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PubMed1998

Metal-catalysed cleavage of Na,K-ATPase as a tool for study of structure-function relations.

This paper describes a novel technique for specific cleavage of renal Na/K-ATPase, based on bound transition metal ions. The approach might have application to other P-type pumps or membrane proteins. In one type of experiment, specific cleavages of the alpha subunit have been observed following incubation with ascorbate plus H2O2. Five fragments with intact C-terminals and complementary fragments with intact N-terminals are detectable. The beta subunit is not cleaved. Cleavages depend on the presence of contaminant or added submicromolar concentrations of Fe2+ ions. The results suggest that Fe2+ (or Fe3+) binds with high affinity at the cytoplasmic surface and catalyze cleavages of peptide bonds close to the Fe2+ (or Fe3+) ion. The rate of cleavage is greatly affected by the conformational state of the protein, E1Na or E2(Rb), respectively. The findings provide information on spatial organization of the protein and suggest that the highly conserved regions of the alpha subunit, within the minor and major cytoplasmic loops, interact in the E2 or E2(Rb) conformations, but move apart in the E1 or E1Na conformations. In a second application of this technique, added Cu2+ ions at micromolar concentrations, have been shown to catalyse specific cleavages of both alpha and beta subunits at the extracellular surface. The experiments provide evidence for trans-membrane topology and proximity between trans-membrane segments M5-M10 within the alpha subunit and for interacting segments of alpha and beta subunits. We discuss the implications of metal-catalysed cleavages for spatial organisation of transmembrane helices of the protein.

Amino Acid Sequence↗
PubMed2003

Direct conversion of ferrous myeloperoxidase to compound II by hydrogen peroxide: an anaerobic stopped-flow study.

Myeloperoxidase (MPO) is one of the essential components of the antimicrobial systems of polymorphonuclear neutrophils. It is unique in having a globin-like standard reduction potential of the ferric/ferrous couple. Here, it is shown that ferrous MPO heterolytically cleaves hydrogen peroxide forming water and oxyferryl MPO (compound II). The two-electron oxidation reaction follows second-order kinetics with the apparent bimolecular rate constant being (6.8+/-0.6)x10(4)M(-1)s(-1) at pH 7.0. After depletion of (micromolar) H(2)O(2) compound II slowly decays to ferric MPO, whereas upon addition of millimolar H(2)O(2) to ferrous MPO, compound III (oxyperoxidase) is formed in a sequence of two reactions involving compound II formation and its direct reaction with H(2)O(2), which also follows second-order kinetics [(78+/-2)M(-1)s(-1) at pH 7.0]. It is discussed how these reactions contribute to the interconversion of compound II and compound III and could explain the catalase activity of MPO.

Anaerobiosis↗
PubMed2004

A novel multistep mechanism for oxygen binding to ferrous hemoproteins: rapid kinetic analysis of ferrous-dioxy myeloperoxidase (compound III) formation.

Myeloperoxidase (MPO), a hemoprotein that uses H(2)O(2) as the electron acceptor in the catalysis of oxidative reactions, is implicated as a participant in inflammatory injury and cardiovascular diseases. Mechanisms for turning off this enzyme once released, preventing unwanted tissue injury, are poorly understood. We recently demonstrated that MPO heme reduction causes collapse of the heme pocket, as monitored by significant reductions in the rates of diatomic ligand binding to the heme iron. Using spectral and rapid kinetic measurements, we now demonstrate that molecular oxygen (O(2)) binds to ferrous MPO (MPO-Fe(II)) in a distinct and novel mechanism. Rather than occurring through a simple, reversible, one-step mechanism, as is typical for O(2) binding to other ferrous hemoproteins, the reaction involves several kinetically and spectroscopically distinguishable intermediates. Diode array spectrophotometric and stopped-flow studies reveal that the formation of the MPO-Fe(II)-O(2) complex consists of at least three elementary steps and includes at least two sequential transient intermediates. The first step involves reversible formation of a transient intermediate via an O(2)-dependent mechanism, followed by two sequential O(2)-independent steps that appear to be conformational in origin. Insights into mechanisms for inactivating MPO and the novel mode of O(2) binding to the hemoprotein may provide important clues toward understanding the catalytic action of MPO.

Binding Sites↗
PubMed2004

Differential protein expression during growth of Acidithiobacillus ferrooxidans on ferrous iron, sulfur compounds, or metal sulfides.

A set of proteins that changed their levels of synthesis during growth of Acidithiobacillus ferrooxidans ATCC 19859 on metal sulfides, thiosulfate, elemental sulfur, and ferrous iron was characterized by using two-dimensional polyacrylamide gel electrophoresis. N-terminal amino acid sequencing and mass spectrometry analysis of these proteins allowed their identification and the localization of the corresponding genes in the available genomic sequence of A. ferrooxidans ATCC 23270. The genomic context around several of these genes suggests their involvement in the energetic metabolism of A. ferrooxidans. Two groups of proteins could be distinguished. The first consisted of proteins highly upregulated by growth on sulfur compounds (and downregulated by growth on ferrous iron): a 44-kDa outer membrane protein, an exported 21-kDa putative thiosulfate sulfur transferase protein, a 33-kDa putative thiosulfate/sulfate binding protein, a 45-kDa putative capsule polysaccharide export protein, and a putative 16-kDa protein of unknown function. The second group of proteins comprised those downregulated by growth on sulfur (and upregulated by growth on ferrous iron): rusticyanin, a cytochrome c(552), a putative phosphate binding protein (PstS), the small and large subunits of ribulose biphosphate carboxylase, and a 30-kDa putative CbbQ protein, among others. The results suggest in general a separation of the iron and sulfur utilization pathways. Rusticyanin, in addition to being highly expressed on ferrous iron, was also newly synthesized, as determined by metabolic labeling, although at lower levels, during growth on sulfur compounds and iron-free metal sulfides. During growth on metal sulfides containing iron, such as pyrite and chalcopyrite, both proteins upregulated on ferrous iron and those upregulated on sulfur compounds were synthesized, indicating that the two energy-generating pathways are induced simultaneously depending on the kind and concentration of oxidizable substrates available.

Acidithiobacillus↗
PubMed1997

Resonance Raman spectroscopic characterization of alpha-hydroxyheme and verdoheme complexes of heme oxygenase.

Heme oxygenase (HO) is the microsomal enzyme that catalyzes the oxidative degradation of protoheme (iron protoporphyrin IX) and the generation of carbon monoxide. The enzyme converts protoheme into biliverdin through two known heme derivatives, alpha-hydroxyheme and verdoheme. To gain insight into the degradation mechanisms of the two intermediates, the resonance Raman spectra were observed for alpha-hydroxyheme and verdoheme complexes of HO and compared with those of apomyoglobin (apo-Mb) complexes. The ferrous alpha-hydroxyheme complexed with both HO and apo-Mb shows a resonance Raman spectral pattern similar to that of the protoheme complexes. On the contrary, the ferric alpha-hydroxyheme and ferrous verdoheme complexes of HO and apo-Mb show atypical Raman patterns, which are interpreted as the result of the symmetry lowering of the porphyrin-conjugated pi-electron system. The comparison of the resonance Raman spectra of the verdoheme complexed with HO and apo-Mb with those of the five- and six-coordinate model complexes of verdoheme shows that the ferrous forms of the verdoheme-protein complexes are six-coordinate. The Fe-CO and Fe-CN stretching frequencies of ferrous verdoheme compounds are distinct from those of ferrous heme compounds. It is inferred that the positive charge of the verdoheme ring possesses some of the charge density on the iron atom, causing unique characteristics of the iron ligand stretching vibrations and altered ligand binding properties.

Heme↗
PubMed1992

Effects of high iron and sulfate ion concentrations on dry matter digestion and volatile fatty acid production by ruminal microorganisms.

The inhibitory effects of iron- and sulfate-containing compounds on the in vitro digestion of a balanced forage diet by mixed populations of ruminal microorganisms were examined in batch cultures. Compounds containing ferrous and ferric cations consistently inhibited DM digestion by up to 36% when added Fe concentrations in cultures were between 100 and 1,000 mg/L. Increased sulfate concentrations of up to 200 mg/L or chloride concentrations of up to 635 mg/L were not associated with decreased DM digestion. Ammonium sulfate additions that provided 200 mg/L of added sulfur increased (P less than .05) digestibility by 10%. Sulfate-containing iron salts tended to be less inhibitory than chloride salts and were associated with increased gas production during digestion. Ferric chloride inhibited (P less than .05) microbial activities at lower concentrations than ferrous chloride. Data suggest that excessive iron supplementation or contamination of feeds with iron-containing pollutants may decrease microbial activities in the rumen.

Ammonium Sulfate↗
PubMed2003

A superoxo-ferrous state in a reduced oxy-ferrous hemoprotein and model compounds.

Cryoreduction of the [FeO2]6 (n = 6 is the number of electrons in 3d orbitals on Fe and pi* orbitals on O2) dioxygen-bound ferroheme through irradiation at 77 K generates an [FeO2]7 reduced oxy-heme. Numerous investigations have examined [FeO2]7 centers that have been characterized as peroxo-ferric centers, denoted [FeO2]per7, in which a ferriheme binds a dianionic peroxo-ligand. The generation of such an intermediate can be understood heuristically if the [FeO2]6 parent is viewed as a superoxo-ferric center and the injected electron localizes on the O-O moiety. We here report EPR/ENDOR experiments which show quite different properties for the [FeO2]7 centers produced by cryoreduction of monomeric oxy-hemoglobin (oxy-GMH3) from Glycera dibranchiata, which is unlike mammalian "globins" in having a leucine in place of the distal histidine; of frozen aprotic solutions of oxy-ferrous octaethyl porphyrin; and of the oxy-ferrous complex of the heme model, cyclidene. These [FeO2]7 centers are characterized as "superoxo-ferrous" centers ([FeO2]sup7), with nearly unit spin density localized on a superoxo moiety which is end-on coordinated to a low-spin ferrous ion. This assignment is based on their g tensors and 17O hyperfine couplings, which are characteristic of the superoxide ion coordinated to a diamagnetic metal ion, and on the absence of detectable ENDOR signals either from the in-plane 14N ligands or from an exchangeable H-bond proton. Such a center would arise if the electron that adds to the [FeO2]6 superoxo-ferric parent localizes on the Fe ion, to make a superoxo-ferrous moiety. Upon annealing to T > 150 K, the [FeO2]sup7 species converts to peroxo/hydroperoxo-ferric ([FeO2H]7) intermediates. These experiments suggest that the primary reduction product is [FeO2]sup7 and that the internal redox transition to [FeO2]per7/[FeO2H]7 states is driven at least in part by H-bonding/proton donation by the environment.

Electron Spin Resonance Spectroscopy↗
PubMed2005

Preparation of a chiral ferrocenyl derivative containing a biphenyl unit via nickel-mediated homocoupling.

Ferrocenyl derivative 2, which possesses a biphenyl unit, was prepared by nickel(0)-mediated homocoupling of a chiral ferrocenyloxazoline. Compound 2 exists as an equilibrium mixture of two diastereomers, in a ratio dependent on temperature and solvent, due to the partially hindered rotation around biaryl bond. Upon complexation with Cu(I) or Zn(II), complete fixation of the stereogenic axis was obtained and a single stable atropoisomer was formed, whose absolute configuration was assigned on the basis of its NOESY spectrum. The 2-Cu(I) complex was used as catalyst in the cyclopropanation of styrene.

Biphenyl Compounds↗
PubMed1999

Nitric oxide reduces nontransferrin-bound iron transport in HepG2 cells.

Nitric oxide (NO) donors S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside (SNP) modulate iron regulatory protein (IRP) activity and may, therefore, affect iron uptake through transferrin receptor expression. However, iron also enters the cell as nontransferrin-bound iron (NTBI), and the aim of this study was to evaluate the effects of NO donors on NTBI transport in HepG2 cells, a model of liver physiology. Incubation with SNP and SNAP led to a time- and concentration-dependent reduction in Fe3+ and Fe2+ uptake, thus indicating an effect on the transporter rather than on the reductase. In terms of Fe2+ uptake, no variations in the Michaelis-Menten constant (Km) and a reduction in maximum uptake (Vmax) (50, 33, and 16.6 fmol/microgram protein/min in control, SNP-, and SNAP-treated cells, respectively) were detected, which suggested a decrease in the number of putative NTBI transport protein(s). Gel shift assays showed that IRP activity was reduced by SNP and slightly increased by SNAP. Northern blot analysis of transferrin receptor messenger RNA (mRNA) levels showed variations similar to those observed for IRPs, but both NO donors increased L-ferritin mRNA levels and had no effect on the stimulator of Fe transport (SFT) mRNA. In conclusion, NO donors significantly reduce NTBI transport in HepG2 cells, an effect that seems to be IRP and SFT independent. Moreover, the reduction in NTBI uptake after NO treatment suggests that this form of iron may play a minor role in the increased hepatic iron stores observed in inflammation or that other liver cells are more involved in this pathological condition.

Apoptosis↗
PubMed2007

Comparative evaluation of heating ability and biocompatibility of different ferrite-based magnetic fluids for hyperthermia application.

In this study, lauric acid-coated, superparamagnetic, nanoparticle-based magnetic fluids of different ferrites (Fe(3)O(4), MnFe(2)O(4), and CoFe(2)O(4)) were prepared and compared in terms of heating ability and biocompatibility to evaluate the feasibility of use in hyperthermia treatment of cancer. All the magnetic fluids prepared had particles of average sizes 9-11 nm. Heating ability of these magnetic fluids was evaluated by calorimetric measurement of specific absorption rate (SAR) at 300 kHz frequency and 15 kA/m field. Fe(3)O(4) and MnFe(2)O(4) showed higher SAR (120 and 97 W/g of ferrite, respectively) than CoFe(2)O(4) (37 W/g of ferrite). In vitro study on BHK 21 cell lines showed dose-dependent cell viability for all the magnetic fluids. Threshold-biocompatible ferrite concentration for all the magnetic fluids was 0.1 mg/mL. Above 0.2 mg/mL, CoFe(2)O(4) was more toxic than the other magnetic fluids. On intravenous injection of different doses (50, 200, and 400 mg/kg body weight) of magnetic fluids in mice, no significant changes in hematological and biochemical parameters were observed for Fe(3)O(4) and MnFe(2)O(4). With CoFe(2)O(4), an increase in SGPT levels at a dose rate of 400 mg/kg body weight was observed, indicating its mild hepatotoxic effect. However, histology of different vital organs showed no pathological changes for all the three magnetic fluids. Further, long term in vivo evaluation of biocompatibility of the lauric acid-coated ferrites is warranted. This study shows that lauric acid-coated, superparamagnetic Fe(3)O(4) and MnFe(2)O(4) may be used for hyperthermia treatment and are to be preferred over CoFe(2)O(4).

Animals↗
PubMed1996

Monoepoxide production from linoleic acid by cytochrome c in the presence of cardiolipin.

We found that cytochrome c (Cyt c) could oxidize cardiolipin (CL), and detected monoepoxides of linoleic acid (LA) in the fatty acids constituting the oxidized CL. We also found that in the presence of CL and Cyt c, free LA was oxidized and LA monoepoxides were produced. The aim of this study was to elucidate the mechanism of this lipid peroxidation. We concluded that ferric Cyt c produced some radical species from water-soluble oxygen in the presence of CL (CL-Cyt c system) and that radicals oxidized free LA or CL. The CL-Cyt c system may be another LA monoepoxide producing system in the neutrophil and may account for the lipid peroxidation observed in the ischemia-reperfusion-induced cardiac injury.

Cardiolipins↗
PubMed1983

The incorporation of iron into apoferritin as mediated by ceruloplasmin.

Ceruloplasmin, a copper ferroxidase, promotes the incorporation of Fe(III) into the iron storage protein, apoferritin. The product formed is identical to ferritin as judged by polyacrylamide electrophoresis and iron/protein measurements. Of several proteins examined, only apoferritin accumulates the Fe(III) produced by ceruloplasmin. When ceruloplasmin was replaced by tyrosinase, which we have shown to have ferroxidase activity, no iron incorporation into apoferritin was observed. It is proposed that Fe(III) is transferred directly and specifically to apoferritin. These data support a more specific role for ceruloplasmin in iron metabolism than has previously been proposed.

Apoferritins↗
PubMed1990

Specificity of the induction of ferritin synthesis by hemin.

We have previously reported that hemin derepresses ferritin mRNA translation in vitro. As noted earlier, pre-incubation of a 90 kDa ferritin repressor protein (FRP) with hemin prevented subsequent repression of ferritin synthesis in a wheat germ extract. The significance of this observation has been investigated further. Evidence is presented here that this inactivation of FRP is temperature dependent. Neither FeCl3, Fe3+ chelated with EDTA, nor protoporphyrin IX caused significant inactivation of FRP under comparable conditions, whereas Zn2(+)-protoporphyrin IX produced an intermediate degree of inhibition. The presence of a glutathione redox buffer (GSB), which was previously shown to minimize non-specific side-effects of hemin, was not necessary for the derepression reaction. Inclusion of mannitol, a free radical scavenger, did not alter the inactivation caused by hemin. Calculation of the expected ratio of hemin monomers to dimers suggests that the active species is the monomer.

Animals↗
PubMed1991

Mössbauer spectroscopic investigation of structure-function relations in ferritins.

Ferritin plays an important role in iron metabolism and our aim is to understand the mechanisms by which iron is sequestered within its protein shell as the mineral ferrihydrite. We present Mössbauer spectroscopic data on recombinant human and horse spleen ferritin from which we draw the following conclusions: (1) that apoferritin catalyses Fe(II) oxidation as a first step in ferrihydrite deposition, (2) that the catalysis of Fe(II) oxidation is associated with residues situated within H chains, at the postulated 'ferroxidase centre' and not in the 3-fold inter-subunit channels previously suggested as the initial Fe(II) binding and oxidation site; (3) that both isolated Fe(III) and Fe(III) mu-oxo-bridged dimers found previously by Mössbauer spectroscopy to be intermediates in iron-core formation in horse spleen ferritin, are located on H chains; and (4) that these dimers form at ferroxidase centres. The importance of the ferroxidase centre is suggested by the conservation of its ligands in many ferritins from vertebrates, invertebrates and plants. Nevertheless iron-core formation does occur in those ferritins that lack ferroxidase centres even though the initial Fe(II) oxidation is relatively slow. We compare the early stages of core formation in such variants and in horse spleen ferritin in which only 10-15% of its chains are of the H type. We discuss our findings in relation to the physiological role of isoferritins in iron storage processes.

Animals↗
PubMed1994

Liver iron quantification: studies in aqueous iron solutions, iron overloaded rats, and patients with hereditary hemochromatosis.

For the noninvasive liver iron quantification by MRI in human iron overload diseases, fundamental proton relaxation mechanisms were studied in aqueous solutions with ferritin and other iron compounds, in experimentally iron overloaded rats, and in patients with iron overload diseases. MR-relaxation rates as a function of iron concentrations in the range of 0-7.5 mg Fe/g aqueous iron solutions, 0-5.4 mg Fe/g rat liver in vivo, and 0.16-4.9 mg Fe/g human liver in vivo were determined from multi- and sets of single-spin echo sequences (1.5 T imager). As predicted by theory, transverse relaxation rates (1/T2) in aqueous iron solutions, in liver tissue of rats, and in human liver tissue increased linearly with the iron concentration. A preliminary calibration for the liver iron quantification by MRI was performed from in vivo measurements of liver 1/T2-relaxation rates and liver iron quantification by atomic absorption spectroscopy in biopsies from 13 patients. With the single spin-echo method, precise in vivo liver iron quantification in humans also above 2.0 mg Fe/g liver tissue (T2 < 15 ms) should be accomplished on any imager with shortest spin-echo time available, at least TE < 20 ms.

Adult↗