Ligand control of low-frequency electron paramagnetic resonance linewidth in Cr(III) complexes

Understanding how the ligand shell controls low-frequency electron paramagnetic resonance (EPR) spectroscopic properties of metal ions is essential if they are to be used in EPR-based bioimaging schemes. In this work, we probe how specific variations in the ligand structure impact L-band (ca. 1.3 GHz) EPR spectroscopic linewidths in the trichloride salts of five Cr(iii) complexes: [Cr(RR-dphen)3]3+ (RR-dphen = (1R,2R)-(+)-diphenylethylenediamine, 1), [Cr(en)3]3+ (en = ethylenediamine, 2), [Cr(me-en)3]3+ (me-en = 1,2-diaminopropane, 3), [Cr(tn)3]3+ (tn = 1,3-diaminopropane, 4) [Cr(trans-chxn)3]3+ (trans-chxn = trans-(±)-1,2-diaminocyclohexane, 5). Spectral broadening varies in a nonintuitive manner across the series, showing the sharpest peaks for 1 and broadest for 5. Molecular dynamics simulations provide evidence that the broadening is correlated to rigidity in the inner coordination sphere and reflected in ligand-dependent distribution of Cr-N bond distances that can be found in frozen solution.

Concentration of Fe(3+)-Triapine in BEAS-2B Cells

An electron paramagnetic resonance (EPR) method was used to determine the concentration of the antitumor agent Triapine in BEAS-2B cells when Triapine was bound to iron (Fe). Knowledge of the concentration of Fe-Triapine in tumor cells may be useful to adjust the administration of the drug or to adjust iron uptake in tumor cells. An EPR spectrum is obtained for Fe(3+)-Triapine, Fe(3+)(Tp)₂⁺, in BEAS-2B cells after addition of Fe(3+)(Tp)₂⁺. Detection of the low spin signal for Fe(3+)(Tp)₂⁺ shows that the Fe(3+)(Tp)₂⁺ complex is intact in these cells. It is proposed that Triapine acquires iron from transferrin in cells including tumor cells. Here, it is shown that iron from purified Fe-transferrin is transferred to Triapine after the addition of ascorbate. To our knowledge, this is the first time that the EPR method has been used to determine the concentration of an iron antitumor agent in cells.

Treatment of Cells and Tissues with Chromate Maximizes Mitochondrial 2Fe2S EPR Signals

In a previous study on chromate toxicity, an increase in the 2Fe2S electron paramagnetic resonance (EPR) signal from mitochondria was found upon addition of chromate to human bronchial epithelial cells and bovine airway tissue ex vivo. This study was undertaken to show that a chromate-induced increase in the 2Fe2S EPR signal is a general phenomenon that can be used as a low-temperature EPR method to determine the maximum concentration of 2Fe2S centers in mitochondria. First, the low-temperature EPR method to determine the concentration of 2Fe2S clusters in cells and tissues is fully developed for other cells and tissues. The EPR signal for the 2Fe2S clusters N1b in Complex I and/or S1 in Complex II and the 2Fe2S cluster in xanthine oxidoreductase in rat liver tissue do not change in intensity because these clusters are already reduced; however, the EPR signals for N2, the terminal cluster in Complex I, and N4, the cluster preceding the terminal cluster, decrease upon adding chromate. More surprising to us, the EPR signals for N3, the cluster preceding the 2Fe2S cluster in Complex I, also decrease upon adding chromate. Moreover, this method is used to obtain the concentration of the 2Fe2S clusters in white blood cells where the 2Fe2S signal is mostly oxidized before treatment with chromate and becomes reduced and EPR detectable after treatment with chromate. The increase of the g = 1.94 2Fe2S EPR signal upon the addition of chromate can thus be used to obtain the relative steady-state concentration of the 2Fe2S clusters and steady-state concentration of Complex I and/or Complex II in mitochondria.

Better Resolution of High-Spin Cobalt Hyperfine at Low Frequency: Co-Doped Ba(Zn1/3Ta2/3)O₃ as a Model Complex

Low-frequency electron paramagnetic resonance (EPR) is used to extract the EPR parameter A-mid and support the approximate X-band value of g-mid for Ba(Co_yZn_1/3−yTa_2/3)O₃. Although the cobalt hyperfine structure for the |±1/2〉 state is often unresolved at X-band or S-band, it is resolved in measurements on this compound. This allows for detailed analysis of the molecular orbital for the |±1/2〉 state, which is often the ground state. Moreover, this work shows that the EPR parameters for Co substituted into Zn compounds give important insight into the properties of zinc binding sites.

Gallium Maltolate Disrupts Tumor Iron Metabolism and Retards the Growth of Glioblastoma by Inhibiting Mitochondrial Function and Ribonucleotide Reductase

Gallium, a metal with antineoplastic activity, binds transferrin (Tf) and enters tumor cells via Tf receptor1 (TfR1); it disrupts iron homeostasis leading to cell death. We hypothesized that TfR1 on brain microvascular endothelial cells (BMEC) would facilitate Tf-Ga transport into the brain enabling it to target TfR-bearing glioblastoma. We show that U-87 MG and D54 glioblastoma cell lines and multiple glioblastoma stem cell (GSC) lines express TfRs, and that their growth is inhibited by gallium maltolate (GaM) in vitro After 24 hours of incubation with GaM, cells displayed a loss of mitochondrial reserve capacity followed by a dose-dependent decrease in oxygen consumption and a decrease in the activity of the iron-dependent M2 subunit of ribonucleotide reductase (RRM2). IHC staining of rat and human tumor-bearing brains showed that glioblastoma, but not normal glial cells, expressed TfR1 and RRM2, and that glioblastoma expressed greater levels of H- and L-ferritin than normal brain. In an orthotopic U-87 MG glioblastoma xenograft rat model, GaM retarded the growth of brain tumors relative to untreated control (P = 0.0159) and reduced tumor mitotic figures (P = 0.045). Tumors in GaM-treated animals displayed an upregulation of TfR1 expression relative to control animals, thus indicating that gallium produced tumor iron deprivation. GaM also inhibited iron uptake and upregulated TfR1 expression in U-87 MG and D54 cells in vitro We conclude that GaM enters the brain via TfR1 on BMECs and targets iron metabolism in glioblastoma in vivo, thus inhibiting tumor growth. Further development of novel gallium compounds for brain tumor treatment is warranted. Mol Cancer Ther; 17(6); 1240-50. ©2018 AACR.

A One-Hole Cu4S Cluster with N2O Reductase Activity: A Structural and Functional Model for CuZ

During bacterial denitrification, two-electron reduction of N₂O occurs at a [Cu₄(μ₄-S)] catalytic site (Cu_Z*) embedded within the nitrous oxide reductase (N₂OR) enzyme. In this Communication, an amidinate-supported [Cu₄(μ₄-S)] model cluster in its one-hole (S = ¹/₂) redox state is thoroughly characterized. Along with its two-hole redox partner and fully reduced clusters reported previously, the new species completes the two-electron redox series of [Cu₄(μ₄-S)] model complexes with catalytically relevant oxidation states for the first time. More importantly, N₂O is reduced by the one-hole cluster to produce N₂ and the two-hole cluster, thereby completing a closed cycle for N₂O reduction. Not only is the title complex thus the best structural model for Cu_Z* to date, but it also serves as a functional Cu_Z* mimic.

A Cu4S model for the nitrous oxide reductase active sites supported only by nitrogen ligands

To model the (His)7Cu4Sn (n = 1 or 2) active sites of nitrous oxide reductase, the first Cu4(μ4-S) cluster supported only by nitrogen donors has been prepared using amidinate supporting ligands. Structural, magnetic, spectroscopic, and computational characterization is reported. Electrochemical data indicates that the 2-hole model complex can be reduced reversibly to the 1-hole state and irreversibly to the fully reduced state.

Di- and Trinuclear Mixed-Valence Copper Amidinate Complexes from Reduction of Iodine

Molecular examples of mixed-valence copper complexes through chemical oxidation are rare but invoked in the mechanism of substrate activation, especially oxygen, in copper-containing enzymes. To examine the cooperative chemistry between two metals in close proximity to each other we began studying the reactivity of a dinuclear Cu(I) amidinate complex. The reaction of [(2,6-Me2C6H3N)2C(H)]2Cu2, 1, with I2 in tetrahydrofuran (THF), CH3CN, and toluene affords three new mixed-valence copper complexes [(2,6-Me2C6H3N)2C(H)]2Cu2(μ2-I3)(THF)2, 2, [(2,6-Me2C6H3N)2C(H)]2Cu2(μ2-I) (NCMe)2, 3, and [(2,6-Me2C6H3N)2C(H)]3Cu3(μ3-I)2, 4, respectively. The first two compounds were characterized by UV-vis and electron paramagnetic resonance spectroscopies, and their molecular structure was determined by X-ray crystallography. Both di- and trinuclear mixed-valence intermediates were characterized for the reaction of compound 1 to compound 4, and the molecular structure of 4 was determined by X-ray crystallography. The electronic structure of each of these complexes was also investigated using density functional theory.

The conserved Trp114 residue of thioredoxin reductase 1 has a redox sensor-like function triggering oligomerization and crosslinking upon oxidative stress related to cell death

The selenoprotein thioredoxin reductase 1 (TrxR1) has several key roles in cellular redox systems and reductive pathways. Here we discovered that an evolutionarily conserved and surface-exposed tryptophan residue of the enzyme (Trp114) is excessively reactive to oxidation and exerts regulatory functions. The results indicate that it serves as an electron relay communicating with the FAD moiety of the enzyme, and, when oxidized, it facilitates oligomerization of TrxR1 into tetramers and higher multimers of dimers. A covalent link can also be formed between two oxidized Trp114 residues of two subunits from two separate TrxR1 dimers, as found both in cell extracts and in a crystal structure of tetrameric TrxR1. Formation of covalently linked TrxR1 subunits became exaggerated in cells on treatment with the pro-oxidant p53-reactivating anticancer compound RITA, in direct correlation with triggering of a cell death that could be prevented by antioxidant treatment. These results collectively suggest that Trp114 of TrxR1 serves a function reminiscent of an irreversible sensor for excessive oxidation, thereby presenting a previously unrecognized level of regulation of TrxR1 function in relation to cellular redox state and cell death induction.

Synthesis, Spectroscopy, and Electrochemistry of (α-Diimine)M(CO)3Br, M = Mn, Re, Complexes: Ligands Isoelectronic to Bipyridyl Show Differences in CO2 Reduction

The synthesis and characterization of new Mn(I)- and Re(I)-centered organometallic complexes fashioned with 1,4-diazabutadiene (DAB) ligands is reported. Ten compounds of the type fac-(α-diimine)M(CO)3Br (M = Mn, Re) were obtained in moderate to excellent yield (35-80%) and high purity from the coordination of the five ligands with M(CO)5Br in refluxing ethanol. Despite the electronic similarity of DAB to 2,2'-bipyridyl, the complexes described herein were poor mediators of electrochemical CO2 conversion to CO, but provide insight into the role of redox-active ligands in catalysis. Additional characterization of the one-electron reduced rhenium compounds, relevant intermediates in CO2 reduction, by EPR and single-crystal X-ray analysis is described.

Redox activation of Fe(III)-thiosemicarbazones and Fe(III)-bleomycin by thioredoxin reductase: specificity of enzymatic redox centers and analysis of reactive species formation by ESR spin trapping

Thiosemicarbazones such as Triapine (Tp) and Dp44mT are tridentate iron (Fe) chelators that have well-documented antineoplastic activity. Although Fe-thiosemicarbazones can undergo redox cycling to generate reactive species that may have important roles in their cytotoxicity, there is only limited insight into specific cellular agents that can rapidly reduce Fe(III)-thiosemicarbazones and thereby promote their redox activity. Here we report that thioredoxin reductase-1 (TrxR1) and glutathione reductase (GR) have this activity and that there is considerable specificity to the interactions between specific redox centers in these enzymes and various Fe(III) complexes. Site-directed variants of TrxR1 demonstrate that the selenocysteine (Sec) of the enzyme is not required, whereas the C59 residue and the flavin have important roles. Although TrxR1 and GR have analogous C59/flavin motifs, TrxR is considerably faster than GR. For both enzymes, Fe(III)(Tp)2 is reduced faster than Fe(III)(Dp44mT)2. This reduction promotes redox cycling and the generation of hydroxyl radical (HO) in a peroxide-dependent manner, even with low-micromolar levels of Fe(Tp)2. TrxR also reduces Fe(III)-bleomycin and this activity is Sec-dependent. TrxR cannot reduce Fe(III)-EDTA at significant rates. Our findings are the first to demonstrate pro-oxidant reductive activation of Fe(III)-based antitumor thiosemicarbazones by interactions with specific enzyme species. The marked elevation of TrxR1 in many tumors could contribute to the selective tumor toxicity of these drugs by enhancing the redox activation of Fe(III)-thiosemicarbazones and the generation of reactive oxygen species such as HO.

Nitric oxide modifies global histone methylation by inhibiting Jumonji C domain-containing demethylases

Methylation of lysine residues on histone tails is an important epigenetic modification that is dynamically regulated through the combined effects of methyltransferases and demethylases. The Jumonji C domain Fe(II) α-ketoglutarate family of proteins performs the majority of histone demethylation. We demonstrate that nitric oxide ((•)NO) directly inhibits the activity of the demethylase KDM3A by forming a nitrosyliron complex in the catalytic pocket. Exposing cells to either chemical or cellular sources of (•)NO resulted in a significant increase in dimethyl Lys-9 on histone 3 (H3K9me2), the preferred substrate for KDM3A. G9a, the primary methyltransferase acting on H3K9me2, was down-regulated in response to (•)NO, and changes in methylation state could not be accounted for by methylation in general. Furthermore, cellular iron sequestration via dinitrosyliron complex formation correlated with increased methylation. The mRNA of several histone demethylases and methyltransferases was also differentially regulated in response to (•)NO. Taken together, these data reveal three novel and distinct mechanisms whereby (•)NO can affect histone methylation as follows: direct inhibition of Jumonji C demethylase activity, reduction in iron cofactor availability, and regulation of expression of methyl-modifying enzymes. This model of (•)NO as an epigenetic modulator provides a novel explanation for nonclassical gene regulation by (•)NO.

Iron-targeting antitumor activity of gallium compounds and novel insights into triapine(®)-metal complexes

SIGNIFICANCE

Despite advances made in the treatment of cancer, a significant number of patients succumb to this disease every year. Hence, there is a great need to develop new anticancer agents.

RECENT ADVANCES

Emerging data show that malignant cells have a greater requirement for iron than normal cells do and that proteins involved in iron import, export, and storage may be altered in cancer cells. Therefore, strategies to perturb these iron-dependent steps in malignant cells hold promise for the treatment of cancer. Recent studies show that gallium compounds and metal-thiosemicarbazone complexes inhibit tumor cell growth by targeting iron homeostasis, including iron-dependent ribonucleotide reductase. Chemical similarities of gallium(III) with iron(III) enable the former to mimic the latter and interpose itself in critical iron-dependent steps in cellular proliferation. Newer gallium compounds have emerged with additional mechanisms of action. In clinical trials, the first-generation-compound gallium nitrate has exhibited activity against bladder cancer and non-Hodgkin's lymphoma, while the thiosemicarbazone Triapine(®) has demonstrated activity against other tumors.

CRITICAL ISSUES

Novel gallium compounds with greater cytotoxicity and a broader spectrum of antineoplastic activity than gallium nitrate should continue to be developed.

FUTURE DIRECTIONS

The antineoplastic activity and toxicity of the existing novel gallium compounds and thiosemicarbazone-metal complexes should be tested in animal tumor models and advanced to Phase I and II clinical trials. Future research should identify biologic markers that predict tumor sensitivity to gallium compounds. This will help direct gallium-based therapy to cancer patients who are most likely to benefit from it.

Dual role of selected antioxidants found in dietary supplements: crossover between anti- and pro-oxidant activities in the presence of copper

Overproduction of reactive oxygen species (ROS) in vivo can result in damage associated with many aging-associated diseases. Defenses against ROS that have evolved include antioxidant enzymes, such as superoxide dismutases, peroxidases, and catalases, which can scavenge ROS. In addition, endogenous and dietary antioxidants play an important role in moderating damage associated with ROS. In this study, we use four common dietary antioxidants to demonstrate that, in the presence of copper (cupric sulfate and cupric gluconate) and physiologically relevant levels of hydrogen peroxide, these antioxidants can also act as pro-oxidants by producing hydroxyl radicals. Using electron spin resonance (ESR) spin trapping techniques, we demonstrate that the level of hydroxyl radical formation is a function of the pH of the medium and the relative amounts of antioxidant and copper. On the basis of the level of hydroxyl radical formation, the relative pro-oxidant potential of these antioxidants is cysteine > ascorbate > EGCG > GSH. It has been reported that copper sequestered by protein ligands, as happens in vivo, loses its redox activity (diminishing/abolishing the formation of free radicals). However, in the presence of hydrogen peroxide, cysteine and GSH efficiently react with cupric sulfate sequestered with bovine serum albumin to generate hydroxyl radicals. Overall, the results demonstrate that in the presence of copper, endogenous and dietary antioxidants can also exhibit pro-oxidative activity.

Damage to mitochondrial complex I during cardiac ischemia reperfusion injury is reduced indirectly by anti-anginal drug ranolazine

Ranolazine, an anti-anginal drug, is a late Na(+) channel current blocker that is also believed to attenuate fatty acid oxidation and mitochondrial respiratory complex I activity, especially during ischemia. In this study, we investigated if ranolazine's protective effect against cardiac ischemia/reperfusion (IR) injury is mediated at the mitochondrial level and specifically if respiratory complex I (NADH Ubiquinone oxidoreductase) function is protected. We treated isolated and perfused guinea pig hearts with ranolazine just before 30 min ischemia and then isolated cardiac mitochondria at the end of 30 min ischemia and/or 30 min ischemia followed by 10 min reperfusion. We utilized spectrophotometric and histochemical techniques to assay complex I activity, Western blot analysis for complex I subunit NDUFA9, electron paramagnetic resonance for activity of complex I Fe-S clusters, enzyme linked immuno sorbent assay (ELISA) for determination of protein acetylation, native gel histochemical staining for respiratory supercomplex assemblies, and high pressure liquid chromatography for cardiolipin integrity; cardiac function was measured during IR. Ranolazine treated hearts showed higher complex I activity and greater detectable complex I protein levels compared to untreated IR hearts. Ranolazine treatment also led to more normalized electron transfer via Fe-S centers, supercomplex assembly and cardiolipin integrity. These improvements in complex I structure and function with ranolazine were associated with improved cardiac function after IR. However, these protective effects of ranolazine are not mediated by a direct action on mitochondria, but rather indirectly via cytosolic mechanisms that lead to less oxidation and better structural integrity of complex I.

The intracellular redox stress caused by hexavalent chromium is selective for proteins that have key roles in cell survival and thiol redox control

Hexavalent chromium [Cr(VI)] compounds (e.g. chromates) are strong oxidants that readily enter cells where they are reduced to reactive Cr intermediates that can directly oxidize some cell components and can promote the generation of reactive oxygen and nitrogen species. Inhalation is a major route of exposure which directly exposes the bronchial epithelium. Previous studies with non-cancerous human bronchial epithelial cells (BEAS-2B) demonstrated that Cr(VI) treatment results in the irreversible inhibition of thioredoxin reductase (TrxR) and the oxidation of thioredoxins (Trx) and peroxiredoxins (Prx). The mitochondrial Trx/Prx system is somewhat more sensitive to Cr(VI) than the cytosolic Trx/Prx system, and other redox-sensitive mitochondrial functions are subsequently affected including electron transport complexes I and II. Studies reported here show that Cr(VI) does not cause indiscriminant thiol oxidation, and that the Trx/Prx system is among the most sensitive of cellular protein thiols. Trx/Prx oxidation is not unique to BEAS-2B cells, as it was also observed in primary human bronchial epithelial cells. Increasing the intracellular levels of ascorbate, an endogenous Cr(VI) reductant, did not alter the effects on TrxR, Trx, or Prx. The peroxynitrite scavenger MnTBAP did not protect TrxR, Trx, Prx, or the electron transport chain from the effects of Cr(VI), implying that peroxynitrite is not required for these effects. Nitration of tyrosine residues of TrxR was not observed following Cr(VI) treatment, further ruling out peroxynitrite as a significant contributor to the irreversible inhibition of TrxR. Cr(VI) treatments that disrupt the TrxR/Trx/Prx system did not cause detectable mitochondrial DNA damage. Overall, the redox stress that results from Cr(VI) exposure shows selectivity for key proteins which are known to be important for redox signaling, antioxidant defense, and cell survival.

The iron-chelating drug triapine causes pronounced mitochondrial thiol redox stress

Triapine (Tp) is an iron chelator with activity against several types of cancer. Iron-Tp [Fe(III)(Tp)(2)] can be redox-cycled to generate reactive oxygen species that may contribute to its cytotoxicity. However, evidence for this mechanism in cells is limited. The cytosolic and mitochondrial thioredoxins (Trx1 and Trx2, respectively) are essential for cell survival. They are normally maintained in the reduced state, and support the function of many intracellular proteins including the peroxiredoxins (Prxs). Their redox status can indicate oxidant stress in their respective subcellular compartments. Tp treatment of human lung A549 cells caused almost complete oxidation of Trx2 and its dependent peroxiredoxin (Prx3), but there was no effect on Trx1 redox status. Significant inhibition of total TrxR activity did not occur until Tp levels were 4-fold above those needed to cause Trx2 oxidation. While Tp caused a 36-45% decline in reduced glutathione (GSH) levels, GSH accounted for >99% of the total glutathione in the absence and presence of Tp. In vitro studies demonstrated that cysteine reduces Fe(III)(Tp)(2) to Fe(II)(Tp)(2), and cysteine was faster and more efficient than reduced glutathione (GSH) in this regard. Fe(III)(Tp)(2) also mediated the oxidation of purified Trx2 in vitro. Thus, Fe(III)(Tp)(2) itself, and/or various reactive species that may result from its redox cycling, could account for Trx2 and Prx3 oxidation in Tp-treated cells. The striking difference between the effects on Trx2 and Trx1 implies a pronounced thiol redox stress that is largely directed at the mitochondria. These previously unrecognized effects of Tp could contribute to its overall cytotoxicity.

The pro-oxidant chromium(VI) inhibits mitochondrial complex I, complex II, and aconitase in the bronchial epithelium: EPR markers for Fe-S proteins

Hexavalent chromium (Cr(VI)) compounds (e.g., chromates) are strong oxidants that readily enter cells, where they are reduced to reactive Cr species that also facilitate reactive oxygen species generation. Recent studies demonstrated inhibition and oxidation of the thioredoxin system, with greater effects on mitochondrial thioredoxin (Trx2). This implies that Cr(VI)-induced oxidant stress may be especially directed at the mitochondria. Examination of other redox-sensitive mitochondrial functions showed that Cr(VI) treatments that cause Trx2 oxidation in human bronchial epithelial cells also result in pronounced and irreversible inhibition of aconitase, a TCA cycle enzyme that has an iron-sulfur (Fe-S) center that is labile with respect to certain oxidants. The activities of electron transport complexes I and II were also inhibited, whereas complex III was not. Electron paramagnetic resonance (EPR) studies of samples at liquid helium temperature (10K) showed a strong signal at g=1.94 that is consistent with the inhibition of electron flow through complex I and/or II. A signal at g=2.02 was also observed, which is consistent with oxidation of the Fe-S center of aconitase. The g=1.94 signal was particularly intense and remained after extracellular Cr(VI) was removed, whereas the g=2.02 signal declined in intensity after Cr(VI) was removed. A similar inhibition of these activities and analogous EPR findings were noted in bovine airways treated ex vivo with Cr(VI). Overall, the data support the hypothesis that Cr(VI) exposure has deleterious effects on a number of redox-sensitive core mitochondrial proteins. The g=1.94 signal could prove to be an important biomarker for oxidative damage resulting from Cr(VI) exposure. The EPR spectra simultaneously showed signals for Cr(V) and Cr(III), which verify Cr(VI) exposure and its intracellular reductive activation.

Neuroprotection by a mitochondria-targeted drug in a Parkinson's disease model

The objective of this study was to assess the neuroprotective effects of a mitochondria-targeted antioxidant, Mito-Q(10), the coenzyme-Q analog attached to a triphenylphosphonium cation that targets the antioxidant to mitochondria, in experimental models of Parkinson's disease (PD). Primary mesencephalic neuronal cells and cultured dopaminergic cells were treated with 1-methyl-4-phenylpyridinium (MPP(+)), an active metabolite of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and mice were used for testing the efficacy of Mito-Q(10). MPP(+) treatment caused a dose-dependent loss of tyrosine hydroxylase and membrane potential and an increase in caspase-3 activation in dopaminergic cells, which were reversed by Mito-Q(10). MPTP treatment induced a loss of striatal dopamine and its metabolites, inactivation of mitochondrial aconitase in the substantia nigra, and a loss of locomotor activity in mice. Treatment with Mito-Q(10) significantly inhibited both MPP(+)- and MPTP-induced neurotoxicity in cell culture and mouse models. Collectively, these results indicate that mitochondrial targeting of antioxidants is a promising neuroprotective strategy in this preclinical mouse model of PD.

Cytotoxic Evaluation of 3-Aminopyridine-2-Carboxaldehyde Thiosemicarbazone, 3-AP, in Peripheral Blood Lymphocytes of Patients with Refractory Solid Tumors using Electron Paramagnetic Resonance

PURPOSE: 3-AP (3-aminopyridine-2-carboxaldehyde thiosemicarbazone, 3-AP) is a metal chelator that potently inhibits the enzyme ribonucleotide reductase, RR, which plays a key role in cell division and tumor progression. A sub-unit of RR has a non-heme iron and a tyrosine free radical, which are required for the enzymatic reduction of ribonucleotides to deoxyribonucleotides. The objective of the study was to determine whether 3-AP affects its targeted action by measuring EPR signals formed either directly or indirectly from low molecular weight ferric-3-AP chelates. METHODS: Peripheral blood lymphocytes were collected from patients with refractory solid tumors at baseline and at 2, 4.5 and 22 hours after 3-AP administration. EPR spectra were used to identify signals from high-spin Fe-transferrin, high-spin heme and low-spin iron or copper ions. RESULTS: An increase in Fe-transferrin signal was observed, suggesting blockage of Fe uptake. It is hypothesized that formation of reactive oxygen species by FeT(2) or CuT damage transferrin or the transferrin receptor. An increase in heme signal was also observed, which is a probable source of cytochrome c release from the mitochondria and potential apoptosis. In addition, increased levels of Fe and Cu were identified. CONCLUSION: These results, which were consistent with our earlier study validating 3-AP-mediated signals by EPR, provide valuable insights into the in vivo mechanism of action of 3-AP.

Oxidation of histidine residues in copper-zinc superoxide dismutase by bicarbonate-stimulated peroxidase and thiol oxidase activities: pulse EPR and NMR studies

In this work, we investigated the oxidative modification of histidine residues induced by peroxidase and thiol oxidase activities of bovine copper-zinc superoxide dismutase (Cu-ZnSOD) using NMR and pulse EPR spectroscopy. 1D NMR and 2D-NOESY were used to determine the oxidative damage at the Zn(II) and Cu(II) active sites as well as at distant histidines. Results indicate that during treatment of SOD with hydrogen peroxide (H(2)O(2)) or cysteine in the absence of bicarbonate anion (HCO(3)(-)), both exchangeable and nonexchangeable protons were affected. Both His-44 and His-46 in the Cu(II) active site were oxidized based on the disappearance of NOESY cross-peaks between CH and NH resonances of the imidazole rings. In the Zn(II) site, only His-69, which is closer to His-44, was oxidatively modified. However, addition of HCO(3)(-) protected the active site His residues. Instead, resonances assigned to the His-41 residue, 11 Å away from the Cu(II) site, were completely abolished during both HCO(3)(-)-stimulated peroxidase activity and thiol oxidase activity in the presence of HCO(3)(-) . Additionally, ESEEM/HYSCORE and ENDOR studies of SOD treated with peroxide/Cys in the absence of HCO(3)(-) revealed that hyperfine couplings to the distal and directly coordinated nitrogens of the His-44 and His-46 ligands at the Cu(II) active site were modified. In the presence of HCO(3)(-), these modifications were absent. HCO(3)(-)-mediated, selective oxidative modification of histidines in SOD may be relevant to understanding the molecular mechanism of SOD peroxidase and thiol oxidase activities.

Pulse Radiolysis Studies of Temperature Dependent Electron Transfers among Redox Centers in ba(3)-Cytochrome c Oxidase from Thermus thermophilus: Comparison of A- and B-Type Enzymes

The functioning of cytochrome c oxidases involves orchestration of long-range electron transfer (ET) events among the four redox active metal centers. We report the temperature dependence of electron transfer from the Cu(A)(r) site to the low-spin heme-(a)b(o) site, i.e., Cu(A)(r) + heme-a(b)(o) → Cu(A)(o) + heme-a(b)(r) in three structurally characterized enzymes: A-type aa(3) from Paracoccus denitrificans (PDB code 3HB3 ) and bovine heart tissue (PDB code 2ZXW ), and the B-type ba(3) from T. thermophilus (PDB codes 1EHK and 1XME ). k,T data sets were obtained with the use of pulse radiolysis as described previously. Semiclassical Marcus theory revealed that λ varies from 0.74 to 1.1 eV, H(ab), varies from ∼2 × 10(-5) eV (0.16 cm(-1)) to ∼24 × 10(-5) eV (1.9 cm(-1)), and βD varies from 9.3 to 13.9. These parameters are consistent with diabatic electron tunneling. The II-Asp111Asn Cu(A) mutation in cytochrome ba(3) had no effect on the rate of this reaction whereas the II-Met160Leu Cu(A)-mutation was slower by an amount corresponding to a decreased driving force of ∼0.06 eV. The structures support the presence of a common, electron-conducting "wire" between Cu(A) and heme-a(b). The transfer of an electron from the low-spin heme to the high-spin heme, i.e., heme-a(b)(r) + heme-a(3)(o) → heme-a(b)(o) + heme-a(3)(r), was not observed with the A-type enzymes in our experiments but was observed with the Thermus ba(3); its Marcus parameters are λ = 1.5 eV, H(ab) = 26.6 × 10(-5) eV (2.14 cm(-1)), and βD = 9.35, consistent also with diabatic electron tunneling between the two hemes. The II-Glu15Ala mutation of the K-channel structure, ∼24 Å between its CA and Fe-a(3), was found to completely block heme-b(r) to heme-a(3)(o) electron transfer. A structural mechanism is suggested to explain these observations.

The selenium-independent inherent pro-oxidant NADPH oxidase activity of mammalian thioredoxin reductase and its selenium-dependent direct peroxidase activities

Mammalian thioredoxin reductase (TrxR) is an NADPH-dependent homodimer with three redox-active centers per subunit: a FAD, an N-terminal domain dithiol (Cys(59)/Cys(64)), and a C-terminal cysteine/selenocysteine motif (Cys(497)/Sec(498)). TrxR has multiple roles in antioxidant defense. Opposing these functions, it may also assume a pro-oxidant role under some conditions. In the absence of its main electron-accepting substrates (e.g. thioredoxin), wild-type TrxR generates superoxide (O ), which was here detected and quantified by ESR spin trapping with 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO). The peroxidase activity of wild-type TrxR efficiently converted the O adduct (DEPMPO/HOO(*)) to the hydroxyl radical adduct (DEPMPO/HO(*)). This peroxidase activity was Sec-dependent, although multiple mutants lacking Sec could still generate O . Variants of TrxR with C59S and/or C64S mutations displayed markedly reduced inherent NADPH oxidase activity, suggesting that the Cys(59)/Cys(64) dithiol is required for O generation and that O is not derived directly from the FAD. Mutations in the Cys(59)/Cys(64) dithiol also blocked the peroxidase and disulfide reductase activities presumably because of an inability to reduce the Cys(497)/Sec(498) active site. Although the bulk of the DEPMPO/HO(*) signal generated by wild-type TrxR was due to its combined NADPH oxidase and Sec-dependent peroxidase activities, additional experiments showed that some free HO(*) could be generated by the enzyme in an H(2)O(2)-dependent and Sec-independent manner. The direct NADPH oxidase and peroxidase activities of TrxR characterized here give insights into the full catalytic potential of this enzyme and may have biological consequences beyond those solely related to its reduction of thioredoxin.

Multifrequency EPR studies of [Cu(1.5)Cu(1.5)](+) for Cu2(mu-NR2)2 and Cu2(mu-PR2)2 diamond cores

Multifrequency electron paramagnetic resonance (EPR) spectroscopy is used to explore the electronic structures of a series of dicopper complexes of the type {(LXL)Cu}(2)(+). These complexes contain two four-coordinate copper centers of highly distorted tetrahedral geometries linked by two [LXL](-) ligands featuring bridging amido or phosphido ligands and associated thioether or phosphine chelate donors. Specific chelating [LXL](-) ligands examined in this study include bis(2-tert-butylsulfanylphenyl)amide (SNS), bis(2-di-iso-butylphosphinophenyl)amide (PNP), and bis(2-di-iso-propylphosphinophenyl)phosphide (PPP). To better map the electronic coupling to copper, nitrogen, and phosphorus in these complexes, X-, S-, and Q-band EPR spectra have been obtained for each complex. The resulting EPR parameters implied by computer simulation are unusual for typical dicopper complexes and are largely consistent with previously published X-ray absorption spectroscopy and density functional theory data, where a highly covalent {Cu(2)(mu-XR(2))(2)}(+) diamond core has been assigned in which removal of an electron from the neutral {Cu(2)(mu-XR(2))(2)} can be viewed as ligand-centered to a substantial degree. To our knowledge, this is the first family of dicopper diamond core model complexes for which the compendium of X-, S-, and Q-band EPR spectra have been collected for comparison to Cu(A).

EPR of Cu2+ prion protein constructs at 2 GHz using the g(perpendicular) region to characterize nitrogen ligation

A double octarepeat prion protein construct, which has two histidines, mixed with copper sulfate in a 3:2 molar ratio provides at most three imidazole ligands to each copper ion to form a square-planar Cu(2+) complex. This work is concerned with identification of the fourth ligand. A new (to our knowledge) electron paramagnetic resonance method based on analysis of the intense features of the electron paramagnetic resonance spectrum in the g( perpendicular) region at 2 GHz is introduced to distinguish between three and four nitrogen ligands. The methodology was established by studies of a model system consisting of histidine imidazole ligation to Cu(2+). In this spectral region at 2 GHz (S-band), g-strain and broadening from the possible rhombic character of the Zeeman interaction are small. The most intense line is identified with the M(I) = +1/2 extra absorption peak. Spectral simulation demonstrated that this peak is insensitive to cupric A(x) and A(y) hyperfine interaction. The spectral region to the high-field side of this peak is uncluttered and suitable for analysis of nitrogen superhyperfine couplings to determine the number of nitrogens. The spectral region to the low-field side of the intense extra absorption peak in the g( perpendicular) part of the spectrum is sensitive to the rhombic distortion parameters A(x) and A(y). Application of the method to the prion protein system indicates that two species are present and that the dominant species contains four nitrogen ligands. A new loop-gap microwave resonator is described that contains approximately 1 mL of frozen sample.

Three-coordinate copper(I) amido and aminyl radical complexes

A three-coordinate Cu-NR(2) system (R = p-tolyl) supported by the anionic bis(phosphino)borate ligand [Ph(2)B(CH(2)P(t)Bu(2))(2)](-) has been isolated and structurally characterized in both its anionic Cu(I) and neutral (formally) Cu(II) oxidation states. A large rate constant for the self-exchange electron-transfer reaction (k(S) >or= 10(7) M(-1) s(-1)) makes this system a functional model for the type-1 active sites in blue copper proteins. Multiedge X-ray absorption spectroscopy, multifrequency electron paramagnetic resonance, and density functional theory analyses collectively indicate that the oxidized form is best regarded as a Cu(I)-aminyl radical complex rather than a Cu(II)-amido species, with about 70% localization of the unpaired electron on the NR(2) unit. Hydrogen-atom transfer and C-C coupling reactions are presented as examples of chemical reactivity manifested by this unusual electronic structure.

Doxorubicin inactivates myocardial cytochrome c oxidase in rats: cardioprotection by Mito-Q

Doxorubicin (DOX) is used for treating various cancers. Its clinical use is, however, limited by its dose-limiting cardiomyopathy. The exact mechanism of DOX-induced cardiomyopathy still remains unknown. The goals were to investigate the molecular mechanism of DOX-induced cardiomyopathy and cardioprotection by mitoquinone (Mito-Q), a triphenylphosphonium-conjugated analog of coenzyme Q, using a rat model. Rats were treated with DOX, Mito-Q, and DOX plus Mito-Q for 12 weeks. The left ventricular function as measured by two-dimensional echocardiography decreased in DOX-treated rats but was preserved during Mito-Q plus DOX treatment. Using low-temperature ex vivo electron paramagnetic resonance (EPR), a time-dependent decrease in heme signal was detected in heart tissues isolated from rats administered with a cumulative dose of DOX. DOX attenuated the EPR signals characteristic of the exchange interaction between cytochrome c oxidase (CcO)-Fe(III) heme a3 and CuB. DOX and Mito-Q together restored these EPR signals and the CcO activity in heart tissues. DOX strongly downregulated the stable expression of the CcO subunits II and Va and had a slight inhibitory effect on CcO subunit I gene expression. Mito-Q restored CcO subunit II and Va expressions in DOX-treated rats. These results suggest a novel cardioprotection mechanism by Mito-Q during DOX-induced cardiomyopathy involving CcO.

Evidence that phosphorylation of iron regulatory protein 1 at Serine 138 destabilizes the [4Fe-4S] cluster in cytosolic aconitase by enhancing 4Fe-3Fe cycling

Iron-sulfur cluster-dependent interconversion of iron regulatory protein 1 (IRP1) between its RNA binding and cytosolic aconitase (c-acon) forms controls vertebrate iron homeostasis. Cluster removal from c-acon is thought to include oxidative demetallation as a required step, but little else is understood about the process of conversion to IRP1. In comparison with c-acon(WT), Ser(138) phosphomimetic mutants of c-acon contain an unstable [4Fe-4S] cluster and were used as tools to further define the pathway(s) of iron-sulfur cluster disassembly. Under anaerobic conditions cluster insertion into purified IRP1(S138E) and cluster loss on treatment with NO regulated aconitase and RNA binding activity over a similar range as observed for IRP1(WT). However, activation of RNA binding of c-acon(S138E) was an order of magnitude more sensitive to NO than for c-acon(WT). Consistent with this, an altered set point between RNA-binding and aconitase forms was observed for IRP1(S138E) when expressed in HEK cells. Active c-acon(S138E) could only accumulate under hypoxic conditions, suggesting enhanced cluster disassembly in normoxia. Cluster disassembly mechanisms were further probed by determining the impact of iron chelation on acon activity. Unexpectedly EDTA rapidly inhibited c-acon(S138E) activity without affecting c-acon(WT). Additional chelator experiments suggested that cluster loss can be initiated in c-acon(S138E) through a spontaneous nonoxidative demetallation process. Taken together, our results support a model wherein Ser(138) phosphorylation sensitizes IRP1/c-acon to decreased iron availability by allowing the [4Fe-4S](2+) cluster to cycle with [3Fe-4S](0) in the absence of cluster perturbants, indicating that regulation can be initiated merely by changes in iron availability.

Addition of DNA to Cr(VI) and cytochrome b5 containing proteoliposomes leads to generation of DNA strand breaks and Cr(III) complexes

Chromium (Cr) is a cytotoxic metal that can be associated with a variety of types of DNA damage, including Cr-DNA adducts and strand breaks. Prior studies with purified human cytochrome b(5) and NADPH:P450 reductase in reconstituted proteoliposomes (PLs) demonstrated rapid reduction of Cr(VI) (hexavalent chromium, as CrO(4)(2-), and the generation of Cr(V), superoxide (O(2)(*-)), and hydroxyl radical (HO(*)). Studies reported here examined the potential for the species produced by this system to interact with DNA. Strand breaks of purified plasmid DNA increased over time aerobically, but were not observed in the absence of O(2). Cr(V) is formed under both conditions, so the breaks are not mediated directly by Cr(V). The aerobic strand breaks were significantly prevented by catalase and EtOH, but not by the metal chelator diethylenetriaminepentaacetic acid (DTPA), suggesting that they are largely due to HO(*) from Cr-mediated redox cycling. EPR was used to assess the formation of Cr-DNA complexes. Following a 10-min incubation of PLs, CrO(4)(2-), and plasmid DNA, intense EPR signals at g=5.7 and g=5.0 were observed. These signals are attributed to specific Cr(III) complexes with large zero field splitting (ZFS). Without DNA, the signals in the g=5 region were weak. The large ZFS signals were not seen, when Cr(III)Cl(3) was incubated with DNA, suggesting that the Cr(III)-DNA interactions are different when generated by the PLs. After 24 h, a broad signal at g=2 is attributed to Cr(III) complexes with a small ZFS. This g=2 signal was observed without DNA, but it was different from that seen with plasmid. It is concluded that EPR can detect specific Cr(III) complexes that depend on the presence of plasmid DNA and the manner in which the Cr(III) is formed.

Reductive activation of hexavalent chromium by human lung epithelial cells: generation of Cr(V) and Cr(V)-thiol species

Chromium(VI) compounds (e.g. chromates) are cytotoxic, mutagenic, and potentially carcinogenic. The reduction of Cr(VI) can yield reactive intermediates such as Cr(V) and reactive oxygen species. Bronchial epithelial cells are the primary site of pulmonary exposure to inhaled Cr(VI) and are the primary cells from which Cr(VI)-associated human cancers arise. BEAS-2B cells were used here as a model of normal human bronchial epithelium for studies on the reductive activation of Cr(VI). Cells incubated with Na(2)CrO(4) exhibited two Cr(V) ESR signals, g=1.979 and 1.985, which persisted for at least 1h. The g=1.979 signal is similar to that generated in vitro by human microsomes and by proteoliposomes containing P450 reductase and cytochrome b(5). Unlike many cells in culture, these cells continued to express P450 reductase and cytochrome b(5). Studies with the non-selective thiol oxidant diamide indicated that the g=1.985 signal was thiol-dependent whereas the g=1.979 signal was not. Pretreatment with phenazine methosulfate eliminated both Cr(V) signals suggesting that Cr(V) generation is largely NAD(P)H-dependent. ESR spectra indicated that a portion of the Cr(VI) was rapidly reduced to Cr(III). Cells incubated with an insoluble chromate, ZnCrO(4), also generated both Cr(V) signals, whereas Cr(V) was not detected with insoluble PbCrO(4). In clonogenic assays, the cells were very sensitive to Na(2)CrO(4) and ZnCrO(4), but considerably less sensitive to PbCrO(4).

Mössbauer studies of the membrane-associated methane monooxygenase from Methylococcus capsulatus bath: evidence for a Diiron center

Two methane monooxygenase (MMO) systems have been identified in methanotrophic bacteria, namely, a soluble or cytoplasmic MMO and a membrane-associated or particulate MMO. The active site of the well-characterized soluble MMO contains a bis-mu-hydroxo-bridged diiron cluster. X-ray crystallographic studies of the particulate enzyme, pMMO, have identified two copper centers on the alpha subunit (pmoB) of the alphabetagamma trimer and a site at the interface of the betagamma subunits filled by a Zn, apparently from the crystallization buffer. In our hands, pMMO preparations containing 1-2 iron atoms per alphabetagamma show the highest catalytic activity. We have employed Mössbauer spectroscopy to characterize the iron in our preparations. Interestingly, we find in pMMO a component with the same spectral properties as the antiferromagnetically coupled diiron(III) cluster in the soluble enzyme. In whole cells, we find nearly 1 diiron center per alphabetagamma of pMMO; in purified enzyme preparations, only 10% of the sites appear to be occupied. These occupancies correlate well with the measured specific activities of purified pMMO and pMMO in whole cells. We suggest that it is the "Zn site" that accommodates the diiron center in active pMMO.

Electron paramagnetic resonance study of peripheral blood mononuclear cells from patients with refractory solid tumors treated with Triapine

The metal chelator Triapine, 3-aminopyridine-2-carboxaldehyde thiosemicarbazone, is a potent inhibitor of ribonucleotide reductase. EPR spectra consistent with signals from Fe-transferrin, heme, and low-spin iron or cupric ion were observed in peripheral blood mononuclear cells (PBMCs) obtained from patients treated with Triapine. One signal that is unequivocally identified is the signal for Fe-transferrin. It is hypothesized that Fe uptake is blocked by reactive oxygen species generated by FeT(2) or CuT that damage transferrin or transferrin receptor. A potential source for the increase in the heme signal is cytochrome c released from the mitochondria. These results provide valuable insight into the in vivo mechanism of action of Triapine.

Generation of novel copper sites by mutation of the axial ligand of amicyanin. Atomic resolution structures and spectroscopic properties

Amicyanin from Paracoccus denitrificans is a type 1 copper protein with three strong equatorial copper ligands provided by nitrogens of His53 and His95 and the sulfur of Cys92, with an additional weak axial ligand provided by the sulfur of Met98. Met98 was replaced with either Gln or Ala. As isolated, the M98A and M98Q mutant proteins contain zinc in the active site. The zinc is then removed and replaced with copper so that the copper-containing proteins may be studied. Each of the mutant amicyanins exhibits a marked decrease in thermal stability relative to that of native amicyanin, consistent with the weaker affinity for copper. Crystal structures were obtained for the oxidized and reduced forms of M98A and M98Q amicyanins at atomic resolution (<or=1.0 A). The crystal structure of oxidized M98A amicyanin exhibits a type 1 ligation geometry but with the axial ligand provided by a water, which fills the void left by the mutation of Met to Ala. The protein undergoes a reversible switch in ligation geometry when going from the aqueous to the frozen state. The visible absorption spectrum in solution is characteristic of type 1 copper, consistent with the crystal structure. On freezing, the blue color is lost, and EPR spectroscopy reveals that the copper is primarily type 2. The crystal structure of reduced M98A amicyanin exhibits an unprecedented ligation geometry in which the His95-Cu coordination is broken, and copper is left with only two ligands from His53 and Cys92 in an almost linear coordination. The replacement of Met98 with Gln yielded a type 1 copper site with increased rhombicity evident from its EPR and visible absorption spectra, and an increase in distance from Cu to the trigonal equatorial plane seen in the crystal structure. Gln98 coordinates more strongly with copper than Met, and the oxidized and reduced forms each exhibit two alternate conformers. EPR and metal analysis of oxidized M98Q amicyanin indicate that a small population of the protein contains weakly bound type 2 copper, which may be removed by washing with EDTA. These results demonstrate that the identity as well as position and rigidity of the axial ligand of the type 1 copper site has a profound influence in the uptake specificity of metal ions, protein stability, and determination of the active site geometry and its spectroscopic properties.

Reduction of hexavalent chromium by human cytochrome b5: generation of hydroxyl radical and superoxide

The reduction of hexavalent chromium, Cr(VI), can generate reactive Cr intermediates and various types of oxidative stress. The potential role of human microsomal enzymes in free radical generation was examined using reconstituted proteoliposomes (PLs) containing purified cytochrome b(5) and NADPH:P450 reductase. Under aerobic conditions, the PLs reduced Cr(VI) to Cr(V) which was confirmed by ESR using isotopically pure (53)Cr(VI). When 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO) was included as a spin trap, a very prominent signal for the hydroxyl radical (HO()) adduct was observed as well as a smaller signal for the superoxide (O(2)(-)) adduct. These adducts were observed even at very low Cr(VI) concentrations (10 muM). NADPH, Cr(VI), O(2), and the PLs were all required for significant HO() generation. Superoxide dismutase eliminated the O(2)(-) adduct and resulted in a 30% increase in the HO() adduct. Catalase largely diminished the HO() adduct signal, indicating its dependence on H(2)O(2). Some sources of catalase were found to have Cr(VI)-reducing contaminants which could confound results, but a source of catalase free of these contaminants was used for these studies. Exogenous H(2)O(2) was not needed, indicating that it was generated by the PLs. Adding exogenous H(2)O(2), however, did increase the amount of DEPMPO/HO() adduct. The inclusion of formate yielded the carbon dioxide radical adduct of DEPMPO, and experiments with dimethyl sulfoxide (DMSO) plus the spin trap alpha-phenyl-N-tert-butylnitrone (PBN) yielded the methoxy and methyl radical adducts of PBN, confirming the generation of HO(). Quantification of the various species over time was consistent with a stoichiometric excess of HO() relative to the net amount of Cr(VI) reduced. This also represents the first demonstration of a role for cytochrome b(5) in the generation of HO(). Overall, the simultaneous generation of Cr(V) and H(2)O(2) by the PLs and the resulting generation of HO() at low Cr(VI) concentrations could have important implications for Cr(VI) toxicity.

Spectral and thermodynamic properties of Ag(I), Au(III), Cd(II), Co(II), Fe(III), Hg(II), Mn(II), Ni(II), Pb(II), U(IV), and Zn(II) binding by methanobactin from Methylosinus trichosporium OB3b

Methanobactin (mb) is a novel chromopeptide that appears to function as the extracellular component of a copper acquisition system in methanotrophic bacteria. To examine this potential physiological role, and to distinguish it from iron binding siderophores, the spectral (UV-visible absorption, circular dichroism, fluorescence, and X-ray photoelectron) and thermodynamic properties of metal binding by mb were examined. In the absence of Cu(II) or Cu(I), mb will bind Ag(I), Au(III), Co(II), Cd(II), Fe(III), Hg(II), Mn(II), Ni(II), Pb(II), U(VI), or Zn(II), but not Ba(II), Ca(II), La(II), Mg(II), and Sr(II). The results suggest metals such as Ag(I), Au(III), Hg(II), Pb(II) and possibly U(VI) are bound by a mechanism similar to Cu, whereas the coordination of Co(II), Cd(II), Fe(III), Mn(II), Ni(II) and Zn(II) by mb differs from Cu(II). Consistent with its role as a copper-binding compound or chalkophore, the binding constants of all the metals examined were less than those observed with Cu(II) and copper displaced other metals except Ag(I) and Au(III) bound to mb. However, the binding of different metals by mb suggests that methanotrophic activity also may play a role in either the solubilization or immobilization of many metals in situ.

Spectral, kinetic, and thermodynamic properties of Cu(I) and Cu(II) binding by methanobactin from Methylosinus trichosporium OB3b

To examine the potential role of methanobactin (mb) as the extracellular component of a copper acquisition system in Methylosinus trichosporium OB3b, the metal binding properties of mb were examined. Spectral (UV-visible, fluorescence, and circular dichroism), kinetic, and thermodynamic data suggested copper coordination changes at different Cu(II):mb ratios. Mb appeared to initially bind Cu(II) as a homodimer with a comparatively high copper affinity at Cu(II):mb ratios below 0.2, with a binding constant (K) greater than that of EDTA (log K = 18.8) and an approximate DeltaG degrees of -47 kcal/mol. At Cu(II):mb ratios between 0.2 and 0.45, the K dropped to (2.6 +/- 0.46) x 10(8) with a DeltaG degrees of -11.46 kcal/mol followed by another K of (1.40 +/- 0.21) x 10(6) and a DeltaG degrees of -8.38 kcal/mol at Cu(II):mb ratios of 0.45-0.85. The kinetic and spectral changes also suggested Cu(II) was initially coordinated to the 4-thiocarbonyl-5-hydroxy imidazolate (THI) and possibly Tyr, followed by reduction to Cu(I), and then coordination of Cu(I) to 4-hydroxy-5-thiocarbonyl imidazolate (HTI) resulting in the final coordination of Cu(I) by THI and HTI. The rate constant (k(obsI)) of binding of Cu(II) to THI exceeded that of the stopped flow apparatus that was used, i.e., >640 s(-)(1), whereas the coordination of copper to HTI showed a 6-8 ms lag time followed by a k(obsII) of 121 +/- 9 s(-)(1). Mb also solubilized and bound Cu(I) with a k(obsI) to THI of >640 s(-)(1), but with a slower rate constant to HTI (k(obsII) = 8.27 +/- 0.16 s(-)(1)), and appeared to initially bind Cu(I) as a monomer.

[Cu3(mu-S)2]3+ clusters supported by N-donor ligands: progress toward a synthetic model of the catalytic site of nitrous oxide reductase

By treating Cu(I) complexes of neutral, bidentate N-donor ligands with S8, clusters with novel delocalized mixed-valence [Cu3(mu-S)2]3+ cores have been isolated. X-ray crystal structures and UV-vis and resonance Raman spectral features of these clusters reveal similarities to the tetracopper-sulfide "CuZ" site in nitrous oxide reductase. A delocalized S = 1 ground state for the mixed-valent CuIIICu2II cores is supported by the observation of high symmetry in the X-ray structures and 10-line hyperfine features arising from coupling to three equivalent Cu ions in EPR spectra obtained at room temperature (shown) and 10 K. The delocalization we observe contrasts with the localization reported previously for a [Cu3(mu-O)2]3+ analogue (Root, D. E.; Henson, M. J.; Machonkin, T.; Mukherjee, P.; Stack, T. D. P.; Solomon, E. I. J. Am. Chem. Soc. 1998, 120, 4982), which we rationalized through DFT calculations.

Effect of methanobactin on the activity and electron paramagnetic resonance spectra of the membrane-associated methane monooxygenase in Methylococcus capsulatus Bath

Improvements in the purification of methanobactin (mb) from eitherMethylosinus trichosporiumOB3bTorMethylococcus capsulatusBath resulted in preparations that stimulated methane-oxidation activity in both whole-cell and cell-free fractions ofMethylococcus capsulatusBath expressing the membrane-associated methane monooxygenase (pMMO). By using washed membrane factions with pMMO activities in the 290 nmol propylene oxidized min−1(mg protein)−1range, activities approaching 400 nmol propylene oxidized min−1(mg protein)−1were commonly observed following addition of copper-containing mb (Cu–mb), which represented 50–75 % of the total whole-cell activity. The stimulation of methane-oxidation activity by Cu–mb was similar to or greater than that observed with equimolar concentrations of Cu(II), without the inhibitory effects observed with high copper concentrations. Stimulation of pMMO activity was not observed with copper-free mb, nor was it observed when the copper-to-mb ratio was <0·5 Cu atoms per mb. The electron paramagnetic resonance (EPR) spectra of mb differed depending on the copper-to-mb ratio. At copper-to-mb ratios of <0·4 Cu(II) per mb, Cu(II) addition to mb showed an initial coordination by both sulfur and nitrogen, followed by reduction to Cu(I) in <2 min. At Cu(II)-to-mb ratios between 0·4 and 0·9 Cu(II) per mb, the intensity of the Cu(II) signal in EPR spectra was more representative of the Cu(II) added and indicated more nitrogen coordination. The EPR spectral properties of mb and pMMO were also examined in the washed membrane fraction following the addition of Cu(II), mb and Cu–mb in the presence or absence of reductants (NADH or duroquinol) and substrates (CH4and/or O2). The results indicated that Cu–mb increased electron flow to the pMMO, increased the free radical formed following the addition of O2and decreased the residual free radical following the addition of O2plus CH4. The increase in pMMO activity and EPR spectral changes to the pMMO following Cu–mb addition represent the first positive evidence of interactions between the pMMO and Cu–mb.

The octarepeat domain of the prion protein binds Cu(II) with three distinct coordination modes at pH 7.4

The prion protein (PrP) binds Cu2+ in its N-terminal octarepeat domain. This unusual domain is comprised of four or more tandem repeats of the fundamental sequence PHGGGWGQ. Previous work from our laboratories demonstrates that at full copper occupancy, each HGGGW segment binds a single Cu2+. However, several recent studies suggest that low copper occupancy favors different coordination modes, possibly involving imidazoles from histidines in adjacent octapeptide segments. This is investigated here using a combination of X-band EPR, S-band EPR, and ESEEM, along with a library of modified peptides designed to favor different coordination interactions. At pH 7.4, three distinct coordination modes are identified. Each mode is fully characterized to reveal a series of copper-dependent octarepeat domain structures. Multiple His coordination is clearly identified at low copper stoichiometry. In addition, EPR detected copper-copper interactions at full occupancy suggest that the octarepeat domain partially collapses, perhaps stabilizing this specific binding mode and facilitating cooperative copper uptake. This work provides the first complete characterization of all dominant copper coordination modes at pH 7.4.

Assembly of cytochrome-c oxidase in the absence of assembly protein Surf1p leads to loss of the active site heme

Surf1p is a protein of the inner membrane of mitochondria that functions in the assembly of cytochrome-c oxidase. The specifics of the role of Surf1p have remained unresolved. Numerous mutations in human Surf1p lead to severe mitochondrial disease. A homolog of human Surf1p is encoded by the genome of the alpha-proteobacterium Rhodobacter sphaeroides, which synthesizes a mitochondrial-like aa(3)-type cytochrome-c oxidase. The gene for Surf1p was deleted from the genome of R. sphaeroides. The resulting aa(3)-type oxidase was purified and analyzed by biochemical methods plus optical and EPR spectroscopy. The oxidase that assembled in the absence of Surf1p was composed of three subpopulations with structurally distinct heme a(3)-Cu active sites. 50% of the oxidase lacked heme a(3), 10-15% contained heme a(3) but lacked Cu(BB), and 35-40% had a normal heme a(3) -Cu(B) active site with normal activity. Cu(A) assembly was unaffected. All of the oxidase contained low-spin heme a, but the environment of the heme a center was slightly altered in the 50% of the enzyme that lacked heme a(3). Introduction of a normal copy of the gene for Surf1p on an exogenous plasmid resulted in a single population of normally assembled, highly active enzyme. The data indicate that Surf1p plays a role in facilitating the insertion of heme a(3) into the active site of cytochrome-c oxidase. The results suggest that maturation of the heme a(3)-Cu(B) center is a step that limits the association of subunits I and II in the assembly of mitochondrial cytochrome oxidase.

Thiol Oxidase Activity of Copper, Zinc Superoxide Dismutase Stimulates Bicarbonate-Dependent Peroxidase Activity via Formation of a Carbonate Radical

Here, we investigated the effect of bicarbonate anion (HCO3-) on the peroxidase activity stimulated by the thiol oxidase activity of copper, zinc superoxide dismutase (SOD1) using electron spin resonance (ESR) and optical techniques. Low temperature direct ESR revealed that cysteine (Cys) caused the reduction of copper(II) to copper(I) that was reoxidized by molecular oxygen to copper(II) at the active site of SOD1. The addition of HCO3- to aerobic incubations containing SOD1, Cys, and DTPA in phosphate buffer enhanced the peroxidase activity of SOD1, as measured by hydroxylation of cyclic nitrone spin traps, dichlorodihydrofluorescein oxidation to dichlorofluorescein, and oxidation of tyrosine to dityrosine. The addition of catalase inhibited the SOD1 peroxidase activity stimulated by the thiol oxidase actvity, implicating an intermediary role for H2O2 in SOD1/Cys/HCO3(-)-mediated oxidation and hydroxylation reactions. Using a competitive kinetic method, rate constants for the reaction between the oxidant formed in the SOD1/Cys/HCO3- system and selected inhibitors were measured. On the basis of these rate constants, we conclude that the thiol oxidase activity of SOD1 stimulates carbonate anion radical (CO3*-) formation in the presence of HCO3- and that the CO3*- formed in the SOD1/Cys/ HCO3- system is responsible for oxidation and hydroxylation reactions. Biological implications of this finding are discussed.

Direct Probing of Copper Active Site and Free Radical Formed during Bicarbonate-dependent Peroxidase Activity of Bovine and Human Copper,Zinc-superoxide Dismutases

Using X-band electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopy at liquid helium temperatures, the Cu(II) coordination geometry at the active site of bovine and human copper,zinc-superoxide dismutases (bSOD1 and hSOD1) treated with H(2)O(2) and bicarbonate (HCO(3)(-)) was examined. The time course EPR of wild type human SOD1 (WT hSOD1), W32F hSOD1 mutant (tryptophan 32 substituted with phenylalanine), and bSOD1 treated with H(2)O(2) and HCO(3)(-) shows an initial reduction of active site Cu(II) to Cu(I) followed by its oxidation back to Cu(II) in the presence of H(2)O(2). However, HCO(3)(-) induced a Trp-32-derived radical from WT hSOD1 but not from bSOD1. The mutation of Trp-32 by phenylalanine totally eliminated the Trp-32 radical signal generated from W32F hSOD1 treated with HCO(3)(-) and H(2)O(2). Further characterization of the free radical was performed by UV irradiation of WT hSOD1 and bSOD1 that generated tryptophanyl and tyrosyl radicals. Both proton ((1)H) and nitrogen ((14)N) ENDOR studies of bSOD1 and hSOD1 in the presence of H(2)O(2) revealed a change in the geometry of His-46 (or His-44) and His-48 (or His-46) coordinated to Cu(II) at the active site of WT hSOD1 and bSOD1, respectively. However, in the presence of HCO(3)(-) and H(2)O(2), both (1)H and (14)N ENDOR spectra were almost identical to those derived from native bSOD1. We conclude that HCO(3)(-)-derived oxidant does not alter significantly the Cu(II) active site geometry and histidine coordination to Cu(II) in SOD1 as does H(2)O(2) alone; however, the oxidant derived from HCO(3)(-) (i.e. carbonate anion radical) reacts with surface-associated Trp-32 in hSOD1 to form the corresponding radical.

Vanadium(V) reduction by Shewanella oneidensis MR-1 requires menaquinone and cytochromes from the cytoplasmic and outer membranes

The metal-reducing bacterium Shewanella oneidensis MR-1 displays remarkable anaerobic respiratory plasticity, which is reflected in the extensive number of electron transport components encoded in its genome. In these studies, several cell components required for the reduction of vanadium(V) were determined. V(V) reduction is mediated by an electron transport chain which includes cytoplasmic membrane components (menaquinone and the tetraheme cytochrome CymA) and the outer membrane (OM) cytochrome OmcB. A partial role for the OM cytochrome OmcA was evident. Electron spin resonance spectroscopy demonstrated that V(V) was reduced to V(IV). V(V) reduction did not support anaerobic growth. This is the first report delineating specific electron transport components that are required for V(V) reduction and of a role for OM cytochromes in the reduction of a soluble metal species.

Evidence That NiNi Acetyl-CoA Synthase Is Active and That the CuNi Enzyme Is Not†

The bifunctional CO dehydrogenase/acetyl-CoA synthase (CODH/ACS) plays a central role in the Wood-Ljungdahl pathway of autotrophic CO(2) fixation. One structure of the Moorella thermoacetica enzyme revealed that the active site of ACS (the A-cluster) consists of a [4Fe-4S] cluster bridged to a binuclear CuNi center with Cu at the proximal metal site (M(p)) and Ni at the distal metal site (M(d)). In another structure of the same enzyme, Ni or Zn was present at M(p). On the basis of a positive correlation between ACS activity and Cu content, we had proposed that the Cu-containing enzyme is active [Seravalli, J., et al. (2003) Proc. Natl. Acad. Sci. U.S.A. 100, 3689-3694]. Here we have reexamined this proposal. Enzyme preparations with a wider range of Ni (1.6-2.8) and Cu (0.2-1.1) stoichiometries per dimer were studied to reexamine the correlation, if any, between the Ni and Cu content and ACS activity. In addition, the effects of o-phenanthroline (which removes Ni but not Cu) and neocuproine (which removes Cu but not Ni) on ACS activity were determined. EXAFS results indicate that these chelators selectively remove M(p). Multifrequency EPR spectra (3-130 GHz) of the paramagnetic NiFeC state of the A-cluster were examined to investigate the electronic state of this proposed intermediate in the ACS reaction mechanism. The combined results strongly indicate that the CuNi enzyme is inactive, that the NiNi enzyme is active, and that the NiNi enzyme is responsible for the NiFeC EPR signal. The results also support an electronic structure of the NiFeC-eliciting species as a [4Fe-4S](2+) (net S = 0) cluster bridged to a Ni(1+) (S = (1)/(2)) at M(p) that is bridged to planar four-coordinate Ni(2+) (S = 0) at M(d), with the spin predominantly on the Ni(1+). Furthermore, these studies suggest that M(p) is inserted during cell growth. The apparent vulnerability of the proximal metal site in the A-cluster to substitution with different metals appears to underlie the heterogeneity observed in samples that has confounded studies of CODH/ACS for many years. On the basis of this principle, a protocol to generate nearly homogeneous preparations of the active NiNi form of ACS was achieved with NiFeC signals of approximately 0.8 spin/mol.

Effect on ribonucleotide reductase of novel lipophilic iron chelators: the desferri-exochelins

Desferri-exochelins are siderophores secreted by Mycobacterium tuberculosis that are both lipid- and water-soluble and have a high binding affinity for iron. Desferri-exochelin 772SM inhibits DNA replication and ribonucleotide reductase activity at 10-fold less concentration than the lipid-insoluble iron chelator deferoxamine, which is currently in clinical use. Neither chelator can extract iron directly from ribonucleotide reductase. However, because of its lipid-solubility and high binding affinity, desferri-exochelin is able to enter cells rapidly and access intracellular iron, while deferoxamine has limited capacity to cross the cell membrane.

The membrane-associated methane monooxygenase (pMMO) and pMMO-NADH:quinone oxidoreductase complex from Methylococcus capsulatus Bath

Improvements in purification of membrane-associated methane monooxygenase (pMMO) have resulted in preparations of pMMO with activities more representative of physiological rates: i.e., >130 nmol.min(-1).mg of protein(-1). Altered culture and assay conditions, optimization of the detergent/protein ratio, and simplification of the purification procedure were responsible for the higher-activity preparations. Changes in the culture conditions focused on the rate of copper addition. To document the physiological events that occur during copper addition, cultures were initiated in medium with cells expressing soluble methane monooxygenase (sMMO) and then monitored for morphological changes, copper acquisition, fatty acid concentration, and pMMO and sMMO expression as the amended copper concentration was increased from 0 (approximately 0.3 microM) to 95 microM. The results demonstrate that copper not only regulates the metabolic switch between the two methane monooxygenases but also regulates the level of expression of the pMMO and the development of internal membranes. With respect to stabilization of cell-free pMMO activity, the highest cell-free pMMO activity was observed when copper addition exceeded maximal pMMO expression. Optimization of detergent/protein ratios and simplification of the purification procedure also contributed to the higher activity levels in purified pMMO preparations. Finally, the addition of the type 2 NADH:quinone oxidoreductase complex (NADH dehydrogenase [NDH]) from M. capsulatus Bath, along with NADH and duroquinol, to enzyme assays increased the activity of purified preparations. The NDH and NADH were added to maintain a high duroquinol/duroquinone ratio.