Rapid purification of rabbit reticulocyte lipoxygenase for electron paramagnetic spectroscopy characterization of the non-heme iron

An efficient three-step purification technique has been developed for the reticulocyte 15-lipoxygenase from rabbit. Ammonium sulfate fractionated reticulocyte lysate was purified by size exclusion chromatography and preparative scale isoelectric focusing. The entire procedure was complete in less than eight hours and was carried out in batches which typically yielded 10 mg of purified enzyme. The identity and purity of the enzyme were evaluated by N-terminal sequencing, sodium dodecylsulfate polyacrylamide gel electrophoresis and specific activity determinations. The enzyme contained approximately one g-atom iron per mole of protein. The iron was present in an electron paramagnetic spectroscopy (EPR) silent, presumably high spin iron(II), form in the isolated enzyme. Treatment with one equivalent of 13-hydroperoxy-9(Z),11(E)-octadecadienoic acid resulted in the appearance of an EPR signal around g6.

Investigation of the mechanism of non-turnover-dependent inactivation of purified human 5-lipoxygenase. Inactivation by H2O2 and inhibition by metal ions

Human 5-lipoxygenase is a non-heme iron protein which is reported to be highly unstable in the presence of oxygen. The results of this investigation demonstrate that H2O2 generated during air oxidation of thiols is the main factor in non-turnover-dependent inactivation of purified recombinant human 5-lipoxygenase for the following reasons: catalase protects against oxygen-dependent inactivation of the enzyme in the presence of dithiothreitol; the active, stable enzyme can be prepared under aerobic conditions with the exclusion of dithiothreitol and contaminating metal ions; 10 microM H2O2 causes the rapid inactivation of the enzyme. The native (ferrous) enzyme is approximately seven times more sensitive to inactivation by H2O2 than the ferric enzyme, suggesting that the mechanism of inactivation involves a Fenton-type reaction of the ferrous enzyme with H2O2, resulting in the formation of an activated oxygen species. Purification of 5-lipoxygenase under aerobic conditions (no dithiothreitol) results in an increase in both the specific activity of the purified protein [up to 70 mumol 5(S)-hydroperoxy-6-trans-8, 11, 14-cis-icosatetraenoic acid (5-HPETE)/mg protein] and in the ratio of specific activity to enzyme iron content compared to enzyme purified under anaerobic conditions in the presence of dithiothreitol. The reaction of the highly active 5-lipoxygenase enzyme shows a dependence on physiological intracellular calcium concentrations, half-maximal product formation being obtained at 0.9 microM free Ca2+. The maximal enzyme activity is also dependent on EDTA and dithiothreitol and low amounts of carrier protein, as well as the known activators PtdCho and ATP. Ca2+ can be substituted by Mn2+, Ba2+ and Sr2+, although lower levels of stimulation are obtained. 5-Lipoxygenase is strongly inhibited by low concentrations (< or = 10 microM) of Zn2+ and Cu2+. The inhibition by Cu2+ is apparently irreversible, whereas that by Zn2+ is slowly reversed (t1/2 = 2 min) in the presence of excess EDTA. These observations on the mechanism of non-turnover-dependent inactivation of 5-lipoxygenase, and the optimisation of assay conditions, have facilitated the purification of large quantities of relatively stable enzyme that will be useful for further kinetic and physical studies.

A molybdenum and a tungsten isoenzyme of formylmethanofuran dehydrogenase in the thermophilic archaeon Methanobacterium wolfei

We have recently reported that the thermophilic archaeon Methanobacterium wolfei contains two formylmethanofuran dehydrogenases, I and II. Formylmethanofuran dehydrogenase II, which is preferentially expressed in tungsten-grown cells, has been purified and shown to be a tungsten-iron-sulfur protein. We have now purified and characterized formylmethanofuran dehydrogenase I from molybdenum-grown cells and shown that it is a molybdenum-iron-sulfur protein. The purified enzyme, with a specific activity of 27 U/mg protein, was found to be composed of three subunits of apparent molecular mass 64 kDa, 51 kDa, and 31 kDa and to contain per mol 146-kDa molecule approximately 0.23 mol molybdenum, 0.46 mol molybdopterin guanine dinucleotide, and 6.6 mol non-heme iron but no tungsten (< 0.01 mol). The molybdenum enzyme differed from the tungsten enzyme (8 U/mg) in that it catalyzed the oxidation of N-furfurylformamide and formate and was inactivated by cyanide. The two enzymes also differed significantly in the pH optimum, in the apparent Km for the electron acceptor, and in the chromatographic behaviour. The molybdenum enzyme and the tungsten enzyme were similar, however, in that the N-terminal amino acid sequences determined for the alpha and beta subunits were identical up to residue 23, indicating that the two proteins are isoenzymes. The molybdenum enzyme, as isolated, was found to display an EPR signal derived from molybdenum as evidenced by isotope substitution.

Iron-sulfur centers as endogenous blue light sensitizers in cells: a study with an artificial non-heme iron protein

The possible involvement of Fe-S clusters in photodynamic reactions as endogenous sensitizing chromophores in cells has been investigated, by using an artificial non-heme iron protein (ANHIP) derived from bovine serum albumin and ferredoxins isolated from spinach and a red marine algae. Ferredoxins and ANHIP, when exposed to visible light, generate singlet oxygen, as measured by the imidazole plus RNO method. Irradiation with intense blue light of the ANHIP-entrapped liposomes caused severe membrane-damage such as liposomal lysis and lipid peroxidation. In the presence of ANHIP, isocitrate dehydrogenase and fructose-1,6-diphosphatase were photoinactivated by blue light. However, all of these photosensitized reactions were significantly suppressed by a singlet oxygen (1O2) quencher, azide, but enhanced by a medium containing deuterium oxide. Further, the Fe-S proteins with the prosthetic groups destroyed did not initiate the blue light-induced reactions. In addition, the action spectrum for 1O2 generation from ANHIP was very similar to the visible absorption spectrum of Fe-S centers. The results obtained in this investigation appear consistent with the suggestion that Fe-S centers are involved in photosensitization in cells via a singlet oxygen mechanism.

EPR detection of heme and nonheme iron-containing protein nitrosylation by nitric oxide during rejection of rat heart allograft

The paramagnetic molecule nitric oxide (NO), produced from L-arginine by a specific enzyme (NO synthase), has been shown to be involved in a surprising variety of mammalian cellular responses, including the regulation of T cell immunity to alloantigens in vitro. In cytotoxic activated macrophages, NO production results in a characteristic pattern of alteration of iron-containing enzyme function that is mimicked by exposure to NO. Electron paramagnetic resonance (EPR) studies have shown the formation of iron-nitrosyl species during macrophage activation and also during sepsis, indicating that alteration of iron-containing protein function may be the result of the well-documented tendency of NO to bind to metal ions. We have recently shown that the NO synthesis induced during alloantigenic activation of rat splenocytes inhibits lymphocyte proliferation and cytotoxic T-lymphocyte generation. This report demonstrates that iron-nitrosyl EPR signals similar to those observed in macrophages and during sepsis are present in the blood and in the grafted tissue of rats during the rejection of allogeneic (but not syngeneic) heart grafts. These signals are found in the blood and at the site of allograft rejection, but are not found in other tissues (such as spleen and lung), and are obliterated by administration of the immunosuppressant FK506. These results directly demonstrate the formation of iron-nitrosyl complexes during vascularized allograft rejection and suggest that consequent destruction of iron-containing protein function plays an important role in the rejection response.

Molecular cloning and sequencing of a non-haem bromoperoxidase gene from Streptomyces aureofaciens ATCC 10762

A bromoperoxidase gene (bpoT), recently cloned from Streptomyces aureofaciens Tü24, was used as a probe in Southern blot hybridization of total DNA from S. aureofaciens ATCC 10762. A single SstI fragment of 5.4 kb was detected, which was cloned via an enriched gene library into Escherichia coli. The functional bromoperoxidase gene was located on a 2.1 kb BamHI-HindIII fragment by subcloning into S. lividans TK64, using the multicopy plasmid pIJ486. The enzyme was overproduced in S. lividans TK64 (up to 30,000 times compared to S. aureofaciens ATCC 10762) and showed the same electrophoretic and immunological properties as the bromoperoxidase BPO-A2 purified from S. aureofaciens ATCC 10762. DNA sequence analysis revealed an open reading frame encoding a predicted polypeptide with the same M(r) and N-terminal amino acid sequence as the purified subunit of BPO-A2.

Fe(II) oxidation and Fe(III) incorporation by the M(r) 66,000 microsomal iron protein that stimulates NADPH oxidation

In a previous study (Minotti, G., and Ikeda-Saito, M. (1991) J. Biol. Chem. 266, 20011-20017) we demonstrated the existence of a M(r) 66,000 microsomal iron protein (MIP) which stimulates NADPH oxidation by shunting electrons from NADPH-cytochrome P-450 reducase to its bound Fe(III). In the present study, purified MIP was depleted of iron and the apoMIP was examined for its ability to incorporate Fe(III) upon an incubation with Fe(II). It was found that apoMIP had an oxygen-dependent ferroxidase activity coupled with the incorporation of Fe(III). The reconstituted MIP exhibited a Fe(III) content and an NADPH oxidation activity similar to those of native MIP. However, the reconstitution of MIP from apoMIP and Fe(II) had to be performed in the presence of detergents to prevent the formation of protein aggregates and the oxidative incorporation of an iron which could not react with NADPH-cytochrome P-450 reductase. This redox inactive iron was probably bound nonspecifically to artifactual sites formed by the protein aggregates.

Mapping the site(s) of MgATP and MgADP interaction with the nitrogenase of Azotobacter vinelandii. Lysine 15 of the iron protein plays a major role in MgATP interaction

Nitrogenase binds and hydrolyzes 2MgATP yielding 2MgADP and 2Pi for each electron that is transferred from the iron protein to the MoFe protein. The iron protein alone binds but does not hydrolyze 2MgATP or 2MgADP and the binding of these nucleotides is competitive. Iron protein amino acid sequences all contain a putatitive mononucleotide-binding region similar to a region found in other mononucleotide-binding proteins. To examine the role of this region in MgATP interaction, we have substituted glutamine and proline for conserved lysine 15. The amino acid substitutions, K15Q and K15P, both yielded a non-N2-fixing phenotype when the genes coding for them were substituted into the Azotobacter vinelandii chromosome in place of the wild-type gene. The iron protein from the K15Q mutant was purified to homogeneity, whereas the protein from the K15P mutant could not be purified in its native form. Unlike wild-type iron protein, the purified K15Q iron protein showed no acetylene reduction, H2 evolution, or ATP hydrolysis activities when complemented with wild-type MoFe protein. The K15Q iron protein and the normal iron protein had a similar total iron content and both proteins showed the characteristic rhombic EPR signal resulting from the reduced state of the single 4Fe-4S cluster bridging the two subunits. Unlike the wild-type iron protein, addition of MgATP to the K15Q iron protein did not result in the perturbation necessary to change the EPR signal of its 4Fe-4S center from a rhombic to an axial line shape. Also unlike the wild-type iron protein, addition of MgATP to K15Q iron protein in the presence of the iron chelator, alpha,alpha'-dipyridyl, did not result in a time-dependent transfer of iron to the chelator. Thus, even though the K15Q iron protein contains a normal 4Fe-4S center, it does not respond to MgATP like the wild-type protein. Examination of the ability of the K15Q iron protein to bind MgADP showed no change from the wild-type iron protein, but its ability to bind MgATP decreased to 35% of the wild-type protein. Thus, in A. vinelandii iron protein, lysine 15 is not needed for interaction with MgADP but is involved in the binding of ATP, presumably through charge-charge interaction with the gamma-phosphate. Based on the above data, this lysine appears to be essential for the MgATP induced conformational change of wild-type iron protein that is required for activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Mutagenesis studies on the amino acid residues involved in the iron-binding and the activity of human 5-lipoxygenase

Human 5-lipoxygenase contains a non-heme iron essential for its activity. In order to determine which amino acid residues are involved in the iron-binding and the lipoxygenase activity, nine amino acid residues in highly homologous regions among the lipoxygenases were individually replaced by means of site-directed mutagenesis. Mutant 5-lipoxygenases in which His-367 or His-550 was replaced by either Asn or Ala, His-372 by either Asn or Ser, or Glu-376 by Gln were completely devoid of the activity. Though mutants containing an alanine residue instead of His-390 or His-399 lacked the activity, the corresponding asparagine substituted mutants exhibited. The other mutants retained the enzyme activity. These results strongly suggest that His-367, His-372, His-550 and Glu-376 are crucial for 5-lipoxygenase activity and coordinate to the essential iron.

Structure-function relationships among the nickel-containing hydrogenases

The enzymology of the heterodimeric (NiFe) and (NiFeSe) hydrogenases, the monomeric nickel-containing hydrogenases plus the multimeric F420-(NiFe) and NAD(+)-(NiFe) hydrogenases are summarized and discussed in terms of subunit localization of the redox-active nickel and non-heme iron clusters. It is proposed that nickel is ligated solely by amino acid residues of the large subunit and that the non-heme iron clusters are ligated by other cysteine-rich polypeptides encoded in the hydrogenase operons which are not necessarily homologous in either structure or function. Comparison of the hydrogenase operons or putative operons and their hydrogenase genes indicate that the arrangement, number and types of genes in these operons are not conserved among the various types of hydrogenases except for the gene encoding the large subunit. Thus, the presence of the gene for the large subunit is the sole feature common to all known nickel-containing hydrogenases and unites these hydrogenases into a large but diverse gene family. Although the different genes for the large subunits may possess only nominal general derived amino acid homology, all large subunit genes sequenced to date have the sequence R-X-C-X-X-C fully conserved in the amino terminal region of the polypeptide chain and the sequence of D-P-C-X-X-C fully conserved in the carboxyl terminal region. It is proposed that these conserved motifs of amino acids provide the ligands required for the binding of the redox-active nickel. The existing EXAFS (Extended X-ray Absorption Fine Structure) information is summarized and discussed in terms of the numbers and types of ligands to the nickel and the various redox species of nickel defined by EPR spectroscopy. New information concerning the ligands to nickel is presented based on site-directed mutagenesis of the gene encoding the large subunit of the (NiFe) hydrogenase-1 of Escherichia coli. Based on considerations of the biochemical, molecular and biophysical information, ligand environments of the nickel in different redox states of the (NiFe) hydrogenase are proposed.

Factors affecting the mRNA levels for the non-heme iron and effector-binding subunits of ribonucleotide reductase

Ribonucleotide reductase which catalyzes the rate-limiting step in the de novo synthesis of dNTPs is composed of two non-identical protein subunits which are not under coordinate control in terms of synthesis and degradation. The mRNAs for the effector-binding (EB) and non-heme iron (NHI) subunits are likewise not under coordinate control during cell cycle traverse. Inhibitors directed at the specific subunits of ribonucleotide reductase block DNA synthesis. These current studies show that drugs such as IMPY or hydroxyurea which specifically inhibit the NHI subunit cause a marked increase in the steady-state level of the mRNA for the NHI subunit while resulting in a decrease in the level of mRNA for the EB subunit. In cells treated with deoxyadenosine, the patterns of the mRNAs for the NHI and EB subunits were different from those seen in the IMPY- or hydroxyurea-treated cells. Control experiments utilizing inhibitors (aphidicolin or araC) directed at DNA polymerase showed that the pattern of changes in the mRNA levels for the NHI and EB subunits were specific for the reductase inhibitors. These changes in the mRNAs for the NHI and EB subunits may be due to drug-induced alterations in transcription rates and/or degradation rates for the specific mRNAs.

Bovine heart microsomes contain an Mr = 66,000 non-heme iron protein which stimulates NADPH oxidation

Bovine heart microsomes have been found to contain a non-heme iron protein which serves as an electron acceptor for NADPH-cytochrome P-450 reductase and therefore stimulates NADPH oxidation. This protein, tentatively referred to as Microsomal Iron Protein (MIP), has been extracted with Triton N-101 and purified by ion exchange chromatography on CM- and DEAE-celluloses and gel filtration on Sepharose 6B. MIP is an Mr = 66,000 monomer with 17 atoms of Fe(III)/molecule. Incubation with dithionite removes iron from MIP and abolishes the stimulation of NADPH oxidation, but subsequent incubation with nitrilotriacetic-Fe(III) reincorporates iron and restores the stimulation of NADPH oxidation. Oxygen is the ultimate electron acceptor. In the presence of oxygen, the enzymatic reduction of MIP Fe(III) is followed by the reoxidation of Fe(II) at the expense of oxygen, generating superoxide anion and regenerating MIP Fe(III) for the continuous oxidation of NADPH. In the absence of oxygen, electron transfer from the reductase to MIP Fe(III) causes the release of Fe(II), which limits the ability of MIP to serve as an electron acceptor and stimulate NADPH oxidation. The--NH2-terminal of MIP has been sequenced, and no homology has been found with the sequence of other iron storage or transport proteins such as ferritin or transferrin.

[Formation of nitric oxide in animal tissues during inflammatory process]

EPR evidence was obtained that more intensive formation of mononitrosyl non-heme iron complexes with diethyl-dithiocarbamate (DETC) took place in mouse liver when inflammation process was initiated in mice by the lipopolysaccharide isolated from Salmonella typhimurium bacterium wall DETC intraperitoneally injected bound with endogenous non-heme iron resulted with DETC-Fe complex formation. These complexes were as a traps of nitric oxide appeared in animal tissues, and NO-Fe-DETC complexes were observed. Phenazone known as a free radical process inhibitor lowered NO production in animal organism. The free radical processes were suggested to intensify under inflammation reactions and to cause the various amino groups oxidation to nitroso groups which were capable to release free nitric oxide.

Use of the stable isotope, 58Fe, for determining availability of nonheme iron in meals

Because of reluctance to use radioisotopes for studies of iron absorption in children, we have explored the feasibility of using the least abundant stable isotope of iron, 58Fe (natural abundance, 0.322 weight %) in a study of nonheme iron absorption. With a balanced cross-over design, each of 16 school-age children was fed a standardized lunch on 3 consecutive days and, 28 days later, an alternate standardized lunch on 3 consecutive days. The lunch included either a beef patty or a beef-soy patty. The mass isotope ratio, 58Fe/57Fe (MIR58/57), was measured in blood by inductively coupled plasma mass spectroscopy before and 14 days after (i.e. study day 15) consuming the three lunches. The MIR58/57 on study day 15 was used as a baseline value for lunches fed on study days 29, 30, and 31. Incorporation of 58Fe into erythrocytes was greater from the lunch with beef patty than from the lunch with beef-soy patty (geometric mean values 2.02 and 1.05% of the dose, p less than 0.03). Based on the similarity of our results with those obtained in adults with radioisotopes, we conclude that 58Fe is a satisfactory tag for studies of nonheme iron absorption from meals.

Uniformity of liver density and nonheme (storage) iron distribution

The extent of variation in tissue density and hepatic nonheme iron concentration has been examined at autopsy in 21 adult livers. Samples were taken from each liver at inferior and superior sites in the midaxillary, anterior axillary, and midclavicular lines. Histologic examination showed diffuse metastatic carcinoma, cirrhosis, fibrosis, necrosis, steatosis, or congestion in 19 livers; two livers were normal. Density was determined by saline displacement of 0.5- to 1.0-g specimens. Nonheme iron concentration was measured at each site in samples of the size obtained by wedge (0.5 to 1.0 g) and percutaneous needle (0.005 to 0.010 g) biopsy using specially developed chemical assays. Density was uniform within each liver. Despite the inclusion of diseased tissues, the variation in density among the 21 livers was small (coefficient of variation, 1.25%). The mean (+/- 1 SD) hepatic density was 1.051 +/- 0.013 g/mL (range, 1.017 to 1.077 g/mL). Within each liver, the nonheme iron also was uniformly distributed among the six sites. Chemical measurements of nonheme iron concentration were not significantly different in samples of the size obtained by wedge or percutaneous liver biopsy. All the hepatic nonheme iron determinations were below the upper 95% confidence limit of concentrations in adult males (480 micrograms/g). In the absence of focal lesions, the uniformity in hepatic density and nonheme iron distribution supports the assumption of several clinical methods for measuring liver storage iron (wedge and needle biopsy, determination of hepatic magnetic susceptibility, computed tomography, and magnetic resonance imaging) that one sample of liver tissue is representative of the whole organ.

Determination of relative spin concentration in some high-spin ferric proteins using E/D-distribution in electron paramagnetic resonance simulations

Lineshape simulations are presented for the multiple, overlapping X-band electron paramagnetic resonance (EPR) spectra in two non-heme, high-spin iron proteins: phenylalanine hydroxylase (PAH) and diferric transferrin. The aim of the calculations is to determine the fraction of iron contributing to each of the sites visible by EPR. The simulations are limited to the experimentally accessible transitions occurring at g-values greater than 1.7. In both PAH and transferrin, at least one of the iron sites is characterized by the ratio of zero-field splitting parameters, E/D, near 1/3 and a broad, asymmetric lineshape. A distribution in E/D-values is used in the simulations to account for this breadth and asymmetry. To test the E/D-distribution model, experimental X-band spectra of diferric transferrin at several salt concentrations are fit by simulation. In this test, first the low-field features arising from transitions between the lowest Kramers doublet levels are simulated using E/D-distributions for two sites. Second, parameters that provide a good fit for the lowest doublet transitions are shown also to fit the resonance near an effective g-value of 4.3 from the middle Kramers doublet transition. When applied to spectra of PAH in the resting state, the E/D-distribution approach accounts for the intensity of one of the two major species of iron. The other species is characterized by E/D = 0.032, and the spectrum of this portion of the resting enzyme may be simulated using a frequency-swept Gaussian lineshape. Spectra for the enzyme in an inhibitor-saturated state are also simulated. The simulations are consistent with previous biochemical studies that indicate that only the E/D = 0.032 form of iron participates in catalysis.

[Purification and characterization of lathosterol 5-desaturase from rat liver microsomes by cytochrome b5-sepharose affinity column chromatography: evidence for the non-heme iron protein]

Lathosterol 5-desaturase, which catalyzes introduction of a delta 5 bond into lathosterol to form 7-dehydrocholesterol, was purified up to 2000-4000-fold with a 13-18% yield from rat liver microsomes by cytochrome b5-Sepharose affinity chromatography followed by isoelectric focusing. The final enzyme preparation was homogenous as judged by SDS-polyacrylamide gel electrophoresis and a single polypeptide of 65,000 daltons. Furthermore, the two molecular forms with isoelectric points of 6.3 and 9.5 were demonstrated by electrofocusing, though they did not show any significant difference with respect to their enzymatic properties. The desaturase was found to be a non-heme iron protein containing one atom of iron per one molecule of the enzyme. The enzyme activity was inhibited strikingly by iron chelators and cyanide. The decreased enzymatic activity, however, was recovered completely by Fe-ion to the original level, suggesting that the iron was essential for the catalytic activity.

Mitochondrial iron not bound in heme and iron-sulfur centers and its availability for heme synthesis in vitro

Rat liver mitochondrial fractions have previously been shown to contain a pool of iron which was bound neither in cytochromes nor in iron-sulfur centers (Tangerås, A., Flatmark, T., Bãckstrõm, D. and Ehrenberg, A. (1980) Biochim. Biophys. Acta 589, 162-175), and in the present study the availability of this iron pool for heme synthesis has been studied in isolated mitochondria. A minor fraction of this iron is here shown to originate from iron-rich lysosomes present as a contaminant in mitochondrial fractions isolated by differential centrifugation, and a method for the selective quantitation of this iron pool was developed. The availability of the mitochondrial iron pool for heme synthesis by mitochondria in vitro was studied using a recently developed HPLC method for the assay of ferrochelatase activity. When deuteroporphyrin was used as the substrate, 1.04 +/- 0.13 nmol/mg protein of deuteroheme was formed after 6 h incubation at 37 degrees C when a plateau was approached, and the initial rate of heme synthesis was 0.3 nmol/h per mg protein. Heme formation from the physiological substrate protoporphyrin was also seen. The heme synthesis increased with the amount of mitochondria used and was blocked by both Fe(II) and Fe(III) chelators. The heme synthesis was independent of mitochondrial oxidizable substrates and no difference was observed between pH 7.4 and 6.5. FMN slightly stimulated the formation of heme from endogenous iron, probably by mobilization of a small amount of contaminating lysosomal iron present in the preparations. The possibility that the mitochondrial iron pool functions as the proximate iron donor for heme synthesis by ferrochelatase in vivo is discussed.

Covalent modification of the iron protein of nitrogenase from Rhodospirillum rubrum by adenosine diphosphoribosylation of a specific arginine residue

Nitrogenase in Rhodospirillum rubrum is inactivated in vivo by the covalent modification of the Fe protein with a nucleotide. The preparation of two modified peptides derived from proteolytic digestion of the inactive Fe protein is described. The modifying group is shown to be adenosine diphosphoribose, linked through the terminal ribose to a guanidino nitrogen of arginine. The structural features were established by using proton and phosphorus NMR, positive- and negative-ion fast atom bombardment mass spectrometry, and fast atom bombardment/collisionally activated decomposition mass spectrometry. Spectral methods along with chromatographic analysis and sequential degradation established the sequence of the modification site of Fe protein as Gly-Arg(ADR-ribose)-Gly-Val-Ile-Thr. This corresponds to the sequence in the Fe protein from Azotobacter vinelandii for amino acid residues 99 to 104.

On the origin of the non-haemic iron transferrin ESR signal: ESR investigations on histidine-iron-citric acid systems

The nature of the high spin ferric iron complex located at g = 4.3 has been investigated by means of electron spin resonance spectroscopy and polarography. It could be shown that two complexes each exist in the acid and alkaline pH region, and that the iron is bound to two histidines, three citric acids, and probably to one bicarbonate. These results agree well with previous findings according to which the ligand field of iron should be composed mainly of oxygen and nitrogen atoms. Another low-field signal located at g = 3.6 appears in the pH range from 2 to 7 only and exhibits its maximum where the g = 4.3 signal has its minimum. Its exact nature is still unknown but it seems to represent some intermediate state of the ternary Fe3+-histidine-citric acid complex. When citric acid is used, the spin concentration seems to be always larger than in the case of ascorbic acid. Since the effect obtained with ascorbic acid and citric acid seems to be similar, it may be concluded that the biological function of both of the acids might be somehow related to each other.

Biological nitrogen fixation: primary structure of the Rhizobium trifolii iron protein gene

Biological nitrogen fixation in the Rhizobium-legume symbiosis is dependent on the induction of a bacterially-encoded enzyme complex, nitrogenase. To examine the organization and expression of the genes encoding the components of nitrogenase in this complex system, these genes have been isolated from the legume symbiont Rhizobium trifolii by molecular cloning. DNA sequence analysis of the entire nifH gene (encoding the Fe-protein component of nitrogenase) and of the amino-terminal 141 codons of the nifD gene (encoding the alpha-subunit of the Mo-Fe protein) indicates that these genes are linked on a single operon in this strain. The Fe-protein amino acid sequence shares considerable homology with the sequence from other organisms, in particular the related organism Rhizobium meliloti (90% homology). The nif structural genes are preceded by a DNA sequence which is repeated at least three times in the Rhizobium trifolii genome.

A novel iron protein from Desulfovibrio gigas

The isolation, purification, and partial characterization of a novel iron-containing protein from the sulfate-reducing anaerobic bacterium, Desulfovibrio gigas, is described. The highly insoluble protein was isolated from the cell debris following osmotic shock of the bacteria. The insoluble fraction consistently contained about 90% of the cell-associated iron. Elemental analysis of a crude protein preparation gave 5.3% iron, 2.9% sulfur and 11.9% nitrogen. An independent colorimetric iron analysis showed 6.4% iron. The iron could be dissociated from the protein by treatment with 5% SDS. The iron-free protein was purified by a combination of organic extraction and DEAE-cellulose chromatography. The purified protein showed only one major band, Mr 14000, by SDS-polyacrylamide gel electrophoresis. The protein could be reconstituted upon treatment with an appropriate mixture of FeS and beta-mercaptoethanol. The reconstituted protein had the same physical and chemical properties as the native protein. The amino acid composition was not unusual except for the high isoleucine content.