The inducible trimethylamine N-oxide reductase of Escherichia coli K12: its localization and inducers

We used an anti-trimethylamine-N-oxide reductase (EC 1.6.6.9) serum and different immunological techniques (Ouchterlony, rocket immunoelectrophoresis, immunoblotting) to show that dimethylsulphoxide (DMSO), tetrahydrothiophene 1-oxide (THTO) and pyridine N-oxide (PNO) were effective inducers of the inducible form of trimethylamine N-oxide reductase. We confirmed this genetically and biochemically using a strain in which phage MudII 1734 carrying lacZ was inserted into torA, the structural gene for inducible trimethylamine-N-oxide reductase. By subcellular fractionation and quantitation with rocket immunoelectrophoresis, we showed that the enzyme was principally localized in the periplasmic fraction. Constitutive trimethylamine-N-oxide reductase was localized in the membrane fraction and, like the inducible enzyme showed a broad specificity with respect to various compounds such as DMSO, THTO and PNO. Apart from their immunological properties, the two enzymes could be clearly differentiated by their temperature stability.     

Purification and further characterization of the second nitrate reductase of Escherichia coli K12

Two nitrate reductases, nitrate reductase A and nitrate reductase Z, exist in Escherichia coli. The nitrate reductase Z enzyme has been purified from the membrane fraction of a strain which is deleted for the operon encoding the nitrate reductase A enzyme and which harbours a multicopy plasmid carrying the nitrate reductase Z structural genes; it was purified 219 times with a yield of about 11%. It is an Mr-230,000 complex containing 13 atoms iron and 12 atoms labile sulfur/molecule. The presence of a molybdopterin cofactor in the nitrate reductase Z complex was demonstrated by reconstitution experiments of the molybdenum-cofactor-deficient NADPH-dependent nitrate reductase activity from a Neurospora crassa nit-1 mutant and by fluorescence emission and excitation spectra of stable derivatives of molybdoterin extracted from the purified enzyme. Both nitrate reductases share common properties such as relative molecular mass, subunit composition and electron donors and acceptors. Nevertheless, they diverge by two properties: their electrophoretic migrations are very different (RF of 0.38 for nitrate reductase Z versus 0.23 for nitrate reductase A), as are their susceptibilities to trypsin. An immunological study performed with a serum raised against nitrate reductase Z confirmed the existence of common epitopes in both complexes but unambiguously demonstrated the presence of specific determinants in nitrate reductase Z. Furthermore, it revealed a peculiar aspect of the regulation of both nitrate reductases: the nitrate reductase A enzyme is repressed by oxygen, strongly inducible by nitrate and positively controlled by the fnr gene product; on the contrary, the nitrate reductase Z enzyme is produced aerobically, barely induced by nitrate and repressed by the fnr gene product in anaerobiosis.     

Characterization of FMN-dependent NADH-quinone reductase induced by menadione in Escherichia coli

It was found that when Escherichia coli is grown in the presence of 0.2-0.3 mM menadione (2-methyl-1,4-naphthoquinone), an FMN-dependent NADH-quinone reductase increases more than 20-fold in the cytoplasmic fraction. The menadione-induced quinone reductase was isolated from the cytoplasmic fraction of induced cells. The purified enzyme had an Mr of 24 kDa on SDS-polyacrylamide gel electrophoresis. The enzyme required flavin as a cofactor and a half-maximum activity was obtained with 0.54 microM FMN or 16.5 microM FAD. The enzyme had a broad pH optimum at pH 7.0-8.0 and reacted with NADH, but not with NADPH. The reaction followed a ping-pong mechanism and the intrinsic Km values for NADH and menadione were estimated to be 132 microM and 2.0 microM, respectively. Dicoumarol was a simple competitive inhibitor with respect to NADH with a Ki value of 0.22 microM. The electron acceptor specificity of this enzyme was very similar to that of NAD(P)H: (quinone acceptor) oxidoreductase (EC 1.6.99.2, DT-diaphorase) from rat liver. Since menadione is reduced by the two-electron reduction pathway to menadiol, the induction of this enzyme is likely to be an adaptive response of E. coli to partially alleviate the toxicity of menadione.     

A common pathway for the activation of several molybdoenzymes in Escherichia coli K12

Three molybdoenzymes, nitrate reductase, formate benzyl-viologen oxidoreductase and trimethylamine-N-oxide reductase which form part of different systems, have been studied in a parental strain of Escherichia coli K12. When the organism is grown in the presence of 10 mM tungstate, these three enzymes are present in an inactive form which may be activated in vivo by the addition of 1 mM sodium molybdate. The mixing of soluble fractions from chlA and chlB mutants grown under the appropriate conditions leads to the activation of nitrate reductase, formate benzyl-viologen oxidoreductase and trimethylamine-N-oxide reductase. The activation of each enzyme is maximal when the mutants are grown under conditions that lead to the induction of that enzyme in the wild-type strain. The employment of purified proteins, the association factor FA and the Protein PA, which are presumed to be the products of the chlA and chlB genes, has shown that these proteins are responsible for the activation of the three enzymes during the complementation process.     

Alkaline phosphatase from adult rat femur

Four alkaline phosphatase forms from adult rat femur were distinguished on polyacrylamide gel electrophoresis: two soluble forms of Mr 165,000 and 110,000 in the water extract, and three membrane-bound forms of Mr 130,000, 110,000 and 100,000 extractable with deoxycholate. Alkaline phosphatase after SDS-treatment disintegrated into three kinds of monomers: of Mr 80,000, 65,000 and 50,000. The soluble fraction (extract I) contained subunits of Mr 80,000 and 55,000--whereas the pellet fraction (extract II), subunits of Mr 65,000 and 50,000. Since for native forms only three types of subunits were found it seems that, apart from homodimers, there are also some heterodimers composed of the Mr 65,000 and 50,000 subunits forming the native enzyme of Mr 110,000-115,000. Two denatured monomers: of Mr 80,000 and 50,000 may form two native homodimeric forms of Mr 165,000 and 100,000 while in the pellet two monomers: of Mr 65,000 and 50,000 may correspond to three native alkaline phosphatase forms: of Mr 130,000, 110,000-115,000 and 100,000. Probably the Mr 110,000-115,000 form is a heterodimer composed of subunits of Mr 65,000 and 50,000.     

Dehydroquinate synthase from Escherichia coli: purification, cloning, and construction of overproducers of the enzyme

Dehydroquinate synthase has been purified 9000-fold from Escherichia coli K-12 (strain MM294). The synthase is encoded by the aroB gene, which is carried by plasmid pLC29-47 from the Carbon-Clarke library. Construction of an appropriate host bearing pLC29-47 results in a strain that produces 20 times more enzyme than strain MM294. Subcloning of the aroB gene behind a tac promoter results in E. coli transformants that produce 1000 times more enzyme than MM294: the synthase constitutes 5% of the soluble protein of the cell. A laborious isolation from 50 g of wild-type E. coli cells yields 80 micrograms of impure enzyme, whereas 50 g of cells containing the subcloned gene yields 150 mg of homogeneous enzyme in a two-column purification. Dehydroquinate synthase is a monomeric protein of Mr 40 000-44 000. The chromosomal enzyme from E. coli K-12, the cloned enzyme encoded by the plasmid pLC29-47, and the subcloned inducible enzyme encoded by pJB14 all comigrate on polyacrylamide gel electrophoresis under denaturing conditions.     

Separation and distribution of thiosulfate-oxidizing enzyme, tetrathionate reductase, and thiosulfate reductase in extracts of marine heterotroph strain 16B.

Thiosulfate-oxidizing enzyme (TSO), tetrathionate reductase (TTR), and thiosulfate reductase (TSR) were demonstrated in cell-free extracts of the marine heterotrophic thiosulfate-oxidizing bacterium strain 16B. Extracts prepared from cells cultured aerobically in the absence of thiosulfate or tetrathionate exhibited constitutive TSO and TTR activity which resided in the soluble fraction of ultracentrifuged crude extracts. Constitutive TSO and TTR cochromatographed on DEAE-Sephadex A-50, Cellex D, Sephadex G-150, and orange A dye-ligand affinity gels. Extracts prepared from cells cultured anaerobically with tetrathionate or aerobically with thiosulfate followed by oxygen deprivation showed an 11- to 30-fold increase in TTR activity, with no increase in TSO activity. The inducible TTR resided in both the ultracentrifuge pellet and supernatant fractions and was readily separated from constitutive TSO and TTR in the latter by DEAE-Sephadex chromatography. Inducible TTR exhibited TSR activity, which was also located in both membrane and soluble extract fractions and which cochromatographed with inducible TTR. The results indicate that constitutive TSO and TTR in marine heterotroph 16B represent reverse activities of the same enzyme whose major physiological function is thiosulfate oxidation. Evidence is also presented which suggests a possible association of inducible TTR and TSR in strain 16B.     

Identity of alanine:glyoxylate aminotransferase with alanine:2-oxoglutarate aminotransferase in rat liver cytosol

Rat liver soluble fraction contained 3 forms of alanine: glyoxylate aminotransferase. One with a pI of 5.2 and an Mr of approx. 110,000 was found to be identical with cytosolic alanine:2-oxoglutarate aminotransferase. The pI 6.0 enzyme with an Mr of approx. 220,000 was suggested to be from broken mitochondrial alanine:glyoxylate aminotransferase 2 and the pI 8.0 enzyme with an Mr of approx. 80,000 enzyme from broken peroxisomal and mitochondrial alanine:glyoxylate aminotransferase 1. These results suggest that the cytosolic alanine: glyoxylate aminotransferase activity is due to cytosolic alanine: 2-oxoglutarate aminotransferase.     

Phosphorylation and modulation of the enzymic activity of native and protease-cleaved purified hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase by a calcium/calmodulin-dependent protein kinase

A Ca2+/calmodulin-dependent kinase has been purified which catalyzed the phosphorylation and concomitant inactivation of both the microsomal native (100,000 Da) and protease-cleaved purified 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase) (53,000 Da) fragments. This low molecular weight brain cytosolic Ca2+/calmodulin-dependent kinase phosphorylates histone H1, synapsin I, and purified HMG-CoA reductase as major substrates. The kinase, purified by sequential chromatography on DEAE-cellulose, calmodulin affinity resin, and high performance liquid chromatography (TSKG 3000 SW) is an electrophoretically homogeneous protein of approximately 110,000 Da. The molecular weight of the holoenzyme, substrate specificity, subunit protein composition, subunit autophosphorylation, subunit isoelectric points, and subunit phosphopeptide analysis suggest that this kinase of Mr 110,000 may be different from other previously reported Ca2+/calmodulin-dependent kinases. Maximal phosphorylation by the low molecular form of Ca2+/calmodulin-dependent kinase of purified HMG-CoA reductase revealed a stoichiometry of approximately 0.5 mol of phosphate/mol of 53,000-Da enzyme. Dephosphorylation of phosphorylated and inactivated native and purified HMG-CoA reductase revealed a time-dependent loss of 32P-bound radioactivity and reactivation of enzyme activity. Based on the results reported here, we propose that HMG-CoA reductase activity may be modulated by yet another kinase system involving covalent phosphorylation. The elucidation of a Ca2+/calmodulin-dependent HMG-CoA reductase kinase-mediated modulation of HMG-CoA reductase activity involving reversible phosphorylation may provide new insights into the molecular mechanisms involved in the regulation of cholesterol biosynthesis.     

A mitochondrial NADP+-dependent reductase related to the 4-aminobutyrate shunt. Purification, characterization, and mechanism

Succinic semialdehyde reductase, a NADP+-dependent enzyme, was purified from whole pig brain homogenates. The enzyme preparation migrates as a single protein and activity band on analytical gel electrophoresis. Succinic semialdehyde reductase (Mr 110,000) catalyzes the reduction of succinic semialdehyde to 4-hydroxybutyrate. The equilibrium constant of the reaction is Keq = 5.8 X 10(7) M-1 at pH 7 and 25 degrees C. The inhibition kinetic patterns obtained when 4-hydroxybutyrate or substrate analogs are used as inhibitors of the reaction catalyzed by the reductase are consistent with an ordered sequential mechanism, in which the coenzyme NADPH adds to the enzyme before the aldehyde substrate. A specific aldehyde reductase was also purified to homogeneity from brain mitochondria preparations. Its catalytic properties are identical to those of the enzyme isolated from whole brain homogenates. It is postulated that two enzymes, i.e. a NAD+-dependent dehydrogenase and a NADP+-dependent reductase, participate in the metabolism of succinic semialdehyde in the mitochondria matrix.     

Isolation and characterisation of a soluble active fragment of hydrogenase isoenzyme 2 from the membranes of anaerobically grown Escherichia coli

An active tryptic fragment of membrane-bound hydrogenase isoenzyme 2 from anaerobically grown Escherichia coli has been purified. The soluble enzyme derivative was released from the membrane fraction by trypsin cleavage. The purification procedure involved ion-exchange, hydroxyapatite and gel permeation chromatography. The enzyme derivative was purified 100-fold from the membrane fraction and the specific activity of the final preparation was 320 mumol benzyl viologen reduced min-1 mg protein-1 (H2:benzyl viologen oxidoreductase). The native enzyme derivative had an Mr of 180,000 and was composed of equimolar amounts of polypeptides of Mr 61,000 and 30,000. It possessed 12.5 mol Fe, 12.8 mol acid-labile S2- and 3.1 mol Ni/180,000 g enzyme. Antibodies were raised to the purified preparation which cross-reacted with hydrogenase isoenzyme 2 but not with isoenzyme 1 in detergent-dispersed preparations. Western immunoblot analysis revealed that isoenzyme 2 which had not been exposed to trypsin contained cross-reacting polypeptides of Mr 61,000 and 35,000. Trypsin treatment of the membrane-bound enzyme to form the soluble derivative of isoenzyme 2, therefore, cleaves a polypeptide of Mr 35,000 to produce the 30,000-Mr fragment. Trypsin treatment of the detergent-dispersed isoenzyme 2 produces the same fragmentation of the enzyme. Neither of the subunits of the enzyme revealed any immunological identity with those of hydrogenase isoenzyme 1.     

Expression of human leukotriene A4 hydrolase cDNA in Escherichia coli

The cDNA clone encoding human leukotriene A4 hydrolase was inserted into a vector pUC9 and expressed in Escherichia coli as a fusion protein containing the first 10 amino acid residues derived from a vector. The leukotriene A4 hydrolase activity was recovered in the soluble fraction of the transformants. The purified enzyme showed kinetic properties similar to the native enzyme, including inactivation by the substrate and sulfhydryl-modifying reagents. The results demonstrate that a protein with an Mr of 70,000 was expressed in Escherichia coli with a full enzyme activity and structural fidelity. Acquisition of the expression system makes it feasible to elucidate the reaction mechanism of the enzyme.     

Quaternary structure and composition of squash NADH:nitrate reductase

NADH:nitrate reductase (EC 1.6.6.1) was isolated from squash cotyledons (Cucurbita maxima L.) by a combination of Blue Sepharose and zinc-chelate affinity chromatographies followed by gel filtration on Bio-Gel A-1.5m. These preparations gave a single protein staining band (Mr = 115,000) on sodium dodecyl sulfate gel electrophoresis, indicating that the enzyme is homogeneous. The native Mr of nitrate reductase was found to be 230,000, with a minor form of Mr = 420,000 also occurring. These results indicate that the native nitrate reductase is a homodimer of Mr = 115,000 subunits. Acidic amino acids predominate over basic amino acids, as shown both by the amino acid composition of the enzyme and an isoelectric point for nitrate reductase of 5.7. The homogeneous nitrate reductase had a UV/visible spectrum typical of a b-type cytochrome. The enzyme was found to contain one each of flavin (as FAD), heme iron, molybdenum, and Mo-pterin/Mr = 115,000 subunit. A model is proposed for squash nitrate reductase in which two Mr = 115,000 subunits are joined to made the native enzyme. Each subunit contains 1 eq of FAD, cytochrome b, and molybdenum/Mo-pterin.     

Cloning and expression of the Clostridium thermosulfurogenes glucose isomerase gene in Escherichia coli and Bacillus subtilis.

The gene that encodes thermostable glucose isomerase in Clostridium thermosulfurogenes was cloned by complementation of glucose isomerase activity in a xylA mutant of Escherichia coli. A new assay method for thermostable glucose isomerase activity on agar plates, using a top agar mixture containing fructose, glucose oxidase, peroxidase, and benzidine, was developed. One positive clone, carrying plasmid pCGI38, was isolated from a cosmid library of C. thermosulfurogenes DNA. The plasmid was further subcloned into a Bacillus cloning vector, pTB523, to generate shuttle plasmid pMLG1, which is able to replicate in both E. coli and Bacillus subtilis. Expression of the thermostable glucose isomerase gene in both species was constitutive, whereas synthesis of the enzyme in C. thermosulfurogenes was inducible by D-xylose. B. subtilis and E. coli produced higher levels of thermostable glucose isomerase (1.54 and 0.46 U/mg of protein, respectively) than did C. thermosulfurogenes (0.29 U/mg of protein). The glucose isomerases synthesized in E. coli and B. subtilis were purified to homogeneity and displayed properties (subunit Mr, 50,000; tetrameric molecular structure; thermostability; metal ion requirement; and apparent temperature and pH optima) identical to those of the native enzyme purified from C. thermosulfurogenes. Simple heat treatment of crude extracts from E. coli and B. subtilis cells carrying the recombinant plasmid at 85 degrees C for 15 min generated 80% pure glucose isomerase. The maximum conversion yield of glucose (35%, wt/wt) to fructose with the thermostable glucose isomerase (10.8 U/g of dry substrate) was 52% at pH 7.0 and 70 degrees C.     

Characterization of mutant TMK368K pig citrate synthase expressed in and isolated from Escherichia coli

A cDNA that encodes pig citrate synthase (PCS) was inserted into a plasmid T7 vector and was expressed in an E. coli gltA mutant. Up to 10 mg of purified PCS was obtained from 2 liters of E. coli. The mammalian protein produced in E. coli comigrated with the enzyme purified from pig heart on a SDS-polyacrylamide gel (SDS-PAGE) with an Mr of 50,000, and reacted with a polyclonal antibody directed against pig heart citrate synthase. The Vmax and Km of the expressed PCS were indistinguishable from those of the pig heart enzyme. The PCS produced in E. coli did not contain the trimethylation modification of Lys 368, characteristic of the pig heart enzyme. These data suggest that the PCS protein produced in E. coli is catalytically similar to the enzyme purified from pig heart and methylation of Lys 368 is not essential for catalysis.     

Cloning and expression of the Clostridium thermosulfurogenes glucose isomerase gene in Escherichia coli and Bacillus subtilis

The gene that encodes thermostable glucose isomerase in Clostridium thermosulfurogenes was cloned by complementation of glucose isomerase activity in a xylA mutant of Escherichia coli. A new assay method for thermostable glucose isomerase activity on agar plates, using a top agar mixture containing fructose, glucose oxidase, peroxidase, and benzidine, was developed. One positive clone, carrying plasmid pCGI38, was isolated from a cosmid library of C. thermosulfurogenes DNA. The plasmid was further subcloned into a Bacillus cloning vector, pTB523, to generate shuttle plasmid pMLG1, which is able to replicate in both E. coli and Bacillus subtilis. Expression of the thermostable glucose isomerase gene in both species was constitutive, whereas synthesis of the enzyme in C. thermosulfurogenes was inducible by D-xylose. B. subtilis and E. coli produced higher levels of thermostable glucose isomerase (1.54 and 0.46 U/mg of protein, respectively) than did C. thermosulfurogenes (0.29 U/mg of protein). The glucose isomerases synthesized in E. coli and B. subtilis were purified to homogeneity and displayed properties (subunit Mr, 50,000; tetrameric molecular structure; thermostability; metal ion requirement; and apparent temperature and pH optima) identical to those of the native enzyme purified from C. thermosulfurogenes. Simple heat treatment of crude extracts from E. coli and B. subtilis cells carrying the recombinant plasmid at 85 degrees C for 15 min generated 80% pure glucose isomerase. The maximum conversion yield of glucose (35%, wt/wt) to fructose with the thermostable glucose isomerase (10.8 U/g of dry substrate) was 52% at pH 7.0 and 70 degrees C.     

The orientation of transhydrogenase in the inner mitochondrial membrane of rat liver

The orientation of the transmembranous enzyme, pyridine dinucleotide transhydrogenase, in the inner mitochondrial membrane of rat liver has been determined by evaluating effects of proteases on the integrity of the enzyme in mitoplasts and submitochondrial particles. Following treatment of these membranes with the nonspecific protease, proteinase K, antigenic proteolytic products were detected by immunoblot analysis using polyclonal antibody prepared against purified bovine heart enzyme. Proteinase K treatment of mitoplasts converted the 110,000 transhydrogenase monomer into a single immunoreactive species having Mr 75,000. This proteolytic product is stable to further incubation with the protease. Treatment of submitochondrial particles with proteinase K resulted in the disappearance of the 110,000 monomer and the transient formation of an intermediate product with Mr 52,000. Information from these proteolysis studies was used to construct a model of the orientation of transhydrogenase in the inner mitochondrial membrane. This model indicates that transhydrogenase (Mr 110,000) contains a core of proteolytically inaccessible proteins within the membrane (Mr 23,000) bounded by extramembranous domains on the matrix (Mr 52,000) and cytoplasmic (Mr 35,000) face of the inner mitochondrial membrane.     

Covalent linkage of 125I-insulin to a cytosolic insulin-degrading enzyme

Cytosol extracts high in insulin-degrading activity were cross-linked to 125I-insulin with the bifunctional cross-linker disuccinimidyl suberate. With cytosols from either rat muscle, liver, kidney or brain or human erythrocytes, only a single protein (Mr = 110,000) was specifically labeled. Three different lines of evidence indicated that this labeled protein is insulin-degrading enzyme, a cysteine protease which accounts for most of the insulin-degrading activity in cell extracts. Firstly, the cross-linking of 125I-insulin to this protein is inhibited by unlabeled insulin over the same concentration range of insulin which inhibits degradation. Separated insulin A and B chain were less potent at inhibiting cross-linking, whereas bovine serum albumin and cytochrome c were without effect. Secondly, antibodies to purified insulin-degrading enzyme precipitated the labeled protein in parallel with their ability to precipitate the insulin-degrading activity of the extracts. Thirdly, when the insulin-degrading activity was purified 40,000-fold from erythrocytes, this Mr 110,000 protein co-purified. These results indicate that cross-linking 125I-insulin may be a convenient method for labeling the insulin-degrading enzyme.     

CO oxidoreductase from Streptomyces strain G26 is a molybdenum hydroxylase.

CO oxidoreductase was purified to 95% homogeneity from crude mycelial extracts of Streptomyces G26. The purified preparation has a specific activity of 25.7 units/mg, a 13-fold improvement on crude soluble mycelial extracts. The native enzyme (Mr 282,000) is composed of non-identical subunits of Mr 110,000 and 33,000. It is a molybdenum hydroxylase containing 1.6 mol of FAD, 7.3 mol of Fe, 8.3 mol of acid-labile sulphide and 1.3 mol of Mo per mol of enzyme. Purified CO oxidoreductase catalyses the reduction of benzyl viologen, confirming the previously reported ability of this enzyme to interact with low-potential acceptors. Cytochrome c reduction cannot be accounted for entirely by non-enzymic reduction by superoxide radicals. NAD+ and NADP+ are not reduced, nor is clostridial ferredoxin.