Nitrogenase of Azotobacter chroococcum. Kinetics of the reduction of oxidized iron-protein by sodium dithionite.

The kinetics of the reduction of oxidized Fe-protein of nitrogenase from Azotobacter chroococcum by sodium dithionite were studied by stopped-flow and rapid-freezing e.p.r. (electron-paramagnetic-resonance) spectroscopy. The appearance of the gav. = 1.94 e.p.r. signal (0.24 electron integrated intensity/mol) was associated with a one-electron reduction by SO2--with k greater than 10(8)M-1-S-1 at 23 degrees C. A value of k = 1.75s-1 was obtained for the rate of dissociation of S2O42- into 2SO2-- at 23 degrees C. Further reductions by SO2-- occurred in three slower phases with rate constants in the range 10(4) -10(6)M-1-S-1. These latter phases have no corresponding e.p.r. signal changes and are probably associated with enzymically inactive protein. The high rate of reduction by SO2-- of the Fe-protein alone (k greater than 10(8)M-1-S-1) relative to the rate of oxidation of the Fe-protein in the catalytically active Fe:Mo-Fe protein complex (k = 2.2 X 1O(2)s-1) and the observation that in the steady state the Fe-protein is substantially oxidized means that at normal assay concentrations another reaction must limit the rate of reduction of Fe-protein during turnover.     

Molybdenum and vanadium nitrogenases of Azotobacter chroococcum. Low temperature favours N2 reduction by vanadium nitrogenase.

A comparison of the effect of temperature on the reduction of N2 by purified molybdenum nitrogenase and vanadium nitrogenase of Azotobacter chroococcum showed differences in behaviour. As the assay temperature was lowered from 30 degrees C to 5 degrees C N2 remained an effective substrate for V nitrogenase, but not Mo nitrogenase, since the specific activity for N2 reduction by Mo nitrogenase decreased 10-fold more than that of V nitrogenase. Activity cross-reactions between nitrogenase components showed the enhanced low-temperature activity to be associated with the Fe protein of V nitrogenase. The lower activity of homologous Mo nitrogenase components, although dependent on the ratio of MoFe protein to Fe protein, did not equal that of V nitrogenase even under conditions of high electron flux obtained at a 12-fold molar excess of Fe protein.     

Comparison of redox and EPR properties of the molybdenum iron proteins of Clostridium pasteurianum and Azotobacter vinelandii nitrogenases

Both heterologous crosses of the Clostridium pasteurianum and Azotobacter vinelandii nitrogenase components are completely inactive, although the reasons for this incompatibility are not known. We have compared a number of properties of the MoFe proteins from these organisms (Cp1 and Av1, respectively) in an attempt to find differences that could explain this lack of functional activity. Optical and CD spectroscopic titrations are similar for both Av1 and Cp1, but EPR titrations are significantly different, suggesting different chemical reactivity patterns and/or magnetic interaction behavior. Similarly, reduction measurements on the six-electron-oxidized state of Cp1 and Av1 at controlled potentials indicate a difference in both the relative reduction sequence of the redox centers and the numerical values for their measured midpoint potentials. EPR measurements as a function of temperature also demonstrate that the relaxation behavior of the S = 3/2 MoFe centers associated with the proteins differ markedly. The Cp1 EPR signal only begins to undergo broadening above 65 K, whereas the Av1 signal is severely broadened above 25 K. These variations in the EPR properties for the two proteins are not likely to be due to differences in the stoichiometry and/or geometry of the MoFe cluster units themselves since similar EPR studies of the isolated cofactors showed them to be essentially identical. Thus, the different EPR behavior of the two proteins seems to arise either from protein constraints imposed on identical cofactors, and/or from magnetic interactions due to neighboring metal clusters.     

The kinetics of the reduction of isocyanides, acetylenes and the cyanide ion by nitrogenase preparation from Azotobacter chroococcum and the effects of inhibitors

1. Nitrogen-fixing preparations from Azotobacter chroococcum reduced substrates with the following K(m) values: methyl isocyanide, 1.8x10(-4)m; ethyl isocyanide, 2.5x10(-2)m; cyanide ion, 1.4x10(-3)m; acetylene, 1.2x10(-4)m. 2. Nitrogen, carbon monoxide or hydrogen competitively inhibited isocyanide reduction with the following K(i) values: hydrogen, 1.3x10(-3)m; carbon monoxide, 6.8x10(-6)m; nitrogen, 4.3x10(-4)m. 3. Living nitrogen-fixing bacteria, and isolated clover nodules, formed methane from methyl isocyanide. 4. These results are discussed in relation to other work and possible mechanisms of nitrogen fixation.     

Azotobacter vinelandii nitrogenases containing altered MoFe proteins with substitutions in the FeMo-cofactor environment: effects on the catalyzed reduction of acetylene and ethylene

Altered MoFe proteins of Azotobacter vinelandii Mo-nitrogenase, with amino acid substitutions in the FeMo-cofactor environment, were used to probe interactions among C(2)H(2), C(2)H(4), CO, and H(2). The altered MoFe proteins used were the alpha-195(Asn) or alpha-195(Gln) MoFe proteins, which have either asparagine or glutamine substituting for alpha-histidine-195, and the alpha-191(Lys) MoFe protein, which has lysine substituting for alpha-glutamine-191. On the basis of K(m) determinations, C(2)H(2) was a particularly poor substrate for the nitrogenase containing the alpha-191(Lys) MoFe protein. Using C(2)D(2), a correlation was shown between the stereospecificity of proton addition to give the products, cis- and trans-C(2)D(2)H(2), and the propensity of nitrogenase to produce ethane. The most extensive loss of stereospecificity occurred with nitrogenases containing either the alpha-195(Asn) or the alpha-191(Lys) MoFe proteins, which also exhibited the highest rate of ethane production from C(2)H(2). These data are consistent with the presence of a common ethylenic intermediate on the enzyme, which is responsible for both ethane production and loss of proton-addition stereochemistry. C(2)H(4) was not a substrate of the nitrogenase with the alpha-191(Lys) MoFe protein and was a poor substrate of the nitrogenases incorporating either the wild-type or the alpha-195(Gln) MoFe protein, both of which had a low V(max) and high K(m) (120 kPa). Ethylene was a somewhat better substrate for the nitrogenase with the alpha-195(Asn) MoFe protein, which exhibited a K(m) of 48 kPa and a specific activity for C(2)H(6) formation from C(2)H(4) 10-fold higher than the others. Neither the wild-type nitrogenase nor the nitrogenase containing the alpha-195(Asn) MoFe protein produced cis-C(2)D(2)H(2) when turned over under trans-C(2)D(2)H(2). These results suggest that the C(2)H(4)-reduction site is affected by substitution at residue alpha-195, although whether the effect is related to the substrate-reduction site directly or is mediated through disturbance of the delivery of electrons/protons is unclear. Ethylene inhibited total electron flux, without uncoupling MgATP hydrolysis from electron transfer, to a similar extent for all four A. vinelandii nitrogenases. This observation indicates that this C(2)H(4) flux-inhibition site is remote from the C(2)H(4)-reduction site. Added CO eliminated C(2)H(4) reduction but did not fully relieve its electron-flux inhibition with all four A. vinelandii nitrogenases, supporting the suggestion that electron-flux inhibition by C(2)H(4) is not directly connected to C(2)H(4) reduction. Thus, C(2)H(4) has two binding sites, and the presence of CO affects only the site at which it binds as a substrate. When C(2)H(2) was added, it also eliminated C(2)H(6) production from C(2)H(4) and also did not relieve electron-flux inhibition fully. Thus, C(2)H(2) and C(2)H(4) are likely reduced at the same site on the MoFe protein. Two schemes are presented to integrate the results of the interactions of C(2)H(2) and C(2)H(4) with the MoFe proteins.     

The reactivity of alpha-hydroxyhaem and verdohaem bound to haem oxygenase-1 to dioxygen and sodium dithionite.

Recently we have shown that ferric alpha-hydroxyhaem bound to haem oxygenase-1 can be converted to ferrous verdohaem by approximately an equimolar amount of O2 in the absence of exogenous electrons [Sakamoto, H., Omata, Y., Palmer, G., and Noguchi, M. (1999) J. Biol. Chem.274, 18196-18200]. Contrary to those results, other studies have claimed that the conversion requires both O2 and an electron. More recently, Migita et al. have reported that the major reaction product of ferric alpha-hydroxyhaem with O2 is a ferric porphyrin cation radical that can be converted to ferrous alpha-hydroxyhaem with sodium dithionite [Migita, C. T., Fujii, H., Matera, K. M., Takahashi, S., Zhou, H., and Yoshida, T. (1999) Biochim. Biophys. Acta1432, 203-213]. To clarify the reason(s) for the discrepancy, we compared the reactions; i.e. alpha-hydroxyhaem to verdohaem and verdohaem to biliverdin, under various conditions as well as according to the procedures of Migita. We find that complex formation of alpha-hydroxyhaem with haem oxygenase may be small and a substantial amount of free alpha-hydroxyhaem may remain, depending on the reconstitution conditions; this could lead to a misinterpretation of the experimental results. We also find that ferrous verdohaem appears to be air-sensitive and is therefore easily converted to a further oxidized species with excess O2. Finally, we find that dithionite seems to be inappropriate for investigating the haem oxygenase reaction, because it reduces ferrous verdohaem to a further reduced species that has not been seen in the haem degradation system driven by NADPH-cytochrome P450 reductase.     

Electron transfer to nitrogenase. Characterization of flavodoxin from Azotobacter chroococcum and comparison of its redox potentials with those of flavodoxins from Azotobacter vinelandii and Klebsiella pneumoniae (nifF-gene product).

Flavodoxin in the hydroquinone state acts as an electron donor to nitrogenase in several nitrogen-fixing organisms. The mid-point potentials for the oxidized-semiquinone and semiquinone-hydroquinone couples of flavodoxins isolated from facultative anaerobe Klebsiella pneumoniae (nifF-gene product, KpFld) and the obligate aerobe Azotobacter chroococcum (AcFld) were determined as a function of pH. The mid-point potentials of the semiquinone-hydroquinone couples of KpFld and AcFld are essentially independent of pH over the range pH 7-9, being -422 mV and -522 mV (normal hydrogen electrode) at pH 7.5 respectively. The mid-point potentials of the quinone-semiquinone couples at pH 7.5 are -200 mV (KpFld) and -133 mV (AcFld) with delta Em/pH of -65 +/- 4 mV (KpFld) and -55 +/- 2 mV (AcFld) over the range pH 7.0-9.5. This indicates that reduction of the quinone is coupled to protonation to yield a neutral semiquinone. The significance of these values with respect to electron transport to nitrogenase is discussed. The amino acid compositions, the N- and C-terminal amino acid sequences and the u.v.-visible spectra of KpFld and AcFld were determined and are compared with published data for flavodoxins isolated from Azotobacter vinelandii.     

The kinetics of suppression of proliferation of oxidized murine lymphoid cells by sodium borohydride reduction.

Murine splenic lymphoid cells are stimulated to proliferate following mild oxidation with sodium periodate. To assess the class of cells responding, we used periodate treatment alone or in association with concanavalin A, a T-cell mitogen, or lipopolysaccharide (LPS), primarily a B-cell mitogen. Brief periodate treatment followed by culturing with concanavalin A gave no additive proliferative response to that seen using concanavalin A alone, while culturing periodate-treated cells with LPS gave approximately an additive response. Furthermore, periodate failed to stimulate spleen cells from neonatally thymectomized mice while LPS produced significant stimulation of proliferation, suggesting that periodate is stimulating a class of T lymphoid cells or a subpopulation of T cells. Studies were performed to determine an optimal concentration of borohydride which would suppress proliferation in lymphoid cells initially oxidized with periodate. It was observed that 2 mM borohydride would suppress proliferation of oxidized cells yet permit a normal response of these cells to another T-cell mitogen, concanavalin A. Higher concentrations of borohydride, from 3 to 5 mM, would also suppress proliferation of oxidized cells but would interfere with the ability of these cells to respond to concanavalin A, perhaps due to cell damage. Studies were performed to determine when it was possible to suppress periodate-induced mitogenesis by reducing with borohydride at various times after the initial oxidation. It was observed that 2 mM borohydride treatment could suppress stimulation through 8 hr after the original periodate oxidation and that from 12 hr through 20 hr after the initial periodate oxidation, borohydride was incapable of inhibiting proliferation. Additional studies demonstrate that optimal mitogenesis induced by periodate or concanavalin A is contingent upon a serum factor.     

Electron paramagnetic resonance studies on nitrogenase. III. Function of magnesium adenosine 5′-triphosphate and adenosine 5′-diphosphate in catalysis by nitrogenase

The electron paramagnetic resonance spectra of azoferredoxin and molybdoferredoxin, components of the nitrogenase of Clostridium pasteurianum, disappear when the proteins are oxidized by certain dyes. When molybdoferredoxin and azoferredoxin were mixed in a 1 to 2 molar ratio, the electron paramagnetic resonance spectrum of the mixture was the sum of the two spectra with the exception of a slight change in the azoferredoxin signal. Addition of magnesium ATP and dithionite to this reconstituted nitrogenase resulted in a rapid change in the spectrum of both nitrogenase components; the molybdoferredoxin spectrum at all g-values decreased with a half-life less than 70 ms to 40% of its original size whereas the azoferredoxin signal changed in shape and size with a half-life of less than 40 ms. If an ATP-generating system was added instead of MgATP so that no ADP accumulated, then the molybdoferredoxin signal almost completely disappeared and the azoferredoxin signal changed in shape and slightly in size. These changes occurred at molar ratios of molybdoferredoxin to azoferredoxin from 1:14 to 1:0.2. If the reaction was allowed to consume the reductant, then the molybdoferredoxin signal(s) was restored but the azoferredoxin signal disappeared. The signal of azoferredoxin was restored and the signal of molybdoferredoxin again disappeared on addition of more reductant. The data suggest that for nitrogenase to catalyze the reduction of substrates, the magnesium ATP-reduced azoferredoxin complex is formed first and this complex then reacts with molybdoferredoxin to allow electron flow. In addition the data suggests that the rate-limiting reaction is an ATP-mediated electron flow from azoferredoxin to molybdoferredoxin. Finally the results show that no flow of electrons from azoferredoxin or molybdoferredoxin occurs when a mixture of ADP and ATP in a molar ratio of 2:1 is added initially or is reached by conversion of ATP to ADP and inorganic phosphate during reduction of protons. A mechanism consistent with these findings is proposed.     

A colorimetric method for the estimation of serum glycated proteins based on differential reduction of free and bound glucose by sodium borohydride

A new colorimetric method based on the phenol-sulfuric acid reaction is described for the estimation of serum glycated proteins by the differential reduction of free glucose and hexose bound nonenzymatically with 2.0 and 20 mg of NaBH4 in 0.02 ml of serum, respectively, at room temperature for 15 min. The values (microgram hexose/mg protein) in control subjects (n = 60) and diabetics (n = 90) were estimated to be 5.60 +/- 0.85 and 10.8 +/- 1.6, respectively. The increase was highly significant (P less than 0.001) in diabetics. The serum glycated protein levels correlate well with fasting blood sugar values (r = 0.77, P less than 0.001, n = 25). There was also a highly significant correlation between glycated protein level and glycated albumin value in individual serum samples (r = 0.85, P less than 0.001, n = 25). Values of borohydride reducible glyco-groups bound to serum proteins also correlated well with serum glycated protein levels (r = 0.96, p less than 0.001, n = 20) determined by the thiobarbituric acid assay method. The method is found to be simple and rapid, with a coefficient of variations of +/- 3.8%.     

The 3'-amido and 5'-amido analogues of adenosine 3':5'-monophosphate; interaction with cAMP-specific proteins.

The sensitivity for recognition of adenosine 3:5'-monophosphate (cAMP) by its coordinate proteins towards chemical changes in the six-membered cyclic phosphate ring has been investigated. A comparison of the interaction parameters of the 3' and 5'-amido analogues (I, II) and of unsubstituted cAMP has been made using two different protein kinases and the phosphodiesterase from bovine heart. Binding affinity and the capacity of the amido analogues to stimulate the phosphotransferase activity of the kinases is greatly reeuced relative to cAMP, the 3'-position being more sensitive towards the modification than the 5'-position. The coordinate noncyclic derivatives, 3'-deoxy-3'-amino-5'-AMP (IV) and 5'-deoxy-5'-amino-3'-amp (iii), were also tested. Surprisingly activity towards protein kinases was found to be considerable for the 5'-deoxy-5'-amino-3'-AMP (III), while the 3'-deoxy-3'-amino-5'-AMP (IV) is practically inactive. A possible reason for this is that the noncylic 5'-analogue (III) may be able to assume a cyclic structure maintained by internal salt formation. The phosphodiesterase splits both cyclic amido analogues but with reduced rates compared to that of natural cAMP. Kinetic data obtained from different methods reveal a stronger affinity for the 5'-analogue (I) than the 3'-analogue (II) for the active site, although the reaction rate at saturated substrate concentration is significantly higher with II than with I. The properties of the amido and the noncyclic amino analogues are discussed with available data from chemotaxis of the cellular slime moulds. Furthermore data of the respective methylene cyclic derivatives are used for a more comprehensive comparison. The above is interpreted in terms of the electronic features of the substitutions and of the changes in bond distances or angles upon replacement of O by NH or CH2 in the cyclic phosphate ring (obtained from X-ray work).     

Inactivation kinetics of model and relevant blood-borne viruses by treatment with sodium hydroxide and heat.

To determine the efficacy of a clean-in-place system for the inactivation of viruses present in human plasma, the effect of 0.1 M sodium hydroxide at 60 degrees C on viral infectivity was investigated. Inactivation of the following model and relevant viruses were followed as a function of time: human hepatitis A virus (HAV), canine parvovirus (CPV; a model for human parvovirus B-19) pseudorabies virus (PRV, a model for hepatitis B virus), and bovine viral diarrhoea virus (BVDV, a model for hepatitis C virus and human immunodeficiency virus). Infectivity of CPV was determined by a novel in situ EIA method which will prove useful for studies to validate parvovirus inactivation or removal. Infectivity of BVDV, PRV and CPV were shown to be reproducibly inactivated below the limit of detection by 0.1 M NaOH at 60 degrees C within 30 s. HAV was inactivated to below the limit of detection within 2 min. Treatment with heat alone also resulted in some log reduction for all viruses tested except for CPV which remained unaffected after heating at 60 degrees C for 16 min. Treatment of HAV with hydroxide alone (up to 1.0 m) at 15 degrees C did not lead to rapid inactivation. Collectively, these data suggest that 0.1 M NaOH at 60 degrees C for two min should be sufficient to inactivate viruses present in process residues.     

Ferric iron reduction by bacteria associated with the roots of freshwater and marine macrophytes.

In vitro assays of washed, excised roots revealed maximum potential ferric iron reduction rates of >100 micromol g (dry weight)(-1) day(-1) for three freshwater macrophytes and rates between 15 and 83 micromol (dry weight)(-1) day(-1) for two marine species. The rates varied with root morphology but not consistently (fine root activity exceeded smooth root activity in some but not all cases). Sodium molybdate added at final concentrations of 0.2 to 20 mM did not inhibit iron reduction by roots of marine macrophytes (Spartina alterniflora and Zostera marina). Roots of a freshwater macrophyte, Sparganium eurycarpum, that were incubated with an analog of humic acid precursors, anthroquinone disulfate (AQDS), reduced freshly precipitated iron oxyhydroxide contained in dialysis bags that excluded solutes with molecular weights of >1,000; no reduction occurred in the absence of AQDS. Bacterial enrichment cultures and isolates from freshwater and marine roots used a variety of carbon and energy sources (e.g., acetate, ethanol, succinate, toluene, and yeast extract) and ferric oxyhydroxide, ferric citrate, uranate, and AQDS as terminal electron acceptors. The temperature optima for a freshwater isolate and a marine isolate were equivalent (approximately 32 degrees C). However, iron reduction by the freshwater isolate decreased with increasing salinity, while reduction by the marine isolate displayed a relatively broad optimum salinity between 20 and 35 ppt. Our results suggest that by participating in an active iron cycle and perhaps by reducing humic acids, iron reducers in the rhizoplane of aquatic macrophytes limit organic availability to other heterotrophs (including methanogens) in the rhizosphere and bulk sediments.