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Electron paramagnetic resonance and magnetic circular dichroism studies of electron-transfer flavoprotein-ubiquinone oxidoreductase from pig liver.

Pig liver electron-transfer flavoprotein-ubiquinone oxidoreductase has been investigated by room temperature UV-visible, low-temperature electron paramagnetic resonance and low-temperature magnetic circular dichroism spectroscopies. The results provide unambiguous evidence for the presence of a single [4Fe-4S] cluster that is diamagnetic in the isolated enzyme and becomes paramagnetic with an S = 1/2 ground state on reduction with dithionite or enzymatically with the physiological electron donor. The EPR data for samples at pH 7.8 indicate that FAD is reduced by one electron to the anionic semiquinone form in the enzymatically reduced enzyme, and by two electrons to the hydroquinone form by excess dithionite. The possibility of weak spin-spin interaction between the FAD semiquinone and the [4Fe-4S]1+ center is discussed in the light of the observation of a small increase in the linewidth of the Fe-S EPR in enzymatically reduced samples.

Animals↗

Molybdenum EXAFS of the Desulfovibrio gigas Mo(2Fe-2S) protein--structural similarity to "desulfo" xanthine dehydrogenase.

The molybdenum EXAFS of the Mo(2Fe-2S) protein from Desulfovibrio gigas has been examined using fluorescence detection and synchrotron radiation. In the oxidized form the molybdenum environment is found to contain two terminal oxo groups and two long (2.47 A) Mo-S bonds. Evidence was also found for an oxygen or nitrogen donor ligand at 1.90 A. Addition of dithionite to the oxidized enzyme results in loss of a terminal oxo group, perhaps due to protonation. In addition, a 0.1 A contraction in the Mo-S bond lengths is observed. The behavior of both oxidized and dithionite-treated forms is similar to that observed previously with "desulfo" xanthine oxidase.

Bacterial Proteins↗

Formate dehydrogenase molybdenum and tungsten sites--observation by EXAFS of structural differences.

Preliminary EXAFS data has been collected on the molybdenum (K-edge) in C. pasteurianum formate dehydrogenase and the tungsten (LIII-edge) in C. thermoaceticum formate dehydrogenase. In the presence of dithionite, the tungsten enzyme was devoid of W = O bonds, and exhibited average W-(O, N) and W-S bond lengths of 2.13 +/- 0.03 A and 2.39 +/- 0.03 A, respectively. In sharp contrast, the C. pasteurianum molybdenum site has three Mo = O bonds with an average bond length of 1.74 +/- 0.03 A. It is also the first molybdenum enzyme found lacking Mo-S bonds, and does not appear to be redox active in the presence of formate or dithionite. Model compounds WO2(8-hydroxyquinoline)2 = WO2(ox)2, and WO2(8 mercaptoquinoline)2 = WO2(tox)2, were also examined. Respective predicted bond lengths for WO2(ox)2 and WO2(tox)2 were W = O of 1.71, 1.73 A; W-N of 2.31, 2.29 A; W-O or W-S of 1.92 or 2.40 A, with estimated uncertainties of +/- 0.03 A.

Aldehyde Oxidoreductases↗

Aerobic, inactive forms of Azotobacter vinelandii hydrogenase: activation kinetics and insensitivity to C2H2 inhibition.

Azotobacter vinelandii hydrogenase (EC class 1.12), either purified or membrane-associated, was obtained aerobically in an inactive state. The kinetics of activation by treatment with a reductant (H2 or dithionite) were determined. Three distinct phases of the activation were observed. Aerobically prepared, inactive hydrogenase was insensitive to acetylene inhibition, but could be rendered acetylene-sensitive by reduction with dithionite. These findings indicate that acetylene inhibition of hydrogenase requires catalytically active enzyme.

Acetylene↗

Absorbance spectral change of the cytochrome P-450S21-phenylisocyanide complex upon binding of reduced NADPH-cytochrome-P-450 reductase.

Reduction of cytochrome P-450S21 (SF) (SF, substrate-free; purified from bovine adrenocortical microsomes) with sodium dithionite (Na2S2O4) in the presence of phenylisocyanide produced a ferrous cytochrome P-450S21 (SF)-phenylisocyanide complex with Soret absorbance maxima at 429 and 456 nm. On the other hand, when a preformed ferric cytochrome P-450S21 (SF)-NADPH-cytochrome-P-450 reductase (Fp2) complex was reduced chemically or enzymatically under the same conditions, the absorbance spectrum of the ferrous cytochrome P-450S21 (SF)-phenylisocyanide complex changed drastically, as characterized by an increase in absorbance intensity at 429 nm and a decrease at 456 nm. Similar spectral changes were observed by addition of reduced Fp2 to the preformed ferrous cytochrome P-450S21 (SF)-phenylisocyanide complex. Experiments to reduce a ferric cytochrome P-450S21 (SF)-phenylisocyanide complex with sodium dithionite in the presence of various amounts of Fp2 showed that; (1), the spectral change reached maxima for both absorption increase at 429 nm and decrease at 456 nm when cytochrome P-450S21 and Fp2 were previously mixed at the cytochrome P-450S21:Fp2 ratio of 1:5; (2), the spectral change was suppressed in 300 mM potassium phosphate buffer (pH 7.4). These results suggest that the absorbance spectral change is due to a conformational change around the heme moiety induced by association with reduced Fp2.

Adrenal Cortex↗

The nitrogenase proteins of Rhizobium meliloti: purification and properties of the MoFe and Fe components.

The alfalfa-Rhizobium meliloti symbiosis contributes a major portion of biologically fixed nitrogen to temperate zone forage crop production. Highly-purified molybdenum-iron (MoFe) and iron (Fe) nitrogenase components were obtained for the first time from extracts of R. meliloti bacteroids. Intact bacteroid cells were isolated anaerobically from 100 g quantities of alfalfa nodules following storage in liquid nitrogen. Centrifuged bacteroid extracts showed a marked reduction in specific activity when assayed at protein concentrations less than 1 mg/ml. Both nitrogenase proteins were resolved and purified to homogeneity as determined spectroscopically and by SDS-PAGE. The purified MoFe protein differed in several respects from previously characterized nitrogenase proteins. Saturation of the acetylene-reducing and proton-reducing activities of the R. meliloti MoFe protein required higher relative concentrations of Fe protein than nitrogenase proteins purified from free living diazotrophs. Electron allocation to dinitrogen reduction was sustained at component ratios similar to those present in bacteroid extracts, suggesting that while the observed saturation effects were not detrimental to physiological function in the symbiotic system, overall activity could be enhanced by higher levels of iron protein. Analyses of the MoFe protein gave 22 Fe, 22 labile sulfide and 1.7 Mo atoms per molecular unit of 215 kDa. Dithionite-reduced MoFe protein contained a spin 3/2 iron centre but had a lower visible absorbance at 360 nm than the equivalent Azotobacter chroococcum component. Amino-acid composition indicated a notably lesser tryptophan content, and cysteine content greater than that of the equivalent tetrameric protein of free living diazotrophs. Ratios of acidic and basic residues were similar to other MoFe proteins. Calculation of hydrophobicity and discriminant parameters gave values midway between those expected for soluble cytoplasmic proteins and peripheral membrane associated proteins. ADP was tightly bound by the dithionite-free MoFe protein containing reduced iron-molybdenum cofactor. The R. meliloti iron protein was found to be a 64 kDa homodimer containing a single 4Fe-4S metal centre.

Amino Acids↗

Identification of an unusual paramagnetic species and of three [2Fe-2S] clusters in the iron-only hydrogenase from the hyperthermophilic bacterium Thermotoga maritima.

Thermotoga maritima is a H2-producing, fermentative anaerobe and is one of the most thermophilic bacteria known. Its iron-only (Fe-)hydrogenase was previously shown to be a homotetramer and to contain two [4Fe-4S] and two [2Fe-2S] clusters per monomer, but the enzyme lacked the characteristic EPR signal of the oxidized H cluster, the proposed site of H2 catalysis in mesophilic Fe-hydrogenases (Juszczak, A., Aono, S. and Adams, M.W.W. (1991) J. Biol. Chem. 266, 13834-13841). The two types of cluster were shown by spectroelectrochemistry to have reduction potentials (Em) of -390 and -440 mV, respectively. We have now identified two additional redox centers in the enzyme, a [2Fe-2S] center with a higher reduction potential (Em = -365 mV) and an unusual paramagnetic species (Em > -200 mV). The higher potential [2Fe-2S] center can be reduced by sodium dithionite at pH 6.0 and exhibits an axial-type EPR signal with gz = 2.026 and gy = gx = 1.940. The two lower potential [2Fe-2S] centers are fully reduced by sodium dithionite only at pH 10.0. Both of these clusters in their reduced states exhibit rhombic-type EPR signals with gz = 2.005, gy = 1.955, and gx = 1.921. This hydrogenase is therefore thought to contain three [2Fe-2S] clusters, as well as two [4Fe-4S] clusters. In addition, a nearly isotropic EPR signal (g = 2.01) was observed when the enzyme was anaerobically oxidized by organic dyes such as thionine (E alpha = 64 mV) or 2,6-dichlorophenolindophenol (E alpha = 217 mV). This resonance was not observed at 20 K due to relaxation broadening and therefore did not arise from a conventional organic radical. The oxidized enzyme was fully active in an H2 production assay, and also reacted directly with H2. In contrast, the air-oxidized enzyme was inactive and did not exhibit the g = 2.01 EPR signal. This resonance was assigned to a novel paramagnetic species with an approximate Em value of -70 mV. It is thought to be associated with the H2 activating site of this atypical Fe-hydrogenase.

Bacterial Proteins↗

Determination of combined benzidine in FD & C yellow no. 5 (tartrazine), using a highly sensitive analytical method.

53 samples of FD & C Yellow No. 5 (tartrazine; Colour Index No. 19140) were examined for combined benzidine. These samples, which represent separate lots from 12 dye distributors, were submitted to the US FDA for certification between 28 February 1990 and 27 June 1991. A method was developed to reduce the dye matrix with dithionite so that combined benzidine present in the form of azo or disazo dyes would be converted to free benzidine. Reduction was followed by extraction, diazotization and coupling with pyrazolone T, and the total benzidine present was quantitated as benzidine-pyrazolone T disazo dye (BZPT) by HPLC with detection at 500 nm. The limit of quantitation for benzidine in FD & C Yellow No. 5 by this method is 5 ng/g. 25 samples of FD & C Yellow No. 5 were found to contain 7-83 ng/g of combined benzidine that was released by dithionite reduction. 23 of these samples were from the same manufacturer. The identify of the BZPT from two FD & C Yellow No. 5 samples was confirmed by spectral analysis using HPLC with a photodiode array detector.

Benzidines↗

Characterization of a soluble catalase-peroxidase hemoprotein b-590, previously identified as 'cytochrome alpha 1' from Bradyrhizobium japonicum bacteroids.

The cytochrome "a1" or P-428, previously proposed to be a high affinity terminal oxidase in nitrogen-fixing bacteroids of Bradyrhizobium japonicum has been purified. The water-soluble native hemoprotein has an Mr of 136,000, lacks heme a and is a high-spin ferric protohemoprotein: It is slowly reduced with dithionite to give a species with an optical spectrum like that of hemoprotein b-590 (Escherichia coli; peak at 555 nm, shoulder at 590 nm), and which reacts slowly with CO. It has catalase and peroxidase activities, again resembling the E. coli b-590. Neither hemoprotein forms a stable oxy complex under conditions in which dithionite-reduced horseradish peroxidase reacts with oxygen to form such a complex. The hemoprotein, which we name hemoprotein b-590 (Bradyrhizobium japonicum), may play a role in removal of peroxides generated during respiration in the bacteroids of several Rhizobium and Bradyrhizobium species. The high-affinity terminal oxidase under nitrogen-fixing conditions remains to be identified.

Catalase↗

Radiolabeled biomolecules with 186Re: potential for radioimmunotherapy.

Rhenium-186 has been theoretically determined to be among the best therapy radiolabels due to its unique half-life, particulate and gamma emissions and chelation properties. Traditionally, rhenium chelates have been synthesized by the tin reduction method at low pH which frequently produces denaturation of acid labile proteins. A comparative study has been carried out to assess three techniques of reducing 186ReO-4 in order to label biologically active macromolecules (i.e. human serum albumin (HSA), anti-human serum albumin antibody (HSA-Ab) and a monoclonal antibody to T-cells [T-101]). These experiments showed that stannous and dithionite reduction methods provide for an overall labeling yield of between 5 and 18% with an associated immunoreactivity of 12-40%. The hypophosphorous acid (H3PO2) reduction method, however, yielded no usable radiolabeled product. The preparation of 186Re-DTPA-HSA produced an 18.2% radiochemical yield (7.4 X 10(6) Bq/mg) and 40% retention of binding affinity. Using the stannous or dithionite reduction methods for the radiolabeling of HSA-Ab and T-101 resulted in a relatively low yield (9%), but the labeled product retained binding affinity of 12.25% with Protein A.

Antibodies, Monoclonal↗

Adsorption of arsenate on soils. Part 2: modeling the relationship between adsorption capacity and soil physiochemical properties using 16 Chinese soils.

An attempt has been made to elucidate the effects of soil properties on arsenate adsorption by modeling the relationships between adsorption capacity and the properties of 16 Chinese soils. The model produced was validated against three Australian and three American soils. The results showed that nearly 93.8% of the variability in arsenate adsorption on the low-energy surface could be described by citrate-dithionite extractable Fe (Fe(CD)), clay content, organic matter content (OM) and dissolved organic carbon (DOC); nearly 87.6% of the variability in arsenate adsorption on the high-energy surface could be described by Fe(CD), DOC and total arsenic in soils. Fe(CD) exhibited the most important positive influence on arsenate adsorption. Oxalate extractable Al (Al(OX)), citrate-dithionite extractable Al (Al(CD)), extractable P and soil pH appeared relatively unimportant for adsorption of arsenate by soils.

Adsorption↗

Time dependent calibration of a sediment extraction scheme.

Sediment extraction methods to quantify metal concentration in aquatic sediments usually present limitations in accuracy and reproducibility because metal concentration in the supernatant is controlled to a large extent by the physico-chemical properties of the sediment that result in a complex interplay between the solid and the solution phase. It is suggested here that standardization of sediment extraction methods using pure mineral phases or reference material is futile and instead the extraction processes should be calibrated using site-specific sediments before their application. For calibration, time dependent release of metals should be observed for each leachate to ascertain the appropriate time for a given extraction step. Although such an approach is tedious and time consuming, using iron extraction as an example, it is shown here that apart from quantitative data such an approach provides additional information on factors that play an intricate role in metal dynamics in the environment. Single step ascorbate, HCl, oxalate and dithionite extractions were used for targeting specific iron phases from saltmarsh sediments and their response was observed over time in order to calibrate the extraction times for each extractant later to be used in a sequential extraction. For surficial sediments, an extraction time of 24 h, 1 h, 2 h and 3 h was ascertained for ascorbate, HCl, oxalate and dithionite extractions, respectively. Fluctuations in iron concentration in the supernatant over time were ubiquitous. The adsorption-desorption behavior is possibly controlled by the sediment organic matter, formation or consumption of active exchange sites during extraction and the crystallinity of iron mineral phase present in the sediments.

Calibration↗

Inhibitory effects of ethanol extract from Radix Ophiopogon japonicus on venous thrombosis linked with its endothelium-protective and anti-adhesive activities.

The in-vivo inhibitory effects of the ethanol extract of Radix Ophiopogon japonicus (ROJ-ext) on venous thrombosis were studied in mouse and rat models and in-vitro endothelial cell-protective and anti-adhesive activities were observed in ECV304 cells injured by sodium dithionite and HL-60 adhesion to ECV304 cells injured by TNF-alpha. The in-vivo results showed that ROJ-ext significantly inhibited venous thrombosis induced by tight ligation of the inferior vena cava for 6 h in mice and for 24 h in rats by once oral administration at doses of 12.5 and 25 mg/kg. Meanwhile, ROJ-ext had no obvious effect on some coagulation parameters, which was different from warfarin, which remarkably prolonged activated partial thromboplastin time (APTT), thrombin time (TT) and prothrombin time (PT) in rats at the same time. Histological analysis under light microscope and scanning electron microscope (SEM) of inferior vena cava indicated that ROJ-ext could protect endothelial cells from anoxic injury and alleviate inflammatory changes in the vein wall. On the other hand, the in-vitro studies approved that ROJ-ext significantly enhanced viability of ECV304 cells injured by sodium dithionite at the concentrations of 0.1, 1.0 and 10 mug/ml when given before and after the anoxic induction. Meanwhile, ROJ-ext remarkably inhibited adhesion of HL-60 cells to ECV304 cells injured by rh TNF-alpha at above concentrations in a dose-dependent manner. The findings of this study showed that ethanol extract of Radix Ophiopogon japonicus (ROJ-ext) inhibited venous thrombosis, which linked with its endothelial cell-protective and anti-adhesive activities. This lends scientific support to the therapeutic use of the plant for thrombotic diseases.

Animals↗

Rapid flip-flop in polyunsaturated (docosahexaenoate) phospholipid membranes.

The transbilayer movement (flip-flop) of 7-nitrobenz-2-oxa-1,3-diazol-4-yl phosphatidylethanolamine (NBD-PE) in phosphatidylcholine (PC) membranes containing various acyl chains was measured by dithionite quenching of NBD fluorescence. Of specific interest was docosahexaenoic acid (DHA), the longest and most unsaturated acyl chain commonly found in membranes. This molecule represents the extreme example of a family of important fatty acids known as omega-3s and has been clearly demonstrated to alter membrane structure and function. One important property that has yet to be reported is the effect of DHA on membrane phospholipid flip-flop. This study demonstrates that as the number of double bonds in the fatty acyl chains comprising the membrane increases, so does the rate of flip-flop of the NBD-PE probe. The increase is particularly marked in the presence of DHA. Half-lives t(1/2) of 0.29 and 0.086 h describe the process in 1-stearoyl-2-docosahexaenoylphosphatidylcholine and 1,2-didocosahexaenoylphosphatidylcholine, respectively, whereas in 1-stearoyl-2-oleoylphosphatidylcholine t(1/2)=11.5h. Enhanced permeability to dithionite with increasing unsaturation was also indicated by our results. We conclude that PC membranes containing DHA support faster flip-flop and permeability rates than those measured for other less-unsaturated PCs.

Docosahexaenoic Acids↗

Separate beta subunits are derivatized with 14C and 3H when the bovine heart mitochondrial F1-ATPase is doubly labeled with 7-chloro-4-nitro[14C]benzofurazan and 5'-p-fluorosulfonylbenzoyl[3H]inosine.

Tyrosine residues 311 and 345 of the beta subunit of the bovine heart mitochondrial F1-ATPase (MF1) are present on the same peptide when the enzyme is fragmented with cyanogen bromide. Maximal inactivation of MF1 with 7-chloro-4-nitro[14C]benzofurazan [( 14C]Nbf-Cl) derivatizes tyrosine-311 in a single beta subunit. Cyanogen bromide digests of MF1 containing the [14C]Nbf-O-derivative of tyrosine-beta 311 were submitted to reversed-phase HPLC, with and without prior reduction of the nitro group on the incorporated reagent with dithionite. The retention time of the radioactive cyanogen bromide peptide was shifted substantially by reduction. When a cyanogen bromide digest of MF1 inactivated with 5'-p-fluorosulfonylbenzoyl[3H]inosine [( 3H]FSBI), which proceeds with derivatization of tyrosine-345 in a single beta subunit, was submitted to HPLC under the same conditions, the fragment labeled with 3H eluted with the same retention time as the [14C]Nbf-O-derivative before reduction. Doubly labeled enzyme was prepared by first derivatizing Tyr-beta 311 with [14C]Nbf-Cl and then derivatizing tyrosine-beta 345 with [3H]FSBI with and without reducing the [14C]Nbf-O-derivative of tyrosine-beta 311 with dithionite before modification with [3H]FSBI. The doubly labeled enzyme preparations were digested with cyanogen bromide and submitted to HPLC. The 14C and 3H in the cyanogen bromide digest prepared from doubly labeled enzyme not submitted to reduction eluted together. In contrast, the 14C and 3H in the digest prepared from doubly labeled enzyme which had been reduced eluted separately. From these results it is concluded that different beta subunits are derivatized when MF1 is doubly labeled with [14C]Nbf-Cl and [3H]FSBI.

Animals↗

Novel inner monolayer fusion assays reveal differential monolayer mixing associated with cation-dependent membrane fusion.

The ability to specifically monitor the behavior of the inner monolayer lipids of membranous vesicles during the membrane fusion process is useful technically and experimentally. In this study, we have identified N-NBD-phosphatidylserine as a reducible probe particularly suitable for inner monolayer fusion assays because of its low rate of membrane translocation after reduction of the outer monolayer probes by dithionite. Data are presented on translocation as a function of temperature, vesicle size, membrane composition, and serum protein concentration. Translocation as a result of the fusion event itself was also characterized. We further show here that a second membrane-localized probe, a long wavelength carbocyanine dye referred to a diI(5)C18ds, appears to form a membrane-bound resonance energy transfer pair with N-NBD-PS, and its outer monolayer fluorescence can also be eliminated by dithionite treatment. Lipid dilution of these probes upon fusion with unlabeled membranes leads to an increase in NBD donor fluorescence, and hence is a new type of inner monolayer fusion assay. These inner monolayer probe mixing assays were compared to random lipid labeling and aqueous contents mixing assays for cation-dependent fusion of liposomes composed of phosphatidylserine and phosphatidylethanolamine. The results showed that the inner monolayer fusion assay eliminates certain artifacts and reflects fairly closely the rate of non-leaky mixing of aqueous contents due to fusion, while outer monolayer mixing always precedes mixing of aqueous contents. In fact, vesicle aggregation and outer monolayer lipid mixing were found to occur over very long periods of time without inner monolayer mixing at low cation concentrations. Externally added lysophosphatidylcholine inhibited vesicle aggregation, outer monolayer mixing and any subsequent fusion. The state of vesicle aggregation and outer monolayer exchange that occurs below the fusion threshold may represent a metastable intermediate state that may be useful for further studies of the mechanism of membrane fusion.

4-Chloro-7-nitrobenzofurazan↗

A spectroscopic analysis of a high fluorescent mutant of Chlamydomonas reinhardi.

Chloroplast fragments of a high fluorescent mutant of Chlamydomonas reinhardi, hfd 91, were compared against those of Acl(+), a low chlorophyll variant of the wild type. The chloroplast fragments of the mutant which have a high invariant fluorescence yield lacked photochemical activities associated with photosystem II (PSII) but retained normal photosystem I (PSI) activities. The mutant fragments also lacked the low temperature (-196 degrees C) light-induced absorbance changes due to the photoreduction of C-550 and the photooxidation of cytochrome (cyt) b-559 which are PSII-mediated reactions. A fourth-derivative analysis of the absolute spectra of the chloroplast fragments at different stages of reduction (obtained with ferricyanide, ascorbate, and dithionite) showed both the oxidized and reduced forms of C-550 and the reduced forms of cyt c-553, b-559, and b-564 in wild-type fragments. The mutant fragments lacked C-550 and an ascorbate-reducible cyt b-559 but contained cyt c-553, a dithionite-reducible cyt b-559, and cyt b-564.

Ascorbic Acid↗

Direct labelling of octreotide with 99mTc: effect of different concentration of reducing agents and amount of sodium pertechnetate on radiolabelling efficiency.

Octreotide, a synthetic analog of natural hormone somatostatin, was labelled with 99mTc. Labelling was accomplished by reduction of the cysteine bridge, which provided sulfhydryl groups for chelating with 99mTc. Sodium ascorbate and sodium dithionite in different concentrations were used as reducing agents. Different amounts of sodium pertechnetate were used for labelling of peptide. When the mass ratio of peptide and sodium ascorbate was 1:100 and the final concentration of dithionite in the labelling vial was 0.2-0.4 microg/microl with 0.18-1.48 GBq sodium pertechnetate more than 80% radiolabelling efficiency was confirmed by RP-HPLC, ITLC-SG and C18 Cartridge analysis. The stability of the 99mTc-peptide bond was evaluated by human serum challenge and that showed the stability was 90% after 4h.

Chromatography, High Pressure Liquid↗