation. At a specific growth rate of 0.1 h-1, 55% of ferrous ions are used for the synthesis of cell biomass, 7.5% for maintainance, 4% of the ions are oxidized to reduce NAD+, and 34% are used to produce ERC necessary for the reduction. Here, 24% of ERC are used for the synthesis of monomers from CO2, 42% for the production of NADH, 24% for the biomass synthesis from monomers, and 10% for maintaining cell activity. The P/2e- for the oxidation of ferrous ions is 0.19 mole of ERC per 2e-. This is possible only when the [Fe3+]/[Fe2+] ratio in the cell periplasm is 1 X 10(3)-1 X 10(4).     

Characterization of a non-haem ferritin of the Archaeon Halobacterium salinarum,homologous to Dps (starvation-induced DNA-binding protein)

An iron-rich protein was isolated from the Archaeon Halobacterium salinarum sharing a sequence identity of 35% with the starvation-induced DNA-binding protein, DpsA, of Synechecoccus sp. PCC 7942. It consists of 20 kDa subunits, forming a dodecameric structure. The protein exhibits a ferric iron loading of up to 103 Fe ions/mol of holoprotein. CD spectra are consistent with an alpha-helical contribution of 58%. The UV/visible spectrum provides no evidence for the presence of haem groups. This protein exhibits features of a non-haem-type bacterial ferritin although it shares only little sequence homology with non-haem bacterial ferritin.