Is hydrogen peroxide produced during iron(II) oxidation in mammalian apoferritins?

Iron Mobilization from Ferritin in Yeast Cell Lysate and Physiological Implications

Most in vitro iron mobilization studies from ferritin have been performed in aqueous buffered solutions using a variety of reducing substances. The kinetics of iron mobilization from ferritin in a medium that resembles the complex milieu of cells could dramatically differ from those in aqueous solutions, and to our knowledge, no such studies have been performed. Here, we have studied the kinetics of iron release from ferritin in fresh yeast cell lysates and examined the effect of cellular metabolites on this process. Our results show that iron release from ferritin in buffer is extremely slow compared to cell lysate under identical experimental conditions, suggesting that certain cellular metabolites present in yeast cell lysate facilitate the reductive release of ferric iron from the ferritin core. Using filtration membranes with different molecular weight cut-offs (3, 10, 30, 50, and 100 kDa), we demonstrate that a cellular component >50 kDa is implicated in the reductive release of iron. When the cell lysate was washed three times with buffer, or when NADPH was omitted from the solution, a dramatic decrease in iron mobilization rates was observed. The addition of physiological concentrations of free flavins, such as FMN, FAD, and riboflavin showed about a two-fold increase in the amount of released iron. Notably, all iron release kinetics occurred while the solution oxygen level was still high. Altogether, our results indicate that in addition to ferritin proteolysis, there exists an auxiliary iron reductive mechanism that involves long-range electron transfer reactions facilitated by the ferritin shell. The physiological implications of such iron reductive mechanisms are discussed.

A Novel Approach for the Synthesis of Human Heteropolymer Ferritins of Different H to L Subunit Ratios

Mammalian ferritins are predominantly heteropolymeric species consisting of 24 structurally similar, but functionally different subunit types, named H and L, that co-assemble in different proportions. Despite their discovery more than 8 decades ago, recombinant human heteropolymer ferritins have never been synthesized, owing to the lack of a good expression system. Here, we describe for the first time a unique approach that uses a novel plasmid design that enables the synthesis of these complex ferritin nanostructures. Our study reveals an original system that can be easily tuned by altering the concentrations of two inducers, allowing the synthesis of a full spectrum of heteropolymer ferritins, from H-rich to L-rich ferritins and any combinations in-between (isoferritins). The H to L subunit composition of purified ferritin heteropolymers was analyzed by SDS-PAGE and capillary gel electrophoresis, and their iron handling properties characterized by light absorption spectroscopy. Our novel approach allows future investigations of the structural and functional differences of isoferritin populations, which remain largely obscure. This is particularly exciting since a change in the ferritin H- to L-subunit ratio could potentially lead to new iron core morphologies for various applications in bio-nanotechnologies.

Ferritin exhibits Michaelis-Menten behavior with oxygen but not with iron during iron oxidation and core mineralization

The excessively high and inconsistent literature values for Km,Fe and Km,O2 prompted us to examine the iron oxidation kinetics in ferritin, the major iron storage protein in mammals, and to determine whether a traditional Michaelis-Menten enzymatic behavior is obeyed. The kinetics of Fe(ii) oxidation and mineralization catalyzed by three different types of ferritins (recombinant human homopolymer 24H, HuHF, human heteropolymer ∼21H:3L, HL, and horse spleen heteropolymer ∼3.3H:20.7L, HosF) were therefore studied under physiologically relevant O2 concentrations, but also in the presence of excess Fe(ii) and O2 concentrations. The observed iron oxidation kinetics exhibited two distinct phases (phase I and phase II), neither of which obeyed Michaelis-Menten kinetics. While phase I was very rapid and corresponded to the oxidation of approximately 2 Fe(ii) ions per H-subunit, phase II was much slower and varied linearly with the concentration of iron(ii) cations in solution, independent of the size of the iron core. Under low oxygen concentration close to physiological, the iron uptake kinetics revealed a Michaelis-Menten behavior with Km,O2 values in the low μM range (i.e. ∼1-2 μM range). Our experimental Km,O2 values are significantly lower than typical cellular oxygen concentration, indicating that iron oxidation and mineralization in ferritin should not be affected by the oxygenation level of cells, and should proceed even under hypoxic events. A kinetic model is proposed in which the inhibition of the protein's activity is caused by bound iron(iii) cations at the ferroxidase center, with the rate limiting step corresponding to an exchange or a displacement reaction between incoming Fe(ii) cations and bound Fe(iii) cations.

Ascorbate and ferritin interactions: Consequences for iron release in vitro and in vivo and implications for inflammation

This review discusses the chemical mechanisms of ascorbate-dependent reduction and solubilization of ferritin's ferric iron core and subsequent release of ferrous iron. The process is accelerated by low concentrations of Fe(II) that increase ferritin's intrinsic ascorbate oxidase activity, hence increasing the rate of ascorbate radical formation. These increased rates of ascorbate oxidation provide reducing equivalents (electrons) to ferritin's core and speed the core reduction rates with subsequent solubilization and release of Fe(II). Ascorbate-dependent solubilization of ferritin's iron core has consequences relating to the interpretation of ⁵⁹Fe uptake sourced from ⁵⁹Fe-lebelled holotransferrin into ferritin. Ascorbate-dependent reduction of the ferritin core iron solubility increases the size of ferritin's iron exchangeable pool and hence the rate and amount of exchange uptake of ⁵⁹Fe into ferritin, whilst simultaneously increasing net iron release rate from ferritin. This may rationalize the inconsistency that ascorbate apparently stabilizes ⁵⁹Fe ferritin and retards lysosomal ferritinolysis and whole cell ⁵⁹Fe release, whilst paradoxically increasing the rate of net iron release from ferritin. This capacity of ascorbate and iron to synergise ferritin iron release has pathological significance, as it lowers the concentration at which ascorbate activates ferritin's iron release to within the physiological range (50-250 μM). These effects have relevance to inflammatory pathology and to the pro-oxidant effects of ascorbate in cancer therapy and cell death by ferroptosis.

Iron Oxidation and Core Formation in Recombinant Heteropolymeric Human Ferritins

In animals, the iron storage and detoxification protein, ferritin, is composed of two functionally and genetically distinct subunit types, H (heavy) and L (light), which co-assemble in various ratios with tissue specific distributions to form shell-like protein structures of 24 subunits within which a mineralized iron core is stored. The H-subunit possesses a ferroxidase center (FC) that catalyzes Fe(II) oxidation, whereas the L-subunit does not. To assess the role of the L-subunit in iron oxidation and core formation, two human recombinant heteropolymeric ferritins, designated H-rich and L-rich with ratios of ∼20H:4L and ∼22L:2H, respectively, were employed and compared to the human homopolymeric H-subunit ferritin (HuHF). These heteropolymeric ferritins have a composition similar to the composition of those found in hearts and brains (i.e., H-rich) and in livers and spleens (i.e., L-rich). As for HuHF, iron oxidation in H-rich ferritin was found to proceed with a 2:1 Fe(II):O₂ stoichiometry at an iron level of 2 Fe(II) atoms/H-subunit with the generation of H₂O₂. The H₂O₂ reacted with additional Fe(II) in a 2:1 Fe(II):H₂O₂ ratio, thus avoiding the production of hydroxyl radical. A μ-1,2-peroxo-diFe(III) intermediate was observed at the FC of H-rich ferritin as for HuHF. Importantly, the H-rich protein regenerated full ferroxidase activity more rapidly than HuHF did and additionally formed larger iron cores, indicating dual roles for the L-subunit in facilitating iron turnover at the FC and in mineralization of the core. The L-rich ferritin, while also facilitating iron oxidation at the FC, additionally promoted oxidation at the mineral surface once the iron binding capacity of the FC was exceeded.

Labile iron potentiates ascorbate-dependent reduction and mobilization of ferritin iron

Ascorbate mobilizes iron from equine spleen ferritin by two separate processes. Ascorbate alone mobilizes ferritin iron with an apparent K_{m (ascorbate)} ≈1.5mM. Labile iron >2μM, complexed with citrate (10mM), synergises ascorbate-dependent iron mobilization by decreasing the apparent K_{m (ascorbate)} to ≈270μM and raising maximal mobilization rate by ≈5-fold. Catalase reduces the apparent K_m(ascorbate) for both ascorbate and ascorbate+iron dependent mobilization by ≈80%. Iron mobilization by ascorbate alone has a higher activation energy (E_a=45.0±5.5kJ/mole) than when mediated by ascorbate with labile iron (10μM) (E_a=13.7±2.2kJ/mole); also mobilization by iron-ascorbate has a three-fold higher pH sensitivity (pH range 6.0-8.0) than with ascorbate alone. Hydrogen peroxide inhibits ascorbate's iron mobilizing action. EPR and autochemiluminescence studies show that ascorbate and labile iron within ferritin enhances radical formation, whereas ascorbate alone produces negligible radicals. These findings suggest that iron catalysed single electron transfer reactions from ascorbate, involving ascorbate or superoxide and possibly ferroxidase tyrosine radicals, accelerate iron mobilization from the ferroxidase centre more than EPR silent, bi-dentate two-electron transfers. These differing modes of electron transference from ascorbate mirror the known mono and bidentate oxidation reactions of dioxygen and hydrogen peroxide with di-ferrous iron at the ferroxidase centre. This study implies that labile iron, at physiological pH, complexed with citrate, synergises iron mobilization from ferritin by ascorbate (50-4000μM). This autocatalytic process can exacerbate oxidative stress in ferritin-containing inflamed tissue.

The Ferritin Superfamily

Iron is very important in many biological processes and the ferritin protein family has evolved to store iron and to maintain cellular iron homeostasis. The deletion of the coding gene for the H subunit of ferritin leads to early embryonic death in mice and mutations in the gene for the L subunits in humans has been observed in neurodegenerative diseases, such as neuroferritinopathy. Thus, understanding how ferritin works is imperative and many studies have been conducted to delineate the molecular mechanism of ferritins and bacterioferritins. In the ferritin protein family, it is clear that a catalytic center for iron oxidation, the routes for iron to reach this center and the ability to nucleate an iron core, are common requirements for all ferritins. However, there are differences in the structural and mechanistic details of iron oxidation and mineralization. Although a common mechanism has been proposed for all ferritins, this mechanism needs to be further explored. There is a mechanistic diversity related to structural variation in the ferritin protein family. It is clear that other factors appear to affect the mechanism of iron oxidation and mineralization. This review focusses on the structural features of the ferritin protein family and its role in the mechanism of iron mineralization.

Significant glial alterations in response to iron loading in a novel organotypic hippocampal slice culture model

Aberrant iron deposition in the brain is associated with neurodegenerative disorders including Multiple Sclerosis, Alzheimer’s disease and Parkinson’s disease. To study the collective response to iron loading, we have used hippocampal organotypic slices as a platform to develop a novel ex vivo model of iron accumulation. We demonstrated differential uptake and toxicity of iron after 12 h exposure to 10 μM ferrous ammonium sulphate, ferric citrate or ferrocene. Having established the supremacy of ferrocene in this model, the cultures were then loaded with 0.1–100 μM ferrocene for 12 h. One μM ferrocene exposure produced the maximal 1.6-fold increase in iron compared with vehicle. This was accompanied by a 1.4-fold increase in ferritin transcripts and mild toxicity. Using dual-immunohistochemistry, we detected ferritin in oligodendrocytes, microglia, but rarely in astrocytes and never in neurons in iron-loaded slice cultures. Moreover, iron loading led to a 15% loss of olig2-positive cells and a 16% increase in number and greater activation of microglia compared with vehicle. However, there was no appreciable effect of iron loading on astrocytes. In what we believe is a significant advance on traditional mono- or dual-cultures, our novel ex vivo slice-culture model allows characterization of the collective response of brain cells to iron-loading.

Functionality of the three-site ferroxidase center of Escherichia coli bacterial ferritin (EcFtnA)

At least three ferritins are found in the bacterium Escherichia coli : the heme-containing bacterioferritin (EcBFR) and two nonheme bacterial ferritins (EcFtnA and EcFtnB). In addition to the conserved A and B sites of the diiron ferroxidase center, EcFtnA has a third iron-binding site (the C site) of unknown function that is nearby the diiron site. In the present work, the complex chemistry of iron oxidation and deposition in EcFtnA was further defined through a combination of oximetry, pH stat, stopped-flow and conventional kinetics, UV-vis, fluorescence, and EPR spectroscopic measurements on both the wild-type protein and site-directed variants of the A, B, and C sites. The data reveal that although H2O2 is a product of dioxygen reduction in EcFtnA and oxidation occurs with a stoichiometry of Fe(2+)/O2 ∼ 3:1 most of the H2O2 produced is consumed in subsequent reactions with a 2:1 Fe(2+)/H2O2 stoichiometry, thus suppressing hydroxyl-radical formation. Although the A and B sites are essential for rapid iron oxidation, the C site slows oxidation and suppresses iron turnover at the ferroxidase center. A tyrosyl radical, assigned to Tyr24 near the ferroxidase center, is formed during iron oxidation, and its possible significance to the function of the protein is discussed. Taken as a whole, the data indicate that there are multiple iron-oxidation pathways in EcFtnA with O2 and H2O2 as oxidants. Furthermore, our data do not support a universal mechanism for iron oxidation in all ferritins whereby the C site acts as transit site, as has been recently proposed.

Semi-artificial and bioactive ferroxidase with nanoparticles as the active sites

L-chain apoferritin can be turned into a more stable and cellular active ferroxidase with nanoparticles as the artificial active sites.

Relationship between iron metabolism, oxidative stress, and insulin resistance in patients with systemic lupus erythematosus

OBJECTIVE

The aim of the present study was to assess oxidative stress and iron metabolism in systemic lupus erythematosus (SLE) patients with and without insulin resistance (IR).

METHOD

This study included 236 subjects (125 controls and 111 SLE patients). Patients with SLE were divided in two groups: with (n = 72) or without (n = 39) IR.

RESULTS

SLE patients with IR showed higher advanced oxidation protein product (AOPP) levels (p = 0.030) and gamma-glutamyltransferase (GGT) levels (p = 0.001) and lower sulfhydryl groups of proteins (p = 0.0002) and total radical-trapping antioxidant parameter (TRAP) corrected by uric acid (UA) levels (p = 0.04) when compared to SLE patients without IR. However, SLE patients with IR presented lower serum 8-isoprostane (p = 0.05) and carbonyl protein levels (p = 0.04) when compared to SLE patients without IR. Serum ferritin levels were significantly higher in SLE patients (p = 0.0006) than in controls, and SLE patients with IR presented higher serum ferritin levels (p = 0.01) than SLE patients without IR. Patients with SLE showed that IR was inversely correlated to TRAP/UA (r = -0.2724, p = 0.0008) and serum ferritin was positively correlated to AOPP (r = 0.2870, p = 0.004).

CONCLUSIONS

This study found that oxidative stress was higher in the group of SLE patients with IR, and increased ferritin, whether caused by the inflammatory process per se or hyperinsulinaemia, can favour the redox process. In addition, the preset data reinforce the need to measure oxidative stress with several methodologies with different assumptions.

Spectroscopic evidence for and characterization of a trinuclear ferroxidase center in bacterial ferritin from Desulfovibrio vulgaris Hildenborough

Ferritins are ubiquitous and can be found in practically all organisms that utilize Fe. They are composed of 24 subunits forming a hollow sphere with an inner cavity of ~80 Å in diameter. The main function of ferritin is to oxidize the cytotoxic Fe(2+) ions and store the oxidized Fe in the inner cavity. It has been established that the initial step of rapid oxidation of Fe(2+) (ferroxidation) by H-type ferritins, found in vertebrates, occurs at a diiron binding center, termed the ferroxidase center. In bacterial ferritins, however, X-ray crystallographic evidence and amino acid sequence analysis revealed a trinuclear Fe binding center comprising a binuclear Fe binding center (sites A and B), homologous to the ferroxidase center of H-type ferritin, and an adjacent mononuclear Fe binding site (site C). In an effort to obtain further evidence supporting the presence of a trinuclear Fe binding center in bacterial ferritins and to gain information on the states of the iron bound to the trinuclear center, bacterial ferritin from Desulfovibrio vulgaris (DvFtn) and its E130A variant was loaded with substoichiometric amounts of Fe(2+), and the products were characterized by Mössbauer and EPR spectroscopy. Four distinct Fe species were identified: a paramagnetic diferrous species, a diamagnetic diferrous species, a mixed valence Fe(2+)Fe(3+) species, and a mononuclear Fe(2+) species. The latter three species were detected in the wild-type DvFtn, while the paramagnetic diferrous species was detected in the E130A variant. These observations can be rationally explained by the presence of a trinuclear Fe binding center, and the four Fe species can be properly assigned to the three Fe binding sites. Further, our spectroscopic data suggest that (1) the fully occupied trinuclear center supports an all ferrous state, (2) sites B and C are bridged by a μ-OH group forming a diiron subcenter within the trinuclear center, and (3) this subcenter can afford both a mixed valence Fe(2+)Fe(3+) state and a diferrous state. Mechanistic insights provided by these new findings are discussed and a minimal mechanistic scheme involving O-O bond cleavage is proposed.

Heme degradation and vascular injury

Heme is an essential molecule in aerobic organisms. Heme consists of protoporphyrin IX and a ferrous (Fe(2+)) iron atom, which has high affinity for oxygen (O(2)). Hemoglobin, the major oxygen-carrying protein in blood, is the most abundant heme-protein in animals and humans. Hemoglobin consists of four globin subunits (alpha(2)beta(2)), with each subunit carrying a heme group. Ferrous (Fe(2+)) hemoglobin is easily oxidized in circulation to ferric (Fe(3+)) hemoglobin, which readily releases free hemin. Hemin is hydrophobic and intercalates into cell membranes. Hydrogen peroxide can split the heme ring and release "free" redox-active iron, which catalytically amplifies the production of reactive oxygen species. These oxidants can oxidize lipids, proteins, and DNA; activate cell-signaling pathways and oxidant-sensitive, proinflammatory transcription factors; alter protein expression; perturb membrane channels; and induce apoptosis and cell death. Heme-derived oxidants induce recruitment of leukocytes, platelets, and red blood cells to the vessel wall; oxidize low-density lipoproteins; and consume nitric oxide. Heme metabolism, extracellular and intracellular defenses against heme, and cellular cytoprotective adaptations are emphasized. Sickle cell disease, an archetypal example of hemolysis, heme-induced oxidative stress, and cytoprotective adaptation, is reviewed.

Effect of hydrogen sulfide in a porcine model of myocardial ischemia-reperfusion: comparison of different administration regimens and characterization of the cellular mechanisms of protection

OBJECTIVE

We investigate the impact of different regimens of parenteral hydrogen sulfide (H2S) administration on myocardium during ischemia-reperfusion (IR) and the molecular pathways involved in its cytoprotective effects.

METHODS

Eighteen male Yorkshire pigs underwent 60 minutes of mid-left anterior descending coronary artery occlusion followed by 120 minutes of reperfusion. Pigs received either placebo (control, n = 6) or H2S as a bolus (bolus group, n = 6, 0.2 mg/kg over 10 seconds at the start of reperfusion) or as an infusion (infusion group, n = 6, 2 mg.kg.h initiated at the onset of ischemia and continued into the reperfusion period). Myocardial function was monitored throughout the experiment. The area at risk and myocardial necrosis was determined by Monastral blue/triphenyl tetrazolium chloride staining. Apoptosis and the expression pattern of various intracellular effector pathways were investigated in the ischemic territory. Coronary microvascular reactivity to endothelium-dependent and endothelium-independent factors was measured.

RESULTS

H2S infusion but not bolus administration markedly reduce myocardial infarct size (P < 0.05) and improve regional left ventricular function, as well as endothelium-dependent and endothelium-independent microvascular reactivity (P < 0.05). The expression of B-cell lymphoma 2 (P = 0.059), heat shock protein 27 and alphaB-crystallin (P < 0.05) were lower in H2S-treated groups. Infusion of H2S caused higher expression of phospho-glycogen synthase kinase-3 beta isoform(P < 0.05) and lower expression of mammalian target of rapamycin and apoptosis-inducing factor (P < 0.05). Bolus of H2S caused higher expression of phospho-p44/42 MAPK extracellular signal-regulated kinase and lower expression of Beclin-1 (P < 0.05). The expression of caspase 3 and cleaved caspase 3 were lower (P < 0.05), whereas the expression of phospho-Bad(Ser136) was higher in the bolus group versus control and infusion groups (P < 0.05). The terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive cell count was lower in both H2S-treated groups compared with the control (P < 0.05).

CONCLUSIONS

This study demonstrates that infusion of H2S is superior to a bolus alone in reducing myocardial necrosis after IR injury, even though some markers of apoptosis and autophagy were affected in both H2S-treated groups. Thus, the current results indicate that infusion of H2S throughout IR may offer better myocardial protection from IR injury.

Facilitated diffusion of iron(II) and dioxygen substrates into human H-chain ferritin. A fluorescence and absorbance study employing the ferroxidase center substitution Y34W

Ferritin is a widespread iron mineralizing and detoxification protein that stores iron as a hydrous ferric oxide mineral core within a shell-like structure of 4/3/2 octahedral symmetry. Iron mineralization is initiated at dinuclear ferroxidase centers inside the protein where Fe(2+) and O(2) meet and react to form a mu-1,2-peroxodiferric intermediate that subsequently decays to form mu-oxo dimeric and oligomeric iron(III) species and ultimately the mineral core. Several types of channels penetrate the protein shell and are possible pathways for the diffusion of iron and dioxygen to the ferroxidase centers. In the present study, UV/visible and fluorescence stopped-flow spectrophotometries were used to determine the kinetics and pathways of Fe(2+) diffusion into the protein shell, its binding at the ferroxidase center and its subsequent oxidation by O(2). Three fluorescence variants of human H-chain ferritin were prepared in which Trp34 was introduced near the ferroxidase center. They included a control variant no. 1 (W93F/Y34W), a "1-fold" channel variant no. 2 (W93F/Y34W/Y29Q) and a 3-fold channel variant no. 3 (Y34W/W93F/D131I/E134F). Anaerobic rapid mixing of Fe(2+) with apo-variant no. 1 quenched the fluorescence of Trp34 with a rate exhibiting saturation kinetics with respect to Fe(2+) concentration, consistent with a process involving facilitated diffusion. A half-life of approximately 3 ms for this process is attributed to the time for diffusion of Fe(2+) across the protein shell to the ferroxidase center. No fluorescence quenching was observed with the 3-fold channel variant no. 3 or when Zn(2+) was prebound in each of the eight 3-fold channels of variant no. 1, observations indicating that the hydrophilic channels are the only avenues for rapid Fe(2+) access to the ferroxidase center. Substitution of Tyr29 with glutamine at the entrance of the "1-fold" hydrophobic channel had no effect on the rate of Fe(2+) oxidation to form the mu-1,2-peroxodiferric complex (t(1/2) approximately 38 ms), a finding demonstrating that Tyr29 and, by inference, the "1-fold" channels do not facilitate O(2) transport to the ferroxidase center, contrary to predictions of DFT and molecular dynamics calculations. O(2) diffusion into ferritin occurs on a time scale that is fast relative to the millisecond kinetics of the stopped-flow experiment.

Anaerobic iron deposition into horse spleen, recombinant human heavy and light and bacteria ferritins by large oxidants

Large-molecule oxidants oxidize Fe(II) to form Fe(III) cores in the interior of ferritins at rates comparable to or faster than the iron deposition reaction using O(2) as oxidant. Iron deposition into horse spleen ferritin (HoSF) occurs using ferricyanide ion, 2,6-dichlorophenol-indophenol, and several redox proteins: cytochrome c, stellacyanin, and ceruloplasmin. Cytochrome c also loads iron into recombinant human H-chain (rHF), human L-chain (rLF), and A. vinelandii bacterioferritin (AvBF). The enzymatic activities of ferritins were monitored anaerobically using stopped-flow kinetic spectrophotometry. The reactions exhibit saturation kinetics with respect to the large oxidant concentrations, giving apparent Michaelis constants for cytochrome c as oxidant: K(m)=39.6 microM for HoSF and 6.9 microM for AvBF. Comparison of the kinetic parameters with that of iron deposition by O(2) shows that large oxidants load iron into HoSF and AvBF more effectively than O(2) and may use a mechanism different than the ferroxidase center. Large oxidants did not deposit iron as efficiently with rHF and rLF. The results suggest that the heme groups in AvBF and the protein redox centers present in heteropolymers may assist in anaerobic iron deposition by large oxidants. The physiological relevance of iron deposition by large molecules, including protein oxidants is discussed.

Ferritin-catalyzed consumption of hydrogen peroxide by amine buffers causes the variable Fe2+ to O2 stoichiometry of iron deposition in horse spleen ferritin

Ferritin catalyzes the oxidation of Fe2+ by O2 to form a reconstituted Fe3+ oxy-hydroxide mineral core, but extensive studies have shown that the Fe2+ to O2 stoichiometry changes with experimental conditions. At Fe2+ to horse spleen ferritin (HoSF) ratios greater than 200, an upper limit of Fe2+ to O2 of 4 is typically measured, indicating O2 is reduced to 2H2O. In contrast, a lower limit of Fe2+ to O2 of approximately 2 is measured at low Fe2+ to HoSF ratios, implicating H2O2 as a product of Fe2+ deposition. Stoichiometric amounts of H2O2 have not been measured, and H2O2 is proposed to react with an unknown system component. Evidence is presented that identifies this component as amine buffers, including 3-N-morpholinopropanesulfonic acid (MOPS), which is widely used in ferritin studies. In the presence of non-amine buffers, the Fe2+ to O2 stoichiometry was approximately 4.0, but at high concentrations of amine buffers (0.10 M) the Fe2+ to O2 stoichiometry is approximately 2.5 for iron loadings of eight to 30 Fe2+ per HoSF. Decreasing the concentration of amine buffer to zero resulted in an Fe2+ to O2 stoichiometry of approximately 4. Direct evidence for amine buffer modification during Fe2+ deposition was obtained by comparing authentic and modified buffers using mass spectrometry, NMR, and thin layer chromatography. Tris(hydroxymethyl)aminomethane, MOPS, and N-methylmorpholine (a MOPS analog) were all rapidly chemically modified during Fe2+ deposition to form N-oxides. Under identical conditions no modification was detected when amine buffer, H2O2, and O2 were combined with Fe2+ or ferritin separately. Thus, a short-lived ferritin intermediate is required for buffer modification by H2O2. Variation of the Fe2+ to O2 stoichiometry versus the Fe2+ to HoSF ratio and the amine buffer concentration are consistent with buffer modification.

Ferritins: iron/oxygen biominerals in protein nanocages

Ferritin protein nanocages that form iron oxy biominerals in the central nanometer cavity are nature's answer to managing iron and oxygen; gene deletions are lethal in mammals and render bacteria more vulnerable to host release of antipathogen oxidants. The multifunctional, multisubunit proteins couple iron with oxygen (maxi-ferritins) or hydrogen peroxide (mini-ferritins) at catalytic sites that are related to di-iron sites oxidases, ribonucleotide reductase, methane monooxygenase and fatty acid desaturases, and synthesize mineral precursors. Gated pores, distributed symmetrically around the ferritin cages, control removal of iron by reductants and chelators. Gene regulation of ferritin, long known to depend on iron and, in animals, on a noncoding messenger RNA (mRNA) structure linked in a combinatorial array to functionally related mRNA of iron transport, has recently been shown to be linked to an array of proteins for antioxidant responses such as thioredoxin and quinone reductases. Ferritin DNA responds more to oxygen signals, and ferritin mRNA responds more to iron signals. Ferritin genes (DNA and RNA) and protein function at the intersection of iron and oxygen chemistry in biology.

Paired Bacillus anthracis Dps (mini-ferritin) have different reactivities with peroxide

Dps (DNA protection during starvation) proteins, mini-ferritins in the ferritin superfamily, catalyze Fe(2+)/H(2)O(2)/O(2) reactions and make minerals inside protein nanocages to minimize radical oxygen-chemistry (metal/osmotic/temperature/nutrient/oxidant) and sometimes to confer virulence. Paired Dps proteins in Bacillus, rare in other bacteria, have 60% sequence identity. To explore functional differences in paired Bacilli Dps protein, we measured ferroxidase activity and DNA protection (hydroxyl radical) for Dps protein dodecamers from Bacillus anthracis (Ba) since crystal structures and iron mineralization (iron-stain) were known. The self-assembled (200 kDa) Ba Dps1 (Dlp-1) and Ba Dps2 (Dlp-2) proteins had similar Fe(2+)/O(2) kinetics, with space for minerals of 500 iron atoms/protein, and protected DNA. The reactions with Fe(2+) were novel in several ways: 1) Ba Dps2 reactions (Fe(2+)/H(2)O(2)) proceeded via an A(650 nm) intermediate, with similar rates to maxi-ferritins (Fe(2+)/O(2)), indicating a new Dps protein reaction pathway, 2) Ba Dps2 reactions (Fe(2+)/O(2) versus Fe(2+)/O(2) + H(2)O(2)) differed 3-fold contrasting with Escherichia coli Dps reactions, with 100-fold differences, and 3) Ba Dps1, inert in Fe(2+)/H(2)O(2) catalysis, inhibited protein-independent Fe(2+)/H(2)O(2) reactions. Sequence similarities between Ba Dps1 and Bacillus subtilis DpsA (Dps1), which is regulated by general stress factor (SigmaB) and Fur, and between Ba Dps2 and B. subtilis MrgA, which is regulated by H(2)O(2) (PerR), suggest the function of Ba Dps1 is iron sequestration and the function of Ba Dps2 is H(2)O(2) destruction, important in host/pathogen interactions. Destruction of H(2)O(2) by Ba Dps2 proceeds via an unknown mechanism with an intermediate similar spectrally (A(650 nm)) and kinetically to the maxi-ferritin diferric peroxo complex.

Oxidation of Good’s buffers by hydrogen peroxide

Good's zwitterionic buffers are widely used in biological and biochemical research in which hydrogen peroxide is a solution component. This study was undertaken to determine whether Good's buffers exhibit reactivity toward H(2)O(2). It is found that H(2)O(2) oxidizes both morpholine ring-containing buffers (e.g., Mops, Mes) and piperazine ring-containing zwitterionic buffers (e.g., Pipes, Hepes, and Epps) to produce their corresponding N-oxide forms. The percentage of oxidized buffer increases as the concentration of H(2)O(2) increases. However, the rate of oxidation is relatively slow. For example, no oxidized Mops was detected 2h after adding 0.1M H(2)O(2) to 0.1M Mops (pH 7.0), and only 5.7% was oxidized after 24h exposure to H(2)O(2). Thus, although all of these buffers can be oxidized by H(2)O(2), their slow reaction does not significantly perturb levels of H(2)O(2) in the time frame and at the concentrations of most biochemical studies. Therefore, the previously reported rapid loss of H(2)O(2) produced from the ferroxidase reaction of ferritin is unlikely due to reaction of H(2)O(2) with buffer, a conclusion supported by the fact that H(2)O(2) is also lost rapidly when the solution pH of the ferroxidase reaction is controlled by a pH stat apparatus in the absence of buffer.

μ-1,2-Peroxo Diferric Complex Formation in Horse Spleen Ferritin. A Mixed H/L-Subunit Heteropolymer

Previous kinetics studies with homopolymer ferritins (bullfrog M-chain, human H-chain and Escherichia coli bacterial ferritins) have established that a mu-1,2-peroxo diferric intermediate is formed during Fe(II) oxidation by O2 at the ferroxidase site of the protein. The present study was undertaken to determine whether such an intermediate is formed also during iron oxidation in horse spleen ferritin (HoSF), a naturally occurring heteropolymer ferritin of H and L-subunits (approximately 3.3 H-chains/HoSF), and to assess its role in the formation of the mineral core. Multi-wavelength stopped-flow spectrophotometry of the oxidative deposition of iron in HoSF demonstrated that a transient peroxo complex (lambda(max) approximately 650 nm) is produced in this protein as for other ferritins. The peroxo complex in HoSF is formed about fourfold slower than in human H-chain (HuHF) and decays more slowly (approximately threefold) as well, at an iron level of two Fe(II)/H-chain. However, as found for HuHF, a second intermediate is formed in HoSF as a decay product of the peroxo complex. Only one-third of the expected peroxo complex forms at the ferroxidase centers of HoSF when two Fe(II)/H-subunits are added to the protein, dropping to only approximately 14% when 20 Fe(II)/H-chain are added, indicating a declining role of the peroxo complex in iron deposition. In contrast to HuHF, HoSF does not enzymatically regenerate the observable peroxo complex. The kinetics of mineralization in HoSF are modeled satisfactorily by a mechanism in which the ferroxidase site rapidly produces an incipient core from a single turnover of iron, upon which subsequent Fe(II) is oxidized autocatalytically to build the Fe(O)OH(s) mineral core. This model supports a role for the L-chain in iron mineralization and helps to explain the widespread occurrence of heteropolymer ferritins in tissues of vertebrates.

Kinetic and Thermodynamic Characterization of the Cobalt and Manganese Oxyhydroxide Cores Formed in Horse Spleen Ferritin

Horse spleen ferritin (HoSF) containing 800-1500 cobalt or 250-1200 manganese atoms as Co(O)OH and Mn(O)OH mineral cores within the HoSF interior (Co-HoSF and Mn-HoSF) was synthesized, and the chemical reactivity, kinetics of reduction, and the reduction potentials were measured. Microcoulometric and chemical reduction of HoSF containing the M(O)OH mineral core (M = Co or Mn) was rapid and quantitative with a reduction stoichiometry of 1.05 +/- 0.10 e/M forming a stable M(OH)(2) mineral core. At pH 9.0, ascorbic acid (AH(2)), a two-electron reductant, effectively reduced the mineral cores; however, the reaction was incomplete and rapidly reached equilibrium. The addition of excess AH(2) shifted the reaction to completion with a M(3+)/AH(2) stoichiometry of 1.9-2.1, consistent with a single electron per metal atom reduction. The rate of reaction between M(O)OH and excess AH(2) was measured by monitoring the decrease in mineral core absorbance with time. The reaction was first order in each reactant with second-order rate constants of 0.53 and 4.74 M(-1) min(-1), respectively, for Co- and Mn-HoSF at pH 9.0. From the variation of absorbance with increasing AH(2) concentration, equilibrium constants at pH 9.0 of 5.0 +/- 1.9 for Co-HoSF and 2.9 +/- 0.9 for Mn-HoSF were calculated for 2M(O)OH + AH(2) = 2M(OH)(2) + D, where AH(2) and D are ascorbic acid and dehydroascorbic acid, respectively. Consistent with these equilibrium constants, the standard potential for the reduction of Co(III)-HoSF is 42 mV more positive than that of the ascorbic acid reaction, while the standard potential of Mn(III)-HoSF is 27 mV positive relative to AH(2). Fe(2+) in solution with Co- and Mn-HoSF under anaerobic conditions was oxidized to form Fe(O)OH within the HoSF interior, resulting in partial displacement of the Co or Mn by iron.

Unique Iron Binding and Oxidation Properties of Human Mitochondrial Ferritin: A Comparative Analysis with Human H-chain Ferritin

Ferritins are ubiquitous iron mineralizing and storage proteins that play an important role in iron homeostasis. Although excess iron is stored in the cytoplasm, most of the metabolically active iron is processed in the mitochondria of the cell. Little is known about how these organelles regulate iron homeostasis and toxicity. The recently discovered human mitochondrial ferritin (MtF), unlike other mammalian ferritins, is a homopolymer of 24 subunits that has a high degree of sequence homology with human H-chain ferritin (HuHF). Parallel experiments with MtF and HuHF reported here reveal striking differences in their iron oxidation and hydrolysis chemistry despite their similar diFe ferroxidase centers. In contrast to HuHF, MtF does not regenerate its ferroxidase activity after oxidation of its initial complement of Fe(II) and generally has considerably slower ferroxidation and mineralization activities as well. MtF exhibits sigmoidal kinetics of mineralization more characteristic of an L-chain than an H-chain ferritin. Site-directed mutagenesis reveals that serine 144, a residue situated near the ferroxidase center in MtF but absent from HuHF, is one player in this impairment of activity. Additionally only one-half of the 24 ferroxidase centers of MtF are functional, further contributing to its lower activity. Stopped-flow absorption spectrometry of Fe(II) oxidation by O(2) in MtF shows the formation of a transient diiron(III) mu-peroxo species (lambda(max) = 650 nm) as observed in HuHF. Also, as for HuHF, minimal hydroxyl radical is produced during the oxidative deposition of iron in MtF using O(2) as the oxidant. However, the 2Fe(II) + H(2)O(2) detoxification reaction found in HuHF does not occur in MtF. The structural differences and the physiological implications of the unique iron oxidation properties of MtF are discussed in light of these results.

Kinetic studies of iron deposition catalyzed by recombinant human liver heavy, and light ferritins and Azotobacter vinelandii bacterioferritin using O2 and H2O2 as oxidants

The discrepancy between predicted and measured H(2)O(2) formation during iron deposition with recombinant heavy human liver ferritin (rHF) was attributed to reaction with the iron protein complex [Biochemistry 40 (2001) 10832-10838]. This proposal was examined by stopped-flow kinetic studies and analysis for H(2)O(2) production using (1) rHF, and Azotobacter vinelandii bacterial ferritin (AvBF), each containing 24 identical subunits with ferroxidase centers; (2) site-altered rHF mutants with functional and dysfunctional ferroxidase centers; and (3) recombinant human liver light ferritin (rLF), containing no ferroxidase center. For rHF, nearly identical pseudo-first-order rate constants of 0.18 s(-1) at pH 7.5 were measured for Fe(2+) oxidation by both O(2) and H(2)O(2), but for rLF, the rate with O(2) was 200-fold slower than that for H(2)O(2) (k = 0.22 s(-1)). A Fe(2+)/O(2) stoichiometry near 2.4 was measured for rHF and its site altered forms, suggesting formation of H(2)O(2). Direct measurements revealed no H(2)O(2) free in solution 0.5-10 min after all Fe(2+) was oxidized at pH 6.5 or 7.5. These results are consistent with initial H(2)O(2) formation, which rapidly reacts in a secondary reaction with unidentified solution components. Using measured rate constants for rHF, simulations showed that steady-state H(2)O(2) concentrations peaked at 14 muM at approximately 600 ms and decreased to zero at 10-30 s. rLF did not produce measurable H(2)O(2) but apparently conducted the secondary reaction with H(2)O(2). Fe(2+)/O(2) values of 4.0 were measured for AvBF. Stopped-flow measurements with AvBF showed that both H(2)O(2) and O(2) react at the same rate (k = 0.34 s(-1)), that is faster than the reactions with rHF. Simulations suggest that AvBF reduces O(2) directly to H(2)O without intermediate H(2)O(2) formation.

Origin of the Unusual Kinetics of Iron Deposition in Human H-Chain Ferritin

From microorganisms to humans, ferritin plays a central role in the biological management of iron. The ferritins function as iron storage and detoxification proteins by oxidatively depositing iron as a hydrous ferric hydroxide mineral core within their shell-like structures. The mechanism by which the mineral core is formed has been the subject of intense investigation for many years. A diiron ferroxidase site located on the H-chain subunit of vertebrate ferritins catalyzes the oxidation of Fe(II) to Fe(III) by molecular oxygen. A previous stopped-flow kinetics study of a transient mu-peroxodiFe(III) intermediate formed at this site revealed very unusual kinetics curves, the shape of which depended markedly on the amount of iron presented to the protein. In the present work, a mathematical model for catalysis is developed that explains the observed kinetics. The model consists of two sequential mechanisms. In the first mechanism, turnover of iron at the ferroxidase site is rapid, resulting in steady-state production of the peroxo intermediate with continual formation of the mineral core until the available Fe(II) in solution is consumed. At this point, the second mechanism comes into play whereby the peroxo intermediate decays and the ferroxidase site is postulated to vacate its complement of iron. The kinetic data reveal for the first time that Fe(II) in excess of that required to saturate the ferroxidase site promotes rapid turnover of Fe(III) at this site and that the ferroxidase site plays a role in catalysis at all levels of iron loading of the protein (48-800 Fe/protein). The data also provide evidence for a second intermediate, a putative hydroperoxodiFe(III) complex, that is a decay product of the peroxo intermediate.

Formation of protein-coated iron minerals

The ability of iron to cycle between Fe(2+) and Fe(3+) forms has led to the evolution, in different forms, of several iron-containing protein cofactors that are essential for a wide variety of cellular processes, to the extent that virtually all cells require iron for survival and prosperity. The redox properties of iron, however, also mean that this metal is potentially highly toxic and this, coupled with the extreme insolubility of Fe(3+), presents the cell with the significant problem of how to maintain this essential metal in a safe and bioavailable form. This has been overcome through the evolution of proteins capable of reversibly storing iron in the form of a Fe(3+) mineral. For several decades the ferritins have been synonymous with the function of iron storage. Within this family are subfamilies of mammalian, plant and bacterial ferritins which are all composed of 24 subunits assembled to form an essentially spherical protein with a central cavity in which the mineral is laid down. In the past few years it has become clear that other proteins, belonging to the family of DNA-binding proteins from starved cells (the Dps family), which are oligomers of 12 subunits, and to the frataxin family, which may contain up to 48 subunits, are also able to lay down a Fe(3+) mineral core. Here we present an overview of the formation of protein-coated iron minerals, with particular emphasis on the structures of the protein coats and the mechanisms by which they promote core formation. We show on the one hand that significant mechanistic similarities exist between structurally dissimilar proteins, while on the other that relatively small structural differences between otherwise similar proteins result in quite dramatic mechanistic differences.

Kinetic studies of iron deposition in horse spleen ferritin using H2O2 and O2 as oxidants

The reaction of horse spleen ferritin (HoSF) with Fe2+ at pH 6.5 and 7.5 using O2, H2O2 and 1:1 a mixture of both showed that the iron deposition reaction using H2O2 is approximately 20- to 50-fold faster than the reaction with O2 alone. When H2O2 was added during the iron deposition reaction initiated with O2 as oxidant, Fe2+ was preferentially oxidized by H2O2, consistent with the above kinetic measurements. Both the O2 and H2O2 reactions were well defined from 15 to 40 degrees C from which activation parameters were determined. The iron deposition reaction was also studied using O2 as oxidant in the presence and absence of catalase using both stopped-flow and pumped-flow measurements. The presence of catalase decreased the rate of iron deposition by approximately 1.5-fold, and gave slightly smaller absorbance changes than in its absence. From the rate constants for the O2 (0.044 s(-1)) and H2O2 (0.67 s(-1)) iron-deposition reactions at pH 7.5, simulations of steady-state H2O2 concentrations were computed to be 0.45 microM. This low value and reported Fe2+/O2 values of 2.0-2.5 are consistent with H2O2 rapidly reacting by an alternate but unidentified pathway involving a system component such as the protein shell or the mineral core as previously postulated [Biochemistry 22 (1983) 876; Biochemistry 40 (2001) 10832].

Iron binding and oxidation kinetics in frataxin CyaY of Escherichia coli

Friedreich's ataxia is associated with a deficiency in frataxin, a conserved mitochondrial protein of unknown function. Here, we investigate the iron binding and oxidation chemistry of Escherichia coli frataxin (CyaY), a homologue of human frataxin, with the aim of better understanding the functional properties of this protein. Anaerobic isothermal titration calorimetry (ITC) demonstrates that at least two ferrous ions bind specifically but relatively weakly per CyaY monomer (K(d) approximately 4 microM). Such weak binding is consistent with the hypothesis that the protein functions as an iron chaperone. The bound Fe(II) is oxidized slowly by O(2). However, oxidation occurs rapidly and completely with H(2)O(2) through a non-enzymatic process with a stoichiometry of two Fe(II)/H(2)O(2), indicating complete reduction of H(2)O(2) to H(2)O. In accord with this stoichiometry, electron paramagnetic resonance (EPR) spin trapping experiments indicate that iron catalyzed production of hydroxyl radical from Fenton chemistry is greatly attenuated in the presence of CyaY. The Fe(III) produced from oxidation of Fe(II) by H(2)O(2) binds to the protein with a stoichiometry of six Fe(III)/CyaY monomer as independently measured by kinetic, UV-visible, fluorescence, iron analysis and pH-stat titrations. However, as many as 25-26 Fe(III)/monomer can bind to the protein, exhibiting UV absorption properties similar to those of hydrolyzed polynuclear Fe(III) species. Analytical ultracentrifugation measurements indicate that a tetramer is formed when Fe(II) is added anaerobically to the protein; multiple protein aggregates are formed upon oxidation of the bound Fe(II). The observed iron oxidation and binding properties of frataxin CyaY may afford the mitochondria protection against iron-induced oxidative damage.

Yeast frataxin sequentially chaperones and stores iron by coupling protein assembly with iron oxidation

We have investigated the mechanism of frataxin, a conserved mitochondrial protein involved in iron metabolism and neurodegenerative disease. Previous studies revealed that the yeast frataxin homologue (mYfh1p) is activated by Fe(II) in the presence of O2 and assembles stepwise into a 48-subunit multimer (alpha48) that sequesters >2000 atoms of iron in 2-4-nm cores structurally similar to ferritin iron cores. Here we show that mYfh1p assembly is driven by two sequential iron oxidation reactions: A ferroxidase reaction catalyzed by mYfh1p induces the first assembly step (alpha --> alpha3), followed by a slower autoxidation reaction that promotes the assembly of higher order oligomers yielding alpha48. Depending on the ionic environment, stepwise assembly is associated with accumulation of 50-75 Fe(II)/subunit. Initially, this Fe(II) is loosely bound to mYfh1p and can be readily mobilized by chelators or made available to the mitochondrial enzyme ferrochelatase to synthesize heme. Transfer of mYfh1p-bound Fe(II) to ferrochelatase occurs in the presence of citrate, a physiologic ferrous iron chelator, suggesting that the transfer involves an intermolecular interaction. If mYfh1p-bound Fe(II) is not transferred to a ligand, iron oxidation, and mineralization proceed to completion, Fe(III) becomes progressively less accessible, and a stable iron-protein complex is formed. Iron oxidation-driven stepwise assembly is a novel mechanism by which yeast frataxin can function as an iron chaperone or an iron store.

Insights into the effects on metal binding of the systematic substitution of five key glutamate ligands in the ferritin of Escherichia coli

Ferritins are nearly ubiquitous iron storage proteins playing a fundamental role in iron metabolism. They are composed of 24 subunits forming a spherical protein shell encompassing a central iron storage cavity. The iron storage mechanism involves the initial binding and subsequent O2-dependent oxidation of two Fe2+ ions located at sites A and B within the highly conserved dinuclear "ferroxidase center" in individual subunits. Unlike animal ferritins and the heme-containing bacterioferritins, the Escherichia coli ferritin possesses an additional iron-binding site (site C) located on the inner surface of the protein shell close to the ferroxidase center. We report the structures of five E. coli ferritin variants and their Fe3+ and Zn2+ (a redox-stable alternative for Fe2+) derivatives. Single carboxyl ligand replacements in sites A, B, and C gave unique effects on metal binding, which explain the observed changes in Fe2+ oxidation rates. Binding of Fe2+ at both A and B sites is clearly essential for rapid Fe2+ oxidation, and the linking of FeB2+ to FeC2+ enables the oxidation of three Fe2+ ions. The transient binding of Fe2+ at one of three newly observed Zn2+ sites may allow the oxidation of four Fe2+ by one dioxygen molecule.

Ferritin: at the crossroads of iron and oxygen metabolism

Iron and oxygen are central to terrestrial life. Aqueous iron and oxygen chemistry will produce a ferric ion trillions of times less soluble than cell iron concentrations, along with radical forms of oxygen that are toxic. In the physiological environment, many proteins have evolved to transport iron or modulate the redox chemistry of iron that transforms oxygen in useful biochemical reactions. Only one protein, ferritin, evolved to concentrate iron to levels needed in aerobic metabolism. Reversible formation and dissolution of a solid nanomineral-hydrated, iron oxide is the main function of ferritin, which additionally detoxifies excess iron and possibly dioxygen and reactive oxygen. Ferritin is a large multifunctional, multisubunit protein with eight Fe transport pores, 12 mineral nucleation sites and up to 24 oxidase sites that produce mineral precursors from ferrous iron and oxygen. Regulation of ferritin synthesis in animals uses both DNA and mRNA controls and genes encoding two types of related subunits with: 1) catalytically active (H) or 2) inactive (L) oxidase sites. Ferritin with varying H/L ratios is related to cell-specific iron and oxygen homeostasis. H-ferritin oxidase activity accelerates rates of iron mineralization in ferritins and, in animals, ferritin produces H(2)O(2) as a byproduct. Properties of ferritin mRNA and ferritin protein pore structure are new targets for manipulating iron homeostasis. Recent observations of the high bioavailability of iron in soybean ferritin and efficient utilization of soybean and ferritin iron by iron-deficient animals, and of soybean iron by humans with borderline deficiency, indicate a role for ferritin in managing global iron deficiency in humans.

Mechanistic studies of the tellurium(II)/tellurium(IV) redox cycle in thiol peroxidase-like reactions of diorganotellurides in methanol

Di-n-hexyl telluride (2), di-p-methoxyphenyl telluride (3), and (S)-2-(1-N,N-dimethylaminoethyl)phenyl phenyl telluride (4) catalyzed the oxidation of PhSH to PhSSPh with H(2)O(2) in MeOH. Telluride 2 displayed greater rate acceleration than the diaryltellurides 3 and 4 as determined by the initial velocities, v(0), for the rate of appearance of PhSSPh determined at 305 nm by stopped-flow spectroscopy. Rate constants for the oxidation of tellurides 2-4 (k(ox)), rate constants for the introduction of PhSH as a ligand to the Te(IV) center (k(PhSH)) of oxidized tellurides 5-7, and thiol-independent (k(1)) and thiol-dependent (k(2)) rate constants for reductive elimination at Te(IV) in oxidized tellurides 5-7 were determined using stopped-flow spectroscopy. Oxidation of the Te atom of the electron-rich dialkyl telluride 2 was more rapid than oxidation of diaryl tellurides 3 and 4. The dimethylaminoethyl substituent of 4, which acts as a chelating ligand to Te(IV), did not affect k(ox). Values of k(PhSH) for the introduction of PhSH to oxidized dialkyl tellurane 5 and oxidized diaryl tellurane 6 were comparable in magnitude, while the chelating dimethylaminoethyl ligand of oxidized telluride 7 diminished k(PhSH) by a fator of 10(3). Reductive elimination by both first-order, thiol-independent (k(1)) and second-order, thiol-dependent (k(2)) pathways was slower from dialkyl Te(IV) species derived from 2 than from diaryl Te(IV) species derived from 3. The chelating dimethylaminoethyl ligand of Te(IV) species derived from 4 diminished k(1) by a factor of 50 and k(2) by a factor of 3 (relative to the 3-derived species).

Defining metal ion inhibitor interactions with recombinant human H- and L-chain ferritins and site-directed variants: an isothermal titration calorimetry study

Zinc and terbium, inhibitors of iron incorporation in the ferritins, have been used for many years as probes of structure-function relationships in these proteins. Isothermal titration calorimetric and kinetic measurements of Zn(II) and Tb(III) binding and inhibition of Fe(II) oxidation were used to identify and characterize thermodynamically ( n, K, Delta H degrees, Delta S degrees, and Delta G degrees ) the functionally important binding sites for these metal ions in recombinant human H-chain, L-chain, and H-chain site-directed variant ferritins. The data reveal at least two classes of binding sites for both Zn(II) and Tb(III) in human H-chain ferritin: one strong, corresponding to binding of one metal ion in each of the eight three-fold channels, and the other weak, involving binding at the ferroxidase and nucleation sites of the protein as well as at other weak unidentified binding sites. Zn(II) and Tb(III) binding to recombinant L-chain ferritin showed similar stoichiometries for the strong binding sites within the channels, but fewer weaker binding sites when compared to the H-chain protein. The kinetics and binding data indicate that the binding of Zn(II) and Tb(III) in the three-fold channels, which is the main pathway of iron(II) entry in ferritin, blocks the access of most of the iron to the ferroxidase sites on the interior of the protein, accounting for the strong inhibition by these metal ions of the oxidative deposition of iron in ferritin.

Noninvasive measurement of iron: report of an NIDDK workshop

An international workshop on the noninvasive measurement of iron was conducted by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) on April 17, 2001, to assess the current state of the science and to identify areas needing further investigation. The workshop concluded that a clear clinical need is evident for quantitative, noninvasive, safe, accurate, and readily available means of measuring body storage iron to improve the diagnosis and management of patients with iron overload from such disorders as hereditary hemochromatosis, thalassemia major, sickle cell disease, aplastic anemia, and myelodysplasia, among others. Magnetic resonance imaging (MRI) potentially provides the best available technique for examining the 3-dimensional distribution of excess iron in the body, but further research is needed to develop means of making measurements quantitative. Biomagnetic susceptometry provides the only noninvasive method to measure tissue iron stores that has been calibrated, validated, and used in clinical studies, but the complexity, cost, and technical demands of the liquid-helium-cooled superconducting instruments required at present have restricted clinical access to the method. The workshop identified basic and clinical research opportunities for deepening our understanding of the physical properties of iron and iron toxicity, for further investigation of MRI as a method for quantitative determinations of tissue iron, especially in liver, heart and brain, and for development of improved methods and more widely available instrumentation for biomagnetic susceptometry.

The ferroxidase activity of yeast frataxin

Frataxin is required for maintenance of normal mitochondrial iron levels and respiration. The mature form of yeast frataxin (mYfh1p) assembles stepwise into a multimer of 840 kDa (alpha(48)) that accumulates iron in a water-soluble form. Here, two distinct iron oxidation reactions are shown to take place during the initial assembly step (alpha --> alpha(3)). A ferroxidase reaction with a stoichiometry of 2 Fe(II)/O(2) is detected at Fe(II)/mYfh1p ratios of < or = 0.5. Ferroxidation is progressively overcome by autoxidation at Fe(II)/mYfh1p ratios of >0.5. Gel filtration analysis indicates that an oligomer of mYfh1p, alpha(3), is responsible for both reactions. The observed 2 Fe(II)/O(2) stoichiometry implies production of H(2)O(2) during the ferroxidase reaction. However, only a fraction of the expected total H(2)O(2) is detected in solution. Oxidative degradation of mYfh1p during the ferroxidase reaction suggests that most H(2)O(2) reacts with the protein. Accordingly, the addition of mYfh1p to a mixture of Fe(II) and H(2)O(2) results in significant attenuation of Fenton chemistry. Multimer assembly is fully inhibited under anaerobic conditions, indicating that mYfh1p is activated by Fe(II) in the presence of O(2). This combination induces oligomerization and mYfh1p-catalyzed Fe(II) oxidation, starting a process that ultimately leads to the sequestration of as many as 50 Fe(II)/subunit inside the multimer.

Iron detoxification properties of Escherichia coli bacterioferritin. Attenuation of oxyradical chemistry

Bacterioferritin (EcBFR) of Escherichia coli is an iron-mineralizing hemoprotein composed of 24 identical subunits, each containing a dinuclear metal-binding site known as the "ferroxidase center." The chemistry of Fe(II) binding and oxidation and Fe(III) hydrolysis using H(2)O(2) as oxidant was studied by electrode oximetry, pH-stat, UV-visible spectrophotometry, and electron paramagnetic resonance spin trapping experiments. Absorption spectroscopy data demonstrate the oxidation of two Fe(II) per H(2)O(2) at the ferroxidase center, thus avoiding hydroxyl radical production via Fenton chemistry. The oxidation reaction with H(2)O(2) corresponds to [Fe(II)(2)-P](Z) + H(2)O(2) --> [Fe(III)(2)O-P](Z) + H(2)O, where [Fe(II)(2)-P](Z) represents a diferrous ferroxidase center complex of the protein P with net charge Z and [Fe(III)(2)O-P](Z) a micro-oxo-bridged diferric ferroxidase complex. The mineralization reaction is given by 2Fe(2+) + H(2)O(2) + 2H(2)O --> 2FeOOH((core)) + 4H(+), where two Fe(II) are again oxidized by one H(2)O(2). Hydrogen peroxide is shown to be an intermediate product of dioxygen reduction when O(2) is used as the oxidant in both the ferroxidation and mineralization reactions. Most of the H(2)O(2) produced from O(2) is rapidly consumed in a subsequent ferroxidase reaction with Fe(II) to produce H(2)O. EPR spin trapping experiments show that the presence of EcBFR greatly attenuates the production of hydroxyl radical during Fe(II) oxidation by H(2)O(2), consistent with the ability of the bacterioferritin to facilitate the pairwise oxidation of Fe(II) by H(2)O(2), thus avoiding odd electron reduction products of oxygen and therefore oxidative damage to the protein and cellular components through oxygen radical chemistry.

Ferritin, iron homeostasis, and oxidative damage

Ferritin is one of the major proteins of iron metabolism. It is almost ubiquitous and tightly regulated by the metal. Biochemical and structural properties of the ferritins are largely conserved from bacteria to man, although the role in the regulation of iron trafficking varies in the different organisms. Recent studies have clarified some of the major aspects of the reaction between iron and ferritin, which results in the formation of the iron core and production of hydrogen peroxide. The characterization of cellular models in which ferritin expression is modulated has shown that the ferroxidase catalytic site on the H-chain has a central role in regulating iron availability. In turn, this has secondary effects on a number of cellular activities, which include proliferation and resistance to oxidative damage. Moreover, the response to apoptotic stimuli is affected by H-ferritin expression. Altered ferritin L-chain expression has been found in at least two types of genetic disorders, although its role in the determination of the pathology has not been fully clarified. The recent discovery of a new ferritin specific for the mitochondria, which is functionally similar to the H-ferritin, opens new perspectives in the study of the relationships between iron, oxidative damage and free radicals.

Iron and hydrogen peroxide detoxification properties of DNA-binding protein from starved cells. A ferritin-like DNA-binding protein of Escherichia coli

The DNA-binding proteins from starved cells (Dps) are a family of proteins induced in microorganisms by oxidative or nutritional stress. Escherichia coli Dps, a structural analog of the 12-subunit Listeria innocua ferritin, binds and protects DNA against oxidative damage mediated by H(2)O(2). Dps is shown to be a Fe-binding and storage protein where Fe(II) oxidation is most effectively accomplished by H(2)O(2) rather than by O(2) as in ferritins. Two Fe(2+) ions bind at each of the 12 putative dinuclear ferroxidase sites (P(Z)) in the protein according to the equation, 2Fe(2+) + P(Z) --> [(Fe(II)(2)-P](FS)(Z+2) + 2H(+). The ferroxidase site (FS) bound iron is then oxidized according to the equation, [(Fe(II)(2)-P](FS)(Z+2) + H(2)O(2) + H(2)O --> [Fe(III)(2)O(2)(OH)-P](FS)(Z-1) + 3H(+), where two Fe(II) are oxidized per H(2)O(2) reduced, thus avoiding hydroxyl radical production through Fenton chemistry. Dps acquires a ferric core of approximately 500 Fe(III) according to the mineralization equation, 2Fe(2+) + H(2)O(2) + 2H(2)O --> 2Fe(III)OOH((core)) + 4H(+), again with a 2 Fe(II)/H(2)O(2) stoichiometry. The protein forms a similar ferric core with O(2) as the oxidant, albeit at a slower rate. In the absence of H(2)O(2) and O(2), Dps forms a ferrous core of approximately 400 Fe(II) by the reaction Fe(2+) + H(2)O + Cl(-) --> Fe(II)OHCl((core)) + H(+). The ferrous core also undergoes oxidation with a stoichiometry of 2 Fe(II)/H(2)O(2). Spin trapping experiments demonstrate that Dps greatly attenuates hydroxyl radical production during Fe(II) oxidation by H(2)O(2). These results and in vitro DNA damage assays indicate that the protective effect of Dps on DNA most likely is exerted through a dual action, the physical association with DNA and the ability to nullify the toxic combination of Fe(II) and H(2)O(2). In the latter process a hydrous ferric oxide mineral core is produced within the protein, thus avoiding oxidative damage mediated by Fenton chemistry.

mu-1,2-Peroxobridged di-iron(III) dimer formation in human H-chain ferritin

Biomineralization of the ferritin iron core involves a complex series of events in which H(2)O(2) is produced during iron oxidation by O(2) at a dinuclear centre, the 'ferroxidase site', located on the H-subunit of mammalian proteins. Rapid-freeze quench Mössbauer spectroscopy was used to probe the early events of iron oxidation and mineralization in recombinant human ferritin containing 24 H-subunits. The spectra reveal that a mu-1,2-peroxodiFe(III) intermediate (species P) with Mössbauer parameters delta (isomer shift)=0.58 mm/s and DeltaE(Q) (quadrupole splitting)=1.07 mm/s at 4.2 K is formed within 50 ms of mixing Fe(II) with the apoprotein. This intermediate accounts for almost all of the iron in the sample at 160 ms. It subsequently decays within 10 s to form a mu-oxodiFe(III)-protein complex (species D), which partially vacates the ferroxidase sites of the protein to generate Fe(III) clusters (species C) at a reaction time of 10 min. The intermediate peroxodiFe(III) complex does not decay under O(2)-limiting conditions, an observation suggesting inhibition of decay by unreacted Fe(II), or a possible role for O(2) in ferritin biomineralization in addition to that of direct oxidation of iron(II).