Sulfite reductase of Escherichia coli is a ferrisiderophore reductase

A soluble ferrisiderophore reductase activity of Escherichia coli was purified to homogeneity and identified as the sulfite reductase. The pure enzyme catalyzes the reduction of ferric citrate, ferriaerobactin, ferrioxamin, ferricrocin, ferrichrome and ferrifusarinin by NADPH. Free flavins, riboflavin, FMN, FAD were absolutely required, suggesting that this activity resides in the flavin reductase activity of sulfite reductase.

Proton NMR of Escherichia coli sulfite reductase: the unligated hemeprotein subunit

The isolated hemeprotein subunit of sulfite reductase (SiR-HP) from Escherichia coli consists of a high spin ferric isobacteriochlorin (siroheme) coupled to a diamagnetic [4Fe-4S]2+ cluster. When supplied with an artificial electron donor, such as methyl viologen cation radical, SiR-HP can catalyze the six electron reductions of sulfite to sulfide and nitrite to ammonia. Thus, the hemeprotein subunit appears to represent the minimal protein structure required for multielectron reductase activity. Proton magnetic resonance spectra are reported for the first time on unligated SiR-HP at 300 MHz in all three redox states. The NMR spectrum of high spin ferric siroheme at pH 6.0 was obtained for the purpose of comparing its spectrum with that of oxidized SiR-HP. On the basis of line widths, T1 measurements, and 1D NOE experiments, preliminary assignments have been made for the oxidized enzyme in solution. The pH profile of oxidized SiR-HP is unusual in that a single resonance shows a 9 ppm shift over a range of only 3 pH units with an apparent pK = 6.7 +/- 0.2. Resonances arising from the beta-CH2 protons of cluster cysteines have been assigned using deuterium substitution for all redox states. One beta-CH2 resonance has been tentatively assigned to the bridging cysteine on the basis of chemical shift, T1, line width, and the presence of NOEs to protons from the siroheme ring. The observed pattern of hyperfine shifts can be used as a probe to measure the degree of coupling between siroheme and cluster in solution. The cluster iron sites of the resting (oxidized) enzyme are found to possess both positive and negative spin density which is in good agreement with Mossbauer results on frozen enzyme. The NMR spectrum of the 1-electron reduced form of SiR-HP is consistent with an intermediate spin (S = 1) siroheme. Intermediate spin Fe(II) hemes have only been previously observed in 4-coordinate model compounds. However, the amount of electron density transferred to the cluster, as measured by the isotropic shift of beta-CH2 resonances, is comparable to that present in the fully oxidized enzyme despite diminution of the total amount of unpaired spin density available. Addition of a second electron to SiR-HP, besides generating a reduced S = 1/2 cluster with both upfield and downfield shifted cysteine resonances, converts siroheme to the high spin (S = 2) ferrous state.(ABSTRACT TRUNCATED AT 400 WORDS)

Physiological analysis of mutants of Saccharomyces cerevisiae impaired in sulphate assimilation

The assimilation of sulphate in Saccharomyces cerevisiae, comprising the reduction of sulphate to sulphide and the incorporation of the sulphur atom into a four-carbon chain, requires the integrity of 13 different genes. To date, the functions of nine of these genes are still not clearly established. A set of strains, each bearing a mutation in one MET gene, was studied. Phenotypic studies and enzyme determinations showed that the products of at least five genes are needed for the synthesis of an enzymically active sulphite reductase. These genes are MET1, MET5, MET8, MET10 and MET20. Wild-type strains of S. cerevisiae can use organic metabolites such as homocysteine, cysteine, methionine and S-adenosylmethionine as sulphur sources. They are also able to use inorganic sulphur sources such as sulphate, sulphite, sulphide or thiosulphate. Here we show that both of the two sulphur atoms of thiosulphate are used by S. cerevisiae. Thiosulphate is cleaved into sulphite and sulphide prior to utilization by the sulphate assimilation pathway, as the metabolism of one sulphur atom from thiosulphate requires the presence of an active sulphite reductase.

Calcium is required for the reduction of sulfite from hydrogen in a reconstituted electron transfer chain from the sulfate reducing bacterium, Desulfovibrio gigas

Calcium is found a strong stimulator of sulfite reduction from hydrogen. A coupling protein of molecular weight 65,000 can be isolated from Desulfovibrio gigas. It functions in a reconstituted electron transfer chain between hydrogenase and sulfite reductase. Its N-terminal sequence shows high homologies with calcium or magnesium binding sites from other calcium-binding proteins.

Enzymatic redox chemistry: a proposed reaction pathway for the six-electron reduction of SO3(2-) to S2- by the assimilatory-type sulfite reductase from Desulfovibrio vulgaris (Hildenborough)

A detailed reaction pathway for the six-electron reduction of SO3(2-) to S2- by the assimilatory-type sulfite reductase (SiR) from Desulfovibrio vulgaris (Hildenborough) has been deduced from experiments with 35S-labeled enzyme and the relative reaction rates of nitrogenous substrates. The ligand bridging the prosthetic [Fe4S4]-siroheme center is apparently exchanged by 35S2- in both oxidized and reduced enzyme. This 35S2- label was retained in the course of SO3(2-) reduction, implicating substrate binding to the nonbridging axial site of the siroheme. A reaction mechanism is proposed in which SO3(2-) binds to Fe2+ through the sulfur atom, followed by a series of two-electron reductive cleavages of S-O bonds. Protonation of oxygen facilitates bond cleavage, giving hydroxide as leaving group. The bridge remains intact throughout the course of the reaction, providing an efficient coupling pathway for electron transfer between the cluster and siroheme.

S = 9/2 EPR signals are evidence against coupling between the siroheme and the Fe/S cluster prosthetic groups in Desulfovibrio vulgaris (Hildenborough) dissimilatory sulfite reductase

Sulfite reductases contain siroheme and iron-sulfur cluster prosthetic groups. The two groups are believed to be structurally linked via a single, common ligand. This chemical model is based on a magnetic model for the oxidized enzyme in which all participating iron ions are exchange coupled. This description leads to two serious discrepancies. Although the iron-sulfur cluster is assumed to be a diamagnetic cubane, [4Fe-4S]2+, all iron appears to be paramagnetic in Mössbauer spectroscopy. On the other hand, EPR spectroscopy has failed to detect anything but a single high-spin heme. We have re-addressed this problem by searching for new EPR spectroscopic clues in concentrated samples of dissimilatory sulfite reductase from Desulfovibrio vulgaris (Hildenborough). We have found several novel signals with effective g values of 17, 15.1, 11.7, 9.4, 9.0, 4. The signals are interpreted in terms of an S = 9/2 system with spin-Hamiltonian parameters g = 2.00, D = -0.56 cm-1, magnitude of E/D = 0.13 for the major component. In a reductive titration with sodium borohydride the spectrum disappears with Em = -205 mV at pH 7.5. Contrarily, the major high-spin siroheme component has S = 5/2, g = 1.99, D = +9 cm-1, magnitude of E/D = 0.042, and Em = -295 mV. The sum of all siroheme signals integrates to 0.2 spin/half molecule, indicating considerable demetallation of this prosthetic group. Rigorous quantification procedures for S = 9/2 are not available, however, estimation by an approximate method indicates 0.6 S = 9/2 spin/half molecule. The S = 9/2 system is ascribed to an iron-sulfur cluster. It follows that this cluster is probably not a cubane, is not necessarily exchange-coupled to the siroheme, and, therefore, is not necessarily structurally close to the siroheme. It is suggested that this iron-sulfur prosthetic group has a novel structure suitable for functioning in multiple electron transfer.

Spectroscopic studies of the molybdenum-containing dimethyl sulfoxide reductase from Rhodobacter sphaeroides f. sp. denitrificans

Absorption and EPR spectroscopic properties of purified dimethyl sulfoxide (Me2SO) reductase from Rhodobacter sphaeroides f. sp. denitrificans have been examined. The absence of prosthetic groups other than the molybdenum center in the enzyme has made it possible to study its absorption properties. The enzyme displays multiple absorbance peaks in both the oxidized and the dithionite-reduced forms. The oxidized enzyme has absorbance peaks at 280, 350, 470, 550, and 720 nm while the dithionite-reduced enzyme has peaks at 280, 374, and 645 nm with a shoulder at 430 nm. A comparison of the absorbance spectrum of oxidized Me2SO reductase with that of the molybdenum fragment of rat liver sulfite oxidase shows that the 350 and 470 peaks are common to both proteins. EPR studies of the Mo(V) form of Me2SO reductase show a rhombic signal with g1 = 1.988, g2 = 1.977, g3 = 1.961, and g(ave) = 1.975. The signal shows evidence of coupling to an exchangeable proton with A1 = 1.05, A2 = 1.13, A3 = 0.98, and Aave = 1.05 millitesla. These parameters are similar to those of other Mo enzymes, however, the epr signal of this enzyme differs from those of other Mo hydroxylases in showing only a slight sensitivity to pH and no detectable anion effect. EPR potentiometric titrations of Me2SO reductase gave midpoint potentials of +144 mV for the Mo(VI)/Mo(V) couple and +160 mV for the Mo(V)/Mo(IV) couple at room temperature and +141 mV for the Mo(VI)/Mo(V) couple and +200 mV for the Mo(V)/Mo(IV) couple at 173 K.

The state of reduction of molybdopterin in xanthine oxidase and sulfite oxidase

Methods have been devised to examine the spectral properties and state of reduction of the pterin ring of molybdopterin (MPT) in milk xanthine oxidase and the Mo-containing domain of rat liver sulfite oxidase. The absorption spectrum of the native pterin was visualized by difference spectroscopy of each protein, denatured anaerobically in 6 M guanidine hydrochloride (GdnHCl), versus a sample containing the respective apoprotein and other necessary components. The state of reduction of MPT was also probed using 2,6-dichlorobenzenoneindophenol (DCIP) to measure reducing equivalents/MPT, after anaerobic denaturation of the protein in GdnHCl in the presence or absence of Hg2+. In the case of xanthine oxidase the data indicate that the terminal sulfide ligand of Mo causes the reduction of a native dihydro form of MPT to the tetrahydro level. This reduction does not occur if Hg2+ is added prior to denaturation of the protein. Based on its observed behavior, the native MPT in the Mo cofactor of xanthine oxidase is postulated to exist as a quinonoid dihydropterin. Quantitation of DCIP reduction by MPT of Mo fragment of sulfite oxidase showed a two-electron oxidation of MPT, even when the Mo fragment was denatured in the presence of Hg2+ to prevent internal reduction reactions due to sulfhydryls or sulfide. Difference spectra of DCIP-treated versus untreated Mo fragment showed that MPT had been fully oxidized. These data indicate that the native MPT in sulfite oxidase must be a dihydro isomer different from that in xanthine oxidase.

Purification and identification of a cytosolic factor required for import of precursors of mitochondrial proteins into mitochondria

A cytosolic factor required for import of the precursor of mitochondrial protein into mitochondria was purified to homogeneity from a rabbit reticulocyte lysate by affinity column chromatography using a synthetic peptide containing the presequence of ornithine amino-transferase as a ligand. The molecular mass of the purified protein was estimated as 28 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The import of precursors of ornithine aminotransferase and sulfite oxidase into mitochondria was inhibited by anti-28-kDa protein IgG raised in guinea pigs. This antibody also blocked the binding of these precursors to mitochondria. These results suggest that the 28-kDa protein is an essential component of the import machinery in the cytosol and that anti-28-kDa protein IgG blocked the binding of the precursor of ornithine aminotransferase to mitochondria, but not the penetration step. Therefore, the 28-kDa protein may be a factor that should be named the "targeting factor" for import of mitochondrial protein.

Purification and characterization of bisulfite reductase (desulfofuscidin) from Desulfovibrio thermophilus and its complexes with exogenous ligands

A dissimilatory bisulfite reductase has been purified from a thermophilic sulfate-reducing bacterium Desulfovibrio thermophilus (DSM 1276) and studied by EPR and optical spectroscopic techniques. The visible spectrum of the purified bisulfite reductase exhibits absorption maxima at 578.5, 392.5 and 281 nm with a weak band around 700 nm. Photoreduction of the native enzyme causes a decrease in absorption at 578.5 nm and a concomitant increase in absorption at 607 nm. When reduced, the enzyme reacts with cyanide, sulfite, sulfide and carbon monoxide to give stable complexes. The EPR spectrum of the native D. thermophilus bisulfite reductase shows the presence of a high-spin ferric signal with g values at 7.26, 4.78 and 1.92. Upon photoreduction the high-spin ferric heme signal disappeared and a typical 'g = 1.94' signal of [4Fe-4S] type cluster appeared. Chemical analyses show that the enzyme contains four sirohemes and eight [4Fe-4S] centers per mol of protein. The molecular mass determined by gel filtration was found to be 175 kDa. On SDS-gel electrophoresis the enzyme presents a main band of 44 to 48 kDa. These results suggest that the bisulfite reductase contains probably one siroheme and two [4Fe-4S] centers per monomer. The dissimilatory bisulfite reductase from D. thermophilus presents some homologous properties with desulfofuscidin, the bisulfite reductase isolated from Thermodesulfobacterium commune (Hatchikian, E.C. and Zeikus, J.G. (1983) J. Bacteriol. 153, 1211-1220).

Occurrence and comparison of sulfite oxidase activity in mammalian tissues

Tissue extracts from six mammalian species have been assayed for sulfite oxidase (sulfite: ferricytochrome c oxidoreductase, EC 1.8.3.1) activity with cytochrome c as electron acceptor. Our results show a large distribution of sulfite oxidase activity in mammalian tissues. Liver, kidney, and heart tissues exhibit high activities whereas brain, spleen, and testis show very low activities. No significant species dependence was observed for the activity of this enzyme.

O2-dependent methionine auxotrophy in Cu,Zn superoxide dismutase-deficient mutants of Saccharomyces cerevisiae

Mutant strains of the yeast Saccharomyces cerevisiae which lack functional Cu,Zn superoxide dismutase (SOD-1) do not grow aerobically unless supplemented with methionine. The molecular basis of this O2-dependent auxotrophy in one of the mutants, Dscd1-1C, has been investigated. Sulfate supported anaerobic but not aerobic mutant growth. On the other hand, cysteine and homocysteine supported aerobic growth while serine, O-acetylserine, and homoserine did not, indicating that the interconversion of cysteine and methionine (and homocysteine) was not impaired. Thiosulfate (S2O3(2-] and sulfide (S2-) also supported aerobic growth; the activities of thiosulfate reductase and sulfhydrylase in the aerobic mutant strain were at wild-type levels. Although the levels of SO4(2-) and adenosine-5'-sulfate (the first intermediate in the SO4(2-) assimilation pathway) were elevated in the aerobically incubated mutant strain, this condition could be attributed to a decrease in protein synthesis caused by the de facto sulfur starvation and not to a block in the pathway. Therefore, the activation of SO4(2-) (to form 3'-phosphoadenosine-5'-phosphosulfate) appeared to be O2 tolerant. Sulfite reductase activity and substrate concentrations [( NADPH] and [SO3(2-)]) were not significantly different in aerobically grown mutant cultures and anaerobic cultures, indicating that SOD-1- mutant strains could reductively assimilate sulfur oxides. However, the mutant strain exhibited an O2-dependent sensitivity to SO3(2-) concentrations of less than 50 microM not exhibited by any SOD-1+ strain or by SOD-1- strains supplemented with a cytosolic O2(-)-scavenging activity. This result suggests that the aerobic reductive assimilation of SO4(2-) at the level of SO3(2-) may generate a cytotoxic compound(s) which persists in SOD-(1-) yeast strains.

Characterization of the flavoprotein moieties of NADPH-sulfite reductase from Salmonella typhimurium and Escherichia coli. Physicochemical and catalytic properties, amino acid sequence deduced from DNA sequence of cysJ, and comparison with NADPH-cytochrome P-450 reductase

NADPH-sulfite reductase flavoprotein (SiR-FP) was purified from a Salmonella typhimurium cysG strain that does not synthesize the hemoprotein component of the sulfite reductase holoenzyme. cysJ, which codes for SiR-FP, was cloned from S. typhimurium LT7 and Escherichia coli B, and both genes were sequenced. Physicochemical analyses and deduced amino acid sequences indicate that SiR-FP is an octamer of identical 66-kDa peptides and contains 4 FAD and 4 FMN per octamer. Potentiometric titrations of SiR holoenzyme, SiR-FP, and FMN-depleted SiR-FP yielded the following redox potentials for the prosthetic groups at pH 7.7: E'1 (FMNH./FMN) = -152 mV; E'2 (FMNH2/FMNH.) = -327 mV; E'3 (FADH./FAD) = -382 mV; E'4 (FADH2/FADH.) = -322 mV. Microcoulometric titration of SiR-FP at 25 degrees C yielded data which were in full agreement with these potentials. Spectroscopic and catalytic studies of native SiR-FP and of SiR-FP depleted of FMN support the following electron flow sequence: NADPH----FAD----FMN. FMN can then contribute electrons to the hemoprotein component of sulfite reductase, as well as to cytochrome c and various diaphorase acceptors. The FMN is postulated to cycle between the FMNH2 and FMNH. oxidation states during catalysis; in this sense SiR-FP shares a catalytic mechanism with NADPH-cytochrome P-450 oxidoreductase. SiR-FP domains involved in binding FMN, FAD, and NADPH are proposed from amino acid sequence homologies with Desulfovibrio vulgaris flavodoxin (Dubourdieu, M., and Fox, J.L. (1977) J. Biol. Chem. 252, 1453-1463) and spinach ferredoxin-NADP+ oxidoreductase (Karplus, P.A., Walsh, K.A., and Herriott, J. R. (1984) Biochemistry 23, 6576-6583). Comparison of the deduced amino acid sequences of SiR-FP and NADPH-cytochrome P-450 oxidoreductase (Porter, T. D., and Kasper, C.B. (1985) Proc. Natl. Acad. Sci. U. S.A. 82, 973-977) also showed identities that suggest these two proteins are descended from a common precursor, which contained binding regions for both FMN and FAD.

Resonance Raman studies of Escherichia coli sulfite reductase hemoprotein. 1. Siroheme vibrational modes

Resonance Raman (RR) spectra are reported for the hemoprotein subunit (SiR-HP) of Escherichia coli NADPH-sulfite reductase (EC 1.8.1.2) in various ligation and redox states. Comparison of the RR spectra of extracted siroheme and the mu-oxo FeIII dimer of octaethylisobacteriochlorin with those of mu-oxo FeIII octaethylchlorin dimer and mu-oxo FeIII octaethylporphyrin dimer demonstrates that many siroheme bands can be correlated with established porphyrin skeletal modes. Depolarization measurements are a powerful tool in this correlation, since the 45 degrees rotation of the C2 symmetry axis of the isobacteriochlorin ring relative to the chlorin system results in reversal of the polarization properties (polarized vs anomalously polarized) of bands correlating with B1g and B2g modes of porphyrin. Various SiR-HP adducts (CO, NO, CN-, SO3(2-] show upshifted high-frequency bands, characteristic of the low-spin state and consistent with the expected core size sensitivity of the skeletal modes. Fully reduced unliganded SiR-HP (both siroheme and Fe4S4 cluster reduced) in liquid solution displays RR features comparable to those of high-spin ferrous porphyrins; on freezing, the RR spectrum changes, reflecting an apparent mixture of siroheme spin states. At intermediate reduction levels in solution a RR species is observed whose high-frequency bands are upshifted relative to oxidized and fully reduced SiR-HP. This spectrum, thought to arise from the "one-electron" state of SiR-HP (siroheme reduced, cluster oxidized), may be due to S = 1 FeII siroheme.

Resonance Raman studies of Escherichia coli sulfite reductase hemoprotein. 3. Bound ligand vibrational modes

The vibrations of the bound diatomic heme ligands CO, CN-, and NO are investigated by resonance Raman spectroscopy in various redox states of Escherichia coli sulfite reductase hemoprotein, and assignments are generated by use of isotopically labeled ligands. For the fully reduced CO complex (ferrous siroheme, reduced Fe4S4 cluster) at room temperature, nu CO is observed at 1904 cm-1, shifting to 1920 cm-1 upon oxidation of the cluster. The corresponding delta FeCO modes are identified at 574 and 566 cm-1, respectively, by virtue of the zigzag pattern of their isotopic shifts. In frozen solution, two species are observed for the cluster-oxidized state, with nu CO at 1910 and 1936 cm-1 and nu FeC at 532 and 504 cm-1, respectively; nu FeC for the fully reduced species is identified at 526 cm-1 in the frozen state. For the ferrous siroheme-NO complex (cluster oxidized), nu NO is identified at 1555 cm-1 in frozen solution and a low-frequency mode is identified at 558 cm-1; this stretching mode is significantly lower than that observed in Mb-NO. For the ferric siroheme cyanide complexes evidence of two ligand-bonding forms is observed, with modes at 451/390 and 451/352 cm-1; they are distinguished by a reversal of the isotopic shift patterns of the upper and lower modes and could arise from a linear and a bent Fe-C unit, respectively. For the ferrous siroheme cyanide complex isotope-sensitive modes observed at 495 and 452 cm-1 are assigned to the FeCN- bending and FeC stretching vibrations, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Resonance Raman studies of Escherichia coli sulfite reductase hemoprotein. 2. Fe4S4 cluster vibrational modes

Resonance Raman (RR) spectra from the hemoprotein subunit of Escherichia coli sulfite reductase (SiR-HP) are examined in the low-frequency (200-500 cm-1) region where Fe-S stretching modes are expected. In spectra obtained with excitation in the siroheme Soret or Q bands, this region is dominated by siroheme modes. Modes assignable to the Fe4S4 cluster are selectively enhanced, however, with excitation at 488.0 or 457.9 nm. The assignments are confirmed by observation of the expected frequency shifts in SiR-HP extracted from E. coli grown on 34S-labeled sulfate. The mode frequencies and isotopic shifts resemble those seen in RR spectra of other Fe4S4 proteins and analogues, but the breathing mode of the cluster at 342 cm-1 is higher than that observed in the other species. Spectra of various ligand complexes of SiR-HP reveal only slight sensitivity of the cluster terminal ligand modes to the presence of exogenous heme ligands, at variance with a model of ligand binding in a bridged mode between heme and cluster. Close examination of RR spectra obtained with siroheme Soret-band excitation reveals additional 34S-sensitive features at 352 and 393 cm-1. These may be attributed to a bridging thiolate ligand.

Molybdenum cofactor biosynthesis in humans. Identification of two complementation groups of cofactor-deficient patients and preliminary characterization of a diffusible molybdopterin precursor

Molybdenum cofactor deficiency is a devastating disease with affected patients displaying the symptoms of a combined deficiency of sulfite oxidase and xanthine dehydrogenase. Because of the extreme lability of the isolated, functional molybdenum cofactor, direct cofactor replacement therapy is not feasible, and a search for stable biosynthetic intermediates was undertaken. From studies of cocultured fibroblasts from affected individuals, two complementation groups were identified. Coculture of group A and group B cells, without heterokaryon formation, led to the appearance of active sulfite oxidase. Use of conditioned media indicated that a relatively stable, diffusible precursor produced by group B cells could be used to repair sulfite oxidase in group A recipient cells. Although the extremely low levels of precursor produced by group B cells preclude its direct characterization, studies with a heterologous, in vitro reconstitution system suggest that the precursor that accumulates in group B cells is the same as a molybdopterin precursor identified in the Neurospora crassa molybdopterin mutant nit-1, and that a converting enzyme is present in group A cells which catalyzes an activation reaction analogous to that of a converting enzyme identified in the Escherichia coli molybdopterin mutant ChlA1.

Effect of molybdenum supplementation on hepatic trace elements and enzymes of female rats

The effects of molybdenum (Mo) supplementation in the drinking water at the levels of 0.1, 0.5, 1.0, 2.0, 5.0 and 10.0 mg/l on the hepatic trace element concentrations and enzyme activities of female Sprague-Dawley rats were studied. The mean hepatic Mo concentration increased significantly in the rats supplemented with 0.1 mg Mo/l as compared to the nonsupplemented rats, but a further significant increase did not occur until the supplementation level reached 5-10 mg Mo/l drinking water. Hepatic copper concentration of the group given 0.1 mg Mo/l and hepatic iron content of the groups given 0.1 or 0.5 mg Mo/l were significantly higher than those of the other groups. The hepatic xanthine dehydrogenase/oxidase activity was not significantly affected by Mo supplementation. The hepatic sulfite oxidase (SOX) activity of the group given 0.1 mg Mo/l was significantly higher than that of the nonsupplemented group. The SOX activities of all the other supplemented groups were at a significantly different level intermediate between the first two. The hepatic superoxide dismutase (SOD) activity was significantly higher in the group given 0.1 mg/l than in the other groups. These results indicated that molybdenum enzymes and SOD might not be participants in previously reported anticarcinogenic activity of Mo, as supplementation at the level of 0.1 mg/l had been observed to be inefficacious in inhibiting N-nitrosomethylurea-induced mammary tumor incidence.

The aldox-2 locus of Drosophila melanogaster also affects sulfite oxidase and molybdenum metabolism

Mutation at the aldox-2 locus in Drosophila melanogaster affects the specific activities of four molybdoenzymes differentially during development. Sulfite oxidase activity is normal during late larval and pupal stages but is reduced during early adult stages in aldox-2 organisms. There was complete concordance among the effects of aldox-2 on sulfite oxidase, aldehyde oxidase, xanthine dehydrogenase, and pyridoxal oxidase, when 38 stocks were analyzed which were derived from single recombination events between c and px, markers which flank aldox-2. Several different biochemical analyses indicate that the active molybdoenzymes present in the aldox-2 strain are normal with respect to size, shape, pH-activity profile, Km, and molecular weight. Significant differences were found between the aldox-2 strain and the OR control strain in their responses to dietary Na2MoO4 and Na2WO4. The mutant strain is much more resistant to the effects of dietary Na2WO4 and much more responsive to the administration of Na2MoO4 than the OR control strain when these effects are quantitated by measurements of molybdoenzyme specific activities. This evidence suggests that the aldox-2+ gene product has a molybdenum binding site which can also bind tungsten and that this site is altered in the mutant strain. The hypothesis presented explains the observed effects of the aldox-2 mutation and relates them to the other mutations reported in this gene-enzyme system.

Electrostatic interactions of 4-carboxy-2,6-dinitrophenyllysine-modified cytochromes c with physiological and non-physiological redox partners

An analysis of the effect of electrostatic properties of 4-carboxy-2,6-dinitrophenyllysine (CDNP-lysine) cytochromes c on their reactions with strongly and weakly binding redox partners is given. For strongly binding systems (cytochrome-c oxidase, cytochrome-c reductase, sulphite oxidase and yeast cytochrome-c peroxidase) the magnitude of the dipole moments of the CDNP cytochromes c determines their relative reactivities. For weakly binding redox agents, such as hexacyanoferrate(III), cobalt(III)tris(1,10-phenanthroline), azurin and plastocyanin, the electrostatic potential at the haem edge accounts for the greater part of the relative activities. Relative rate data were obtained from the literature. It is concluded that the dipole moment of native cytochromes c may account for an approx. 50-fold increase in the efficiency of its physiological activity towards membrane-bound enzymes. A correction on a formula to describe the contribution of a molecular dipole moment to the ionic strength dependence of a bimolecular rate constant (Koppenol, W. H. (1980) Biophys. J. 29, 493-508) leads to an equation nearly identical to that obtained by Van Leeuwen et al. (Van Leeuwen, J.W., Mofers, F.J.M. and Verrman, E.C.I. (1981) Biochim. Biophys. Acta 635, 434-439).

The nature of the phosphate complex of sulphite oxidase from electron-paramagnetic-resonance studies

The phosphate complex of sulphite oxidase in the Mo(V) oxidation state was investigated by e.p.r. spectroscopy. Third-derivative spectra reveal a wealth of structural detail previously unobserved in this spectrum. Most notable is the presence of hyperfine coupling from two inequivalent I = 1/2 nuclei, which we tentatively attribute to two 31P nuclei. Unresolved hyperfine interactions from at least one exchangeable 1H nucleus are also present.

Superoxidized states of Escherichia coli sulfite reductase heme protein subunit

The oxidized forms of resting and sulfite-complexed Escherichia coli sulfite reductase heme protein subunit react with near-stoichiometric amounts of porphyrexide to produce what is best characterized as a ferrisiroheme pi cation radical. Addition of either sodium ascorbate or NADPH completely regenerates the parent form. Implications of these findings with respect to mechanisms of metal-radical J coupling and catalysis are discussed.

Effect of dietary protein and methionine on sulfite oxidase activity in rats

Sulfite oxidase catalyzes the oxidation of sulfite to sulfate. To investigate whether or not sulfite oxidase activity (EC 1.8.3.1) is regulated by the amount of sulfur from dietary protein or excess methionine, we fed rats diets containing 5, 10, 20 and 50% casein with or without excess methionine and measured sulfite oxidase activity in liver and intestinal mucosa. Hepatic sulfite oxidase activity was significantly lower in rats fed 5 or 10% casein diets and significantly higher in rats fed 50% casein than in rats fed the control diet containing 20% casein, but activity did not change in response to the addition of methionine at any level of protein. Sulfite oxidase activity in the intestinal mucosa was only 5% of that seen in liver and did not change in response to dietary protein or methionine. Activity did not change in rats fed low iron diets (5 mg Fe/kg diet) at any level of protein tested or in response to glycine. These results show that sulfite oxidase activity can adapt to different levels of dietary protein but is unaffected by the level of methionine, total amino nitrogen or iron in the diet.

The inorganic biochemistry of molybdoenzymes

Molybednum-containing enzymes (Coughlan, 1980; Spiro, 1985) occupy a significant place in the development of the field now termed inorganic biochemistry. The importance of the metal as a biological trace element depends on its involvement in the known, and perhaps other as yet unknown, molybdoenzymes. That it plays a role in biological nitrogen fixation, the process whereby the enzyme nitrogenase in the root nodules of plants converts atmospheric nitrogen into ammonia, was recognized in the 1930s. The metal is also a constituent of a variety of other enzymes, having first been found in a mammalian enzyme, xanthine oxidase, in the 1950s.

Activated conformers of Escherichia coli sulfite reductase heme protein subunit

The heme protein subunit of Escherichia coli sulfite reductase shows enhanced reactivity with its substrate and a number of other ligands after a cycle of reduction and reoxidation at alkaline pH. At pH 9.5 this variant of the enzyme possesses at least four EPR-detectable, chloride-sensitive high-spin conformers, in contrast to the single chloride-insensitive species observed in the oxidized, resting enzyme at pH 7.7. Quantitative reversal of the spectral and ligand-binding properties of the "activated" enzyme to those of the resting enzyme is observed on reacidification to pH 7.7. At intermediate pH values, there occurs an acid-catalyzed relaxation of the activated enzyme to the resting form. This reaction is distinct from the one responsible for the accelerated ligand binding and production of multiple EPR conformers, which appears to be regulated by a process with a pK of 8.5.

Trimethylamine oxidation in liver tissue is not catalyzed by a molybdenum cofactor-dependent enzyme

The oxidation of trimethylamine to trimethylamine N-oxide in animals is catalyzed by an enzyme which has not yet been fully characterized. The discovery that a bacterial enzyme catalyzing the reverse reaction, the reduction of trimethylamine N-oxide to trimethylamine, utilizes the molybdenum cofactor to carry out this function raised the possibility that trimethylamine oxidation may also be dependent on this cofactor. It was found, however, that liver tissue from tungsten-treated rats contained normal levels of trimethylamine oxidase. In addition, analysis of a urine sample from a patient with trimethylamine oxidase deficiency revealed the presence of normal levels of urothione, the degradation product of the molybdenum cofactor. These results suggest that trimethylamine oxidase is not a molybdoenzyme and that oxidation of trimethylamine proceeds by a mechanism which differs considerably from a simple reversal of trimethylamine N-oxide reduction.

Chicken liver sulfite oxidase. Kinetics of reduction by laser-photoreduced flavins and intramolecular electron transfer

Laser flash photolysis was used to study the reaction of photoproduced 5-deazariboflavin (dRFH.), lumiflavin (LFH.), and riboflavin (RFH.) semiquinone radicals with the redox centers of purified chicken liver sulfite oxidase. Kinetic studies of the native enzyme with dRFH. yielded a second-order rate constant of 4.0 X 10(8) M-1 s-1 for direct reduction of the heme and a first-order rate constant of 310 s-1 for intramolecular electron transfer from the Mo center to the heme. The reaction with LFH. gave a second-order rate constant of 2.9 X 10(7) M-1 s-1 for heme reduction. Reoxidation of the reduced heme due to intramolecular electron transfer to the Mo center gave a first-order rate constant of 155 s-1. The direction of intramolecular electron transfer using dRFH. and LFH. was independent of the buffer used for the experiment. The different first-order rate constants observed for intramolecular electron transfer using dRFH. and LFH. are proposed to result from chemical differences at the Mo site. Flash photolysis studies with cyanide-inactivated sulfite oxidase using dRFH. and LFH. resulted in second-order reduction of the heme center with rate constants identical with those obtained with the native enzyme, whereas the first-order intramolecular electron-transfer processes seen with the native enzyme were absent. The isolated heme peptide of sulfite oxidase gave only second-order kinetics upon laser photolysis and confirmed that the first-order processes observed with the native enzyme involve the Mo site.(ABSTRACT TRUNCATED AT 250 WORDS)

Magnetization of the sulfite and nitrite complexes of oxidized sulfite and nitrite reductases: EPR silent spin S = 1/2 states

The saturation magnetizations of the sulfite complex of oxidized sulfite reductase and the nitrite complex of oxidized nitrite reductase have been measured to determine their spin state. Each shows the saturation magnetization signal of a spin S = 1/2 state with sigma g2 = 16, which is typical of low-spin ferrihemes. However, the EPR spectra of these complexes lack the expected signal intensity of a spin S = 1/2 state. Indeed, one of these complexes is EPR silent. The reasons for this unexpectedly low EPR signal intensity are considered.

The structure of the molybdenum cofactor. Characterization of di-(carboxamidomethyl)molybdopterin from sulfite oxidase and xanthine oxidase

A di-(carboxamidomethyl) derivative of molybdopterin, the organic component of the molybdenum cofactor, has been prepared under conditions favoring retention of all of the structural features of the molecule. The specific radioactivity of [1-14C]iodoacetamide incorporated relative to the amount of phosphate indicated two alkylation sites per pterin. Energy-dispersive x-ray analysis of the derivative showed the presence of 2 sulfurs in the derivative. An exact mass corresponding to the molecular formula C14H18N7O5S2 was obtained for the MH+ ion of the alkylated, dephosphorylated compound by fast atom bombardment mass spectroscopy. 1H NMR spectra of the phosphorylated and dephosphorylated forms of alkylated molybdopterin, in conjunction with the other data, have provided strong corroboration of the validity of the proposed structure of molybdopterin (Johnson, J. L., and Rajagopalan, K. V. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 6856-6860) as a 6-alkylpterin with a 4-carbon side chain containing an enedithiol on C-1' and C-2', a secondary alcohol on C-3', and a phosphorylated primary alcohol on C-4'. As isolated, the di-(carboxamido-methyl)molybdopterin was found to be a 5,6,7,8-tetrahydropterin.

Spectroscopic properties of siroheme extracted from sulfite reductases

Siroheme has been extracted from sulfite reductases and its properties in aqueous solution have been investigated by optical absorption, electron paramagnetic resonance (EPR), and magnetic circular dichroism (MDC) spectroscopy. The absorption spectrum of siroheme exhibits a marked pH dependence, and two pK values, 4.2 and 9.0, were determined by pH titration in the range 2-12. The first pK (4.2) is thought to correspond to the ionization of the carboxylic acid side-chains on the tetrapyrrole rings, and the second pK (9.0) is attributed to displacement of the axial ligand chloride by hydroxide. The binding of the strong field ligands, CO, NO, and cyanide, were investigated by UV-visible absorption and, in the case of the cyanide complex, by low-temperature EPR and MCD spectroscopies. CO and NO were able to reduce and bind to siroheme without additional reducing agent. The EPR spectrum of the isolated siroheme (chloride-ferrisiroheme) exhibits an axial signal with g perpendicular = 6.0 and g parallel = 2.0, typical of high-spin ferric hemes (S = 5/2), whereas the cyanide-complexed siroheme exhibits an approximately axial signal with g perpendicular = 2.38 and g parallel = 1.76 that is indicative of a low-spin ferric heme (S = 1/2). The low-temperature MCD spectra and magnetization data for the as-isolated and cyanide-complexed ferrisiroheme are entirely consistent with the interpretation of the EPR spectra. The results for ferrosiroheme indicate that the siroheme remains high spin (S = 2) and low spin (S = 0) on reduction of the as-isolated and cyanide-complexed siroheme, respectively. The isolated siroheme expressed sulfite reductase activity but the assessable catalytic cycle was much less than that of the native enzyme, showing the importance of the protein environment.

Distribution, metabolism and toxicity of inhaled sulfur dioxide and endogenously generated sulfite in the respiratory tract of normal and sulfite oxidase-deficient rats

We report on the distribution, metabolism, and toxicity of sulfite in the respiratory tract and other tissues of rats exposed to endogenously generated sulfite or to inhaled sulfur dioxide (SO2). Graded sulfite oxidase deficiency was induced in several groups of rats by manipulating their tungsten to molybdenum intake ratio. Endogenously generated sulfite and S-sulfonate compounds (a class of sulfite metabolite) accumulated in the respiratory tract tissues and in the plasma of these rats in inverse proportion to hepatic sulfite oxidase activity. In contrast to this systemic mode of exposure, sulfite exposure of normal, sulfite oxidase-competent rats via inhaled SO2 (10 and 30 ppm) was restricted to the airways. Minor pathological changes consisting of epithelial hyperplasia, mucoid degeneration, and desquamation of epithelium were observed only in the tracheas and bronchi of the rats inhaling SO2, even though the concentration of sulfite plus S-sulfonates in the tracheas and bronchi of these rats was considerably lower than that in the endogenously exposed rats. We attribute this histological damage to hydrogen ions stemming from inhaled SO2, not to the sulfite/bisulfite ions that are also a product of inhaled SO2. In addition to the lungs and trachea, all other tissues examined, except the testes, appeared to be refractory to high concentrations of endogenously generated sulfite. The testes of grossly sulfite oxidase-deficient rats were severely atrophied and devoid of spermatogenic cells.

Low-spin sulfite reductases: a new homologous group of non-heme iron-siroheme proteins in anaerobic bacteria

Two new low molecular weight proteins with sulfite reductase activity, isolated from Methanosarcina barkeri (DSM 800) and Desulfuromonas acetoxidans (strain 5071), were studied by EPR and optical spectroscopic techniques. Both proteins have visible spectra similar to that of the low-spin sulfite reductase of Desulfovibrio vulgaris strain Hildenborough and no band at 715 nm, characteristic of high-spin Fe3+ complexes in isobacteriochlorins is observed. EPR shows that as isolated the siroheme is in a low-spin ferric state (S = 1/2) with g-values at 2.40, 2.30 and 1.88 for the Methanosarcina barkeri enzyme and g-values at 2.44, 2.33 and 1.81 for the Desulfuromonas acetoxidans enzyme. Chemical analysis shows that both proteins contain one siroheme and one [Fe4S4] center per polypeptidic chain. These results suggest that the low molecular weight, low-spin non-heme iron siroheme proteins represent a new homologous class of sulfite reductases common to anaerobic microorganisms.

Molybdenum in enzymatic and heterogeneous catalysis

The chemistry common to molybdenum at the active centers of molybdoenzymes and at the surface of heterogeneous catalysts is described. Oxomolybdenum(VI) compounds catalyze selective oxidation of unsaturated hydrocarbons, e.g., propene to acrolein. Similarly, oxomolybdenum species take part in reactions catalyzed by molybdoenzymes, e.g., xanthine oxidase, sulfite oxidase, nitrate reductase. In these reactions H+, O2- or HO-, and electrons transfer between substrate molecules and molybdenum atoms and groups at the active centres. The chemistry involved is the acid-base and redox chemistry of molybdenum. Molybdenum disulfide catalyzes hydrogenation of unsaturated hydrocarbons, e.g., acetylene. The active site is a coordinately unsaturated molybdenum atom in a sulfur-ligand environment. The enzyme nitrogenase, which is a protein-bound iron-molybdenum sulfide, is also an excellent hydrogenation catalyst. Both catalysts exploit the chemistry of lower-valent molybdenum coordinated by sulfur. The extent to which understanding of the catalysis can be transferred between the two types of catalyst is assessed.

Sulfite oxidase from chicken liver. The role of imidazole and carboxyl groups for the reaction with cytochrome c

Oxidation of sulfite to sulfate by sulfite oxidase is inhibited when the enzyme is treated with reagents known to modify imidazole and carboxyl groups. Modification inhibits the oxidation of sulfite by the physiological electron acceptor cytochrome c, but not by the artificial acceptor ferricyanide. This indicates interference with reaction steps that follow the oxidation of sulfite by the enzyme's molybdenum cofactor. Reaction with diethylpyrocarbonate modifies ten histidines per enzyme monomer. Loss of activity is concomitant to the modification of only a single histidine residue. Inactivation takes place at the same rate in free sulfite oxidase and in the sulfite-oxidase--cytochrome-c complex. Blocking of carboxyl groups with water-soluble carbodiimides inactivates the enzyme. But none of the enzyme's carboxyl groups seems to be essential in the sense that its modification fully abolishes activity. The pattern of inactivation by chemical modification of sulfite oxidase is quite similar to that observed previously for cytochrome c peroxidase from yeast [Bosshard, H. R., Bänziger, J., Hasler, T. and Poulos, T. L. (1984) J. Biol. Chem. 259, 5683-5690; Bechtold, R. and Bosshard, H. R. (1985) J. Biol. Chem. 260, 5191-5200]. The two enzymes have very different structures yet share cytochrome c as a common substrate of which they recognize the same electron-transfer domain around the exposed heme edge.

57Fe and 1H electron-nuclear double resonance of three doubly reduced states Escherichia coli sulfite reductase

We have employed electron-nuclear double resonance (ENDOR) spectroscopy to study the 57Fe hyperfine interactions in the bridged-siroheme [4Fe-4S] cluster that forms the catalytically active center of the two-electron-reduced hemoprotein subunit of Escherichia coli NADPH-sulfite reductase (SiR2-). Previous electron paramagnetic resonance (EPR) and Mössbauer studies have shown that this enzyme oxidation state can exist in three distinct spectroscopic forms: (1) a "g = 2.29" EPR species that predominates in unligated SiR2-, in which the siroheme Fe2+ is believed to be in an S = 1 state; (2) a "g = 4.88" type of EPR species that predominates in SiR2- in the presence of small amounts of guanidinium sulfate, in which the siroheme Fe2+ is in an S = 2 state; and (3) a classical "g = 1.94" type of EPR species that is seen in SiR2- ligated with CO, in which the siroheme Fe2+ is in an S = 0 state. In all three species, the cluster is in the [4Fe-4S]1+ state, and two distinct types of Fe site are seen in Mössbauer spectroscopy. ENDOR studies confirm the Mössbauer assignments for the cluster 57Fe in the g = 1.94 state, with A values of 37, 37, and 32 MHz for site I and ca. 19 MHz for site II. The hyperfine interactions are not too different on the g = 2.29 state, with site I Fe showing more anisotropic A values of 32, 24, and 20 MHz (site II was not detected).(ABSTRACT TRUNCATED AT 250 WORDS)

Electron-nuclear double resonance studies of oxidized Escherichia coli sulfite reductase: 1H, 14N, and 57Fe measurements

We have employed electron-nuclear double resonance (ENDOR) spectroscopy to study the bridged siroheme--[Fe4S4] cluster that forms the catalytically active center of the oxidized hemoprotein subunit (SiRo) of Escherichia coli NADPH-sulfite reductase. The siroheme 57Fe hyperfine coupling (Az = 27.6 MHz, Ay = 26.8 MHz) is similar to that of other high-spin heme systems (A approximately equal to 27 MHz). Bonding parameters obtained from the 14N hyperfine coupling constants of the siroheme pyrrole nitrogens are consistent with a model of a nonplanar pi system of reduced aromaticity. The absence of hyperfine coupling to the 14N of an axial ligand, such as is observed for the histidine 14N of metmyoglobin (Az = 11.55 MHz), rules out the possibility that imidazolate acts as the bridge between the siroheme and the [Fe4S4] cluster. Proton ENDOR of the deuterium-exchanged protein indicates that H2O does not function as a sixth axial ligand and suggests that the ferrisiroheme is five-coordinate. 57Fe ENDOR measurements confirm the results of Mössbauer spectroscopy for the [Fe4S4] cluster. They also disclose a slight anisotropy of the cluster 57Fe coupling that may be associated with the mechanism by which the siroheme and cluster spins are coupled.

Effect of sulfite on the energy metabolism of mammalian tissues in correlation to sulfite oxidase activity

Mammalian tissues show significant differences in the activity of sulfite oxidase (EC 1.8.3.1) which detoxifies sulfite by oxidation to sulfate. Lung tissue and phagocytic cells such as alveolar macrophages, peritoneal macrophages, Kupffer cells and granulocytes show very low activities of sulfite oxidase. Liver tissue and hepatocytes, however, exhibit high activities of sulfite oxidase. Lung tissue and macrophages show an almost 100% decrease of the intracellular ATP levels when incubated with 1 mM sulfite at pH 6 for 30 min. In addition, the O2 consumption of lung tissue is inhibited by 1 mM sulfite at pH 6 by more than 80%. This sulfite-induced decrease of the ATP level and of the O2 consumption of lung tissue is enhanced between pH 6.0 and pH 7.4 with decreasing pH value of the incubation medium. In contrast, the ATP levels in liver tissue and hepatocytes are not affected by 1 mM sulfite at pH 6. The O2 consumption of liver tissue and hepatocytes is significantly increased by sulfite due to the high activities of sulfite oxidase. Therefore, the activity of the 'sulfite-detoxifying enzyme' sulfite oxidase and the sensitivity of the energy metabolism to sulfite show a reciprocal relationship in the tissues and cells studied.

Characterization of a sulfite reductase from Desulfovibrio vulgaris. Evidence for the presence of a low-spin siroheme and an exchange-coupled siroheme-[4Fe-4S] unit

We have studied a low-molecular-weight (Mr = 27,200) sulfite reductase from Desulfovibrio vulgaris (Hildenborough, NCIB 8303) with Mössbauer, EPR, and chemical techniques. This sulfite reductase was found to contain one siroheme and one [4Fe-4S] cluster. As purified, the siroheme is low-spin ferric (S = 1/2) which exhibits characteristic EPR resonances at g = 2.44, 2.36, and 1.77. At 150 K, the observed Mössbauer parameters, delta EQ = 2.49 +/- 0.02 mm/s and delta = 0.31 +/- 0.02 mm/s, for the siroheme are typical for low-spin ferric complexes. The [4Fe-4S] cluster is in the 2+ state. The Mössbauer parameters, delta EQ = 0.95 +/- 0.02 mm/s and delta = 0.38 +/- 0.02 mm/s, for the cluster are almost identical to those observed for the [4Fe-4S]2+ cluster in the hemoprotein subunit of the sulfite reductase from Escherichia coli. Similar to the hemoprotein subunit of E. coli sulfite reductase, low-temperature Mössbauer spectra of D. vulgaris sulfite reductase recorded with weak and strong applied fields also show evidence for an exchange-coupled siroheme-[4Fe-4S] unit.

Sulphur nutrition and metabolism in various species of Neisseria

Most Neisseria strains are able to grow with sulphate as a unique source of sulphur. Nevertheless, a cysteine requirement was present in a few strains of N. meningitidis and in 30% of N. flava strains isolated in our laboratory. All strains of N. gonorrhoeae exhibited such a requirement. In every strain tested, the need for cysteine (which can be satisfied by thiosulphate) was linked to the lack of sulphite-reducing-activity. The implications of these findings for the taxonomy and identification of Neisseria are discussed.

The pterin component of the molybdenum cofactor. Structural characterization of two fluorescent derivatives

Two stable fluorescent derivatives of molybdopterin have been structurally characterized. Form A is an oxidized pterin with a 6-alkyl substituent. Results of chemical, mass spectral, and NMR studies are consistent with the side chain formulation -C identical to C--CH-OHCH2OPO2-3. Similar studies on the Form B derivative indicate that it is the phosphorylated analog of urothione but lacks the 3-methylthio function. Form B (dephospho) can be synthesized from urothione by desulfuration with Raney nickel and oxidation with SeO2. Chicken liver sulfite oxidase (sulfite:ferricytochrome c oxidoreductase, EC 1.8.2.1) contains one phosphate residue (as molybdopterin) per subunit. The phosphate is noncovalently bound but is not released by trichloroacetic acid at 4 degrees C. The yield of Form A and Form B from sulfite oxidase is 0.50 and 0.27/subunit, respectively. The phosphate ester bond in isolated molybdopterin (Form B) is partially hydrolyzed by 1 N HCl at 100 degrees C (33% in 1 h). The release of inorganic phosphate from sulfite oxidase was more rapid (35% in 10 min) due to the presence of molybdate in the denatured enzyme mixture but slower than expected from a high energy phosphate bond. The presence of molybdopterin in a wide variety of molybdenum- and tungsten-containing enzymes has been demonstrated. Glucose oxidase and the iron and manganese superoxide dismutases are devoid of molybdopterin.

Purification and characterization of an inducible dissimilatory type sulfite reductase from Clostridium pasteurianum

An inducible sulfite reductase was purified from Clostridium pasteurianum. The pH optimum of the enzyme is 7.5 in phosphate buffer. The molecular weight of the reductase was determined to be 83,600 from sodium dodecyl sulfate gel electrophoresis with a proposed molecular structure: alpha 2 beta 2. Its absorption spectrum showed a maximum at 275 nm, a broad shoulder at 370 nm and a very small absorption maximum at 585 nm. No siroheme chromophore was isolated from this reductase. The enzyme could reduce the following substrates in preferential order: NH2OH greater than SeO3 (2-) greater than NO(2-) 2 at rates 50% or less of its preferred substrate SO3(2-). The proposed dissimilatory intermediates, S3O6 (2-) or S2O3(2-), were not utilized by this reductase while KCN inhibited its activity. Varying the substrate concentration [SO3(2-)] from 1 to 2.5 mumol affected the stoichiometry of the enzyme reaction by alteration of the ratio of H2 uptake to S2- formed from 2.5:1 to 3.1:1. The inducible sulfite reductase was found to be linked to ferredoxin which could be completely replaced by methyl viologen or partially by benzyl viologen. Some of the above-mentioned enzyme properties and physiological considerations indicated that it was a dissimilatory type sulfite reductase.

Quantitative transfer of the molybdenum cofactor from xanthine oxidase and from sulphite oxidase to the deficient enzyme of the nit-1 mutant of Neurospora crassa to yield active nitrate reductase

An assay method is described for measurement of absolute concentrations of the molybdenum cofactor, based on complementation of the defective nitrate reductase ('apo nitrate reductase') in extracts of the nit-1 mutant of Neurospora crassa. A number of alternative methods are described for preparing, anaerobically, molybdenum-cofactor-containing solutions from sulphite oxidase, xanthine oxidase and desulpho xanthine oxidase. For assay, these were mixed with an excess of extract of the nit-1 mutant, incubated for 24 h at 3.5 degrees C then assayed for NADPH:nitrate reductase activity. In all cases, the specific activity of the molybdenum cofactor, expressed as mumol of NO2-formed/min per ng-atom of Mo added from the denatured molybdoenzyme , was 25 +/- 4, a value that agrees with the known catalytic activity of the nitrate reductase of wild-type Neurospora crassa. This indicates that, under our conditions, there was quantitative transfer of the molybdenum cofactor from denatured molybdoenzyme to yield fully active nitrate reductase. Comparable cofactor assay methods of previous workers, apparently indicating transfer efficiencies of at best a few per cent, have never excluded satisfactorily the possibility that cofactor activity arose, not from stoichiometric constituents of the molybdoenzymes , but from contaminants. The following factors were investigated separately in developing the assay:the efficiency of extraction of the cofactor from the original enzyme, the efficiency of the complementation reaction between cofactor and apo nitrate reductase, and the assay of the resultant nitrate reductase, which must be carried out under non-inhibitory conditions. Though the cofactor is unstable in air (t1/2 about 15 min at 3.5 degrees C), it is stable when kept anaerobic in the presence of sodium dithionite, in aqueous solution or in dimethyl sulphoxide (activity lost at the rate of about 3%/24 h at 20-25 degrees C). Studies of stabilities, and investigations of the effect of added molybdate on the assay, permit conclusions to be drawn about the ligation of molybdenum to the cofactor and about steps in incorporation of the cofactor into the apoenzyme. Though the development of nitrate reductase activity is slow at 3.5 degrees C (t1/2 1.5-3 h) the complementation reaction may be carried out in high yield, aerobically. This is ascribed to rapid formation of an air-stable but catalytically inactive complex of the cofactor, as a precursor of the active nitrate reductase.(ABSTRACT TRUNCATED AT 400 WORDS)

Transport of the precursor for sulfite oxidase into intermembrane space of liver mitochondria: binding of the precursor to outer mitochondrial membrane

Rat liver mitochondria were incubated with in vitro translation products programmed by liver RNA, then disrupted by sonication and subjected to sucrose density gradient centrifugation. Pre-sulfite oxidase bound preferentially to the outer mitochondrial membrane recovered with the inner membrane. This outer membrane could not be separated from the inner membrane by recentrifugation, suggesting the tight association of both membranes. The binding was not affected by pretreatment of mitochondria with proteolytic enzymes. The mature enzyme and its precursor synthesized in isolated hepatocytes have isoelectric points of 4.2 and 5.5, respectively. The molecular size of the precursor in cytosol was estimated to be about 100,000 daltons (dimer) by gel filtration.

Transport of the precursor for sulfite oxidase into intermembrane space of liver mitochondria: characterization of import and processing activities

Sulfite oxidase, a soluble enzyme in mitochondrial intermembrane space, was synthesized as a precursor protein larger than the authentic mature enzyme when rat liver total RNA was translated in a cell-free system. When the in vitro translation products were incubated with isolated rat liver mitochondria, pre-sulfite oxidase was recovered in mitochondria and converted to the size of the mature enzyme. The in vitro-processed mature enzyme was recovered in the intermembrane space of mitochondria (Ono, H. & Ito, A. (1981) Biochem. Biophys. Res. Commun. 107, 258-264). Mature sulfite oxidase was not imported into mitochondria, and did not affect the import of pre-sulfite oxidase. When mitochondria were incubated with gel-filtered translation products, the import was dependent on ATP, and the activity restored by the addition of ATP was blocked by valinomycin and K+ ion. These results suggest that the import of pre-sulfite oxidase into mitochondrial intermembrane space requires an electrochemical potential across the inner membrane. When mitochondria were fractionated, most of the processing activity was recovered in the mitoplast, whereas the inner membrane (after being mostly inverted by sonication) exhibited only slight activity. The processing activity was strongly inhibited by some metal chelators including EDTA, GTP, and Zincon. It was not inhibited by phenyl methyl sulfonyl fluoride, aprotinin, or various microbial protease inhibitors including pepstatin, antipain, leupeptin, and chymostatin. The processing enzyme seems to be a metal protease. The processing of pre-sulfite oxidase by mitoplasts was energy-dependent.

Reduction of bisulfite by the trithionate pathway by cell extracts from Desulfotomaculum nigrificans

Bisulfite was reduced to sulfide by cell extracts of Desulfotomaculum nigrificans. When trithionate was added to reaction mixtures reducing bisulfite, sulfide formation was inhibited with accumulation of thiosulfate. The thiosulfate reductase activity of cell extracts was found to be inhibited by trithionate. Trithionate alone was reduced to thiosulfate and purified bisulfite reductase (P582) was not affected by trithionate. It is concluded that the pathway for bisulfite reduction in Dt. nigrificans includes trithionate and thiosulfate as intermediate compounds.

Sulfite oxidase activity in Thiobacillus novellus

Thiobacillus novellus shows a maximum induction of sulfite oxidase activity and a maximum growth rate as a result of supplementing the autotrophic growth medium with 4.0 microM ammonium molybdate. Cells grown in the presence of molybdate showed approximately 10-fold increases in the amount of enzyme-associated molybdenum and in the sulfite-to-cytochrome c and sulfite-to-ferricyanide reductase activities. The effect of exogenous molybdate was not discernible with cells grown in the absence of thiosulfate. Tungsten inhibited the growth of T. novellus and the expression of sulfite oxidase activity.