Purification and characterization of aromatic amine dehydrogenase from Alcaligenes xylosoxidans

Aromatic amine dehydrogenase was purified and characterized from Alcaligenes xylosoxidans IFO13495 grown on beta-phenylethylamine. The molecular mass of the enzyme was 95.5 kDa. The enzyme consisted of heterotetrameric subunits (alpha2beta2) with two different molecular masses of 42.3 kDa and 15.2 kDa. The N-terminal amino acid sequences of the alpha-subunit (42.3-kDa subunit) and the beta-subunit (15.2-kDa subunit) were DLPIEELXGGTRLPP and APAAGNKXPQMDDTA respectively. The enzyme had a quinone cofactor in the beta-subunit and showed a typical absorption spectrum of tryptophan tryptophylquinone-containing quinoprotein showing maxima at 435 nm in the oxidized form and 330 nm in the reduced form. The pH optima of the enzyme activity for histamine, tyramine, and beta-phenylethylamine were the same at 8.0. The enzyme retained full activity after incubation at 70 degrees C for 40 min. It readily oxidized various aromatic amines as well as some aliphatic amines. The Michaelis constants for phenazine methosulfate, beta-phenylethylamine, tyramine, and histamine were 48.1, 1.8, 6.9, and 171 microM respectively. The enzyme activity was strongly inhibited by carbonyl reagents. The enzyme could be stored without appreciable loss of enzyme activity at 4 degrees C for one month at least in phosphate buffer (pH 7.0).     

Purification and Characterization of Aromatic Amine Dehydrogenase from Alcaligenes xylosoxidans

Aromatic amine dehydrogenase was purified and characterized from Alcaligenes xylosoxidans IFO13495 grown on β-phenylethylamine. The molecular mass of the enzyme was 95.5 kDa. The enzyme consisted of heterotetrameric subunits (α₂β₂) with two different molecular masses of 42.3 kDa and 15.2 kDa. The N-terminal amino acid sequences of the α-subunit (42.3-kDa subunit) and the β-subunit (15.2-kDa subunit) were DLPIEELXGGTRLPP and APAAGNKXPQMDDTA respectively. The enzyme had a quinone cofactor in the β-subunit and showed a typical absorption spectrum of tryptophan tryptophylquinone-containing quinoprotein showing maxima at 435 nm in the oxidized form and 330 nm in the reduced form. The pH optima of the enzyme activity for histamine, tyramine, and β-phenylethylamine were the same at 8.0. The enzyme retained full activity after incubation at 70 °C for 40 min. It readily oxidized various aromatic amines as well as some aliphatic amines. The Michaelis constants for phenazine methosulfate, β-phenylethylamine, tyramine, and histamine were 48.1, 1.8, 6.9, and 171 μM respectively. The enzyme activity was strongly inhibited by carbonyl reagents. The enzyme could be stored without appreciable loss of enzyme activity at 4 °C for one month at least in phosphate buffer (pH 7.0).     

Purification and some properties of glyceraldehyde 3-phosphate dehydrogenase fromSynechococcus sp.

Glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.13) was purified 386 fold to apparent homogeneity from the thermophilic cyanobacteriumSynechococcus sp. grown at optimum light intensities in batch cultures. The molecular mass of the tetrameric form of the enzyme was 160 kDa as determined by gel filtration and sucrose gradient centrifugation in a phosphate buffer containing DTT. The pH optimum for the oxidation of NADPH was broad (6–8) and the enzyme had a pI of 4.5. The turnover number was 36,000 min^–1 at 40° C. The activation energy was 12.4 Kcal for t>29° C and 20.6 Kcal for t<29° C. The specific absorption coefficient, A _{280 mm} ^{1% 1cm} of the pure enzyme in phosphate buffer at pH 6.8 was 15.2.By SDS gel electrophoresis molecular masses of 78 kDa and 39 kDa were found, indicating that the purified enzyme is a tetramer, probably a homotetramer.When Tris was used as buffer in the homogenization and phosphate and DTT were omitted, a high molecular form with a molecular mass above 500 kDa was found. This form was less active than the purified tetrameric form. Acetone and other organic solvents stimulated the native enzyme several fold.     

Characterization of an inducible phenylserine aldolase from Pseudomonas putida 24-1

An inducible phenylserine aldolase (L-threo-3-phenylserine benzaldehyde-lyase, EC 4.1.2.26), which catalyzes the cleavage of L-3-phenylserine to yield benzaldehyde and glycine, was purified to homogeneity from a crude extract of Pseudomonas putida 24-1 isolated from soil. The enzyme was a hexamer with the apparent subunit molecular mass of 38 kDa and contained 0.7 mol of pyridoxal 5' phosphate per mol of the subunit. The enzyme exhibited absorption maxima at 280 and 420 nm. The maximal activity was obtained at about pH 8.5. The enzyme acted on L-threo-3-phenylserine (Km, 1.3 mM), l-erythro-3-phenylserine (Km, 4.6 mM), l-threonine (Km, 29 mM), and L-allo-threonine (Km, 22 mM). In the reverse reaction, threo- and erythro- forms of L-3-phenylserine were produced from benzaldehyde and glycine. The optimum pH for the reverse reaction was 7.5. The structural gene coding for the phenylserine aldolase from Pseudomonas putida 24-1 was cloned and overexpressed in Escherichia coli cells. The nucleotide sequence of the phenylserine aldolase gene encoded a peptide containing 357 amino acids with a calculated molecular mass of 37.4 kDa. The recombinant enzyme was purified and characterized. Site-directed mutagenesis experiments showed that replacement of K213 with Q resulted in a loss of the enzyme activity, with a disappearance of the absorption maximum at 420 nm. Thus, K213 of the enzyme probably functions as an essential catalytic residue, forming a Schiff base with pyridoxal 5'-phosphate.     

Existence of aa3-type ubiquinol oxidase as a terminal oxidase in sulfite oxidation of Acidithiobacillus thiooxidans

It was found that Acidithiobacillus thiooxidans has sulfite:ubiquinone oxidoreductase and ubiquinol oxidase activities in the cells. Ubiquinol oxidase was purified from plasma membranes of strain NB1-3 in a nearly homogeneous state. A purified enzyme showed absorption peaks at 419 and 595 nm in the oxidized form and at 442 and 605 nm in the reduced form. Pyridine ferrohaemochrome prepared from the enzyme showed an alpha-peak characteristic of haem a at 587 nm, indicating that the enzyme contains haem a as a component. The CO difference spectrum of ubiquinol oxidase showed two peaks at 428 nm and 595 nm, and a trough at 446 nm, suggesting the existence of an aa(3)-type cytochrome in the enzyme. Ubiquinol oxidase was composed of three subunits with apparent molecular masses of 57 kDa, 34 kDa, and 23 kDa. The optimum pH and temperature for ubiquinol oxidation were pH 6.0 and 30 degrees C. The activity was completely inhibited by sodium cyanide at 1.0 mM. In contrast, the activity was inhibited weakly by antimycin A(1) and myxothiazol, which are inhibitors of mitochondrial bc(1) complex. Quinone analog 2-heptyl-4-hydoroxyquinoline N-oxide (HOQNO) strongly inhibited ubiquinol oxidase activity. Nickel and tungstate (0.1 mM), which are used as a bacteriostatic agent for A. thiooxidans-dependent concrete corrosion, inhibited ubiquinol oxidase activity 100 and 70% respectively.     

Isolation and characterization of a novel superoxide dismutase from fungal strain Humicola lutea 110.

A novel thermostable MnSOD was purified to electrophoretic homogeneity from the fungal strain Humicola lutea 110. The preparation of the pure metalloenzyme was performed using treatment with acetone followed by ion exchange and gel permeation chromatography. We found that the activity of this enzyme comprises about 80% of the total superoxide dismutase activity in the crude extract, containing two proteins: MnSOD and Cu/ZnSOD. The MnSOD has a molecular mass of approximately 76 kDa and 7200 U/mg protein specific activity. It is a tetrameric enzyme with four identical subunits of 18 860 Da each as indicated by SDS-PAGE, amino acid analysis and mass spectrometry. N-terminal sequence analysis of MnSOD from the fungal strain revealed a high degree of structural homology with enzymes from other eukaryotic sources. Physicochemical properties were determined by absorption spectroscopy and circular dichroism measurements. The UV absorption spectrum was typical for an MnSOD enzyme, but displayed an increased absorption in the 280 nm region (epsilon280 = 10.4 mM(-1). cm(-1)), attributed to aromatic amino acid residues. The CD data show that MnSOD has two negative Cotton effects at 208 and 222 nm allowing the calculation of its helical content. The ellipticity at 222 nm is 6800 deg. x m(2) x dmol(-1) and thus similar to the values reported for other MnSODs. The MnSOD from H. lutea 110 is stable over a wide range of pH (4.5-8), even in the presence of EDTA. The enzyme is thermostable at 70-75 degrees C, and more stable than MnSODs from other sources.     

Petunia peroxidase a: isolation, purification and characteristics

The fast-moving anionic peroxidase isoenzyme variant PRXa was purified from leaves of petunia (Petunia hybrida). Over 1300-fold purification was achieved by subjecting extracellular extracts to two sequential acetone precipitations and resuspending the pellets at pH 5.0 and pH 8.0, respectively, followed by gel filtration and chromatofocusing. The purified enzyme had an absorbance ratio (A405 nm/A280 nm) of 3.6, a molecular mass of about 37 kDa and a pI of 3.8. Three molecular forms with slightly different molecular masses were separated by concanavalin-A--Sepharose affinity chromatography, indicating that these three forms differ in their carbohydrate moieties. The absorption spectrum of PRXa had maxima at 496 and 636 nm and a Soret band at 405 nm. Spectra of compounds I and IV were obtained by titrating a batch of PRXa stored for several months at -20 degrees C with H2O2. The addition of 1 mol H2O2/mol freshly purified PRXa caused the formation of compound II, indicating that freshly isolated PRXa contains a bound hydrogen donor which is lost upon storage. Compound III was obtained from both preparations in the presence of excess H2O2. The pH optimum of PRXa for the reaction with H2O2 and guaiacol was 5.0 and its specific activity 61 mkat/g protein. Among various aromatic compounds, coniferyl alcohol was polymerized by PRXa to presumed lignin-like material. The extracellular localization and high affinity of PRXa for the cinnamic acid derivatives suggest that this isoenzyme functions in the polymerization or cross-linking of lignin in the plant cell wall.     

Purification and biochemical characterization of some of the properties of recombinant human kynureninase.

Recombinant human kynureninase (L-kynurenine hydrolase, EC 3.7.1.3) was purified to homogeneity (60-fold) from Spodoptera frugiperda (Sf9) cells infected with baculovirus containing the kynureninase gene. The purification protocol comprised ammonium sulfate precipitation and several chromatographic steps, including DEAE-Sepharose CL-6B, hydroxyapatite, strong anionic and cationic separations. The purity of the enzyme was determined by SDS/PAGE, and the molecular mass verified by MALDI-TOF MS. The monomeric molecular mass of 52.4 kDa determined was > 99.99% of the predicted molecular mass. A UV absorption spectrum of the holoenzyme resulted in a peak at 432 nm. The optimum pH was 8.25 and the enzyme displayed a strong dependence on the ionic strength of the buffer for optimum activity. This cloned enzyme was highly specific for 3-hydroxykynurenine (Km = 3.0 microm +/- 0.10) and was inhibited by L-kynurenine (Ki = 20 microm), d-kynurenine (Ki = 12 microm) and a synthetic substrate analogue D,L-3,7-dihydroxydesaminokynurenine (Ki = 100 nm). The activity/concentration profile for kynureninase from this source was sigmoidal in all instances. There appeared to be partial inhibition by substrate, and excess pyridoxal 5'-phosphate was found to be inhibitory.     

Purification and characterization of methylamine oxidase induced in Aspergillus niger AKU 3302

Crude extract of Aspergillus niger AKU 3302 mycelia incubated with methylamine showed a single amine oxidase activity band in a developed polyacrylamide gel that weakly cross-reacted with the antibody against a copper/topa quinone-containing amine oxidase (AO-II) from the same strain induced by n-butylamine. Since the organism cannot grow on methylamine and the already known quinoprotein amine oxidases of the organism cannot catalyze oxidation of methylamine, the organism was forced to produce another enzyme that could oxidize methylamine when the mycelia were incubated with methylamine. The enzyme was separated and purified from the already known two quinoprotein amine oxidases formed in the same mycelia. The purified enzyme showed a sharp symmetric sedimentation peak in analytical ultracentrifugation showing S20,w0 of 6.5s. The molecular mass of 133 kDa estimated by gel chromatography and 66.6 kDa found by SDS-PAGE confirmed the dimeric structure of the enzyme. The purified enzyme was pink in color with an absorption maximum at 494 nm. The enzyme readily oxidized methylamine, n-hexylamine, and n-butylamine, but not benzylamine, histamine, or tyramine, favorite substrates for the already known two quinoprotein amine oxidases. Inactivation by carbonyl reagents and copper chelators suggested the presence of a copper/topa quinone cofactor. Spectrophotometric titration by p-nitrophenylhydrazine showed one reactive carbonyl group per subunit and redox-cyclic quinone staining confirmed the presence of a quinone cofactor. pH-dependent shift of the absorption spectrum of the enzyme-p-nitrophenylhydrazone (469 nm at neutral to 577 nm at alkaline pH) supported the identity of the cofactor with topaquinone. Nothern blot analysis indicated that the methylamine oxidase encoding gene is largely different from the already known amine oxidase in the organism.     

Overexpression,one-step purification,and characterization of a type II cholesterol oxidase from a local isolate <Emphasis Type="Italic">Rhodococcus</Emphasis> sp. PTCC 1633

A 1.6 kb gene encoding a cholesterol oxidase (choR) from a local isolate, Rhodococcus sp. PTCC 1633 was cloned into pET23a and the highly expressed recombinant enzyme was purified from the cell lysate of IPTG-induced Escherichia coli BL21(DE3)pLysS with one-step absorption on cholesterol. The purified protein had a molecular mass of 55 kDa, isoelectric point at about pH 9.0 and absorption peaks at 280, 380 and 460 nm, indicating that the enzyme is a flavoprotein. The optimum pH and temperature for the recombinant enzyme were 7.0 and 50°C, respectively. Steady-state kinetic revealed that the cholesterol oxidase had a K _m of 32 μM. This study is the first report concerning expression and one-step purification of a gene encoding cholesterol oxidase from Rhodococcus spp. This study revealed that this enzyme is a type II cholesterol oxidase.     

Purification and Some Properties of An Aldehyde Oxidase from Streptomyces Rimosus ATCC10970

Summary An aldehyde oxidase was purified from a cell-free extract of Streptomyces rimosus ATCC10970 to an electrophoretically homogeneous state. The molecular mass of the native enzyme was estimated to be 150 kDa by a gel filtration. SDS-polyacryamide gel electrophoresis showed that the enzyme consisted of three non-identical subunits with molecular masses of 79, 39 and 23 kDa. The absorption spectrum revealed a distinctive feature as an enzyme belonging to the xanthine oxidase family with maxima at 277, 325, 365, 415, 450, 480, and 550 nm. A variety of aliphatic and aromatic aldehydes were oxidized, but nitrogen-containing heterocyclic compounds were not. Among the substrates tested, n-heptanal was most rapidly acted on. Its optimum pH and temperature were pH 7.0 and 30 °C, respectively.     

Purification and characterization of thermostable aspartate aminotransferase from a thermophilic Bacillus species.

Aspartate aminotransferase (EC 2.6.1.1) was purified to homogeneity from cell extracts of a newly isolated thermophilic bacterium, Bacillus sp. strain YM-2. The enzyme consisted of two subunits identical in molecular weight (Mr, 42,000) and showed microheterogeneity, giving two bands with pIs of 4.1 and 4.5 upon isoelectric focusing. The enzyme contained 1 mol of pyridoxal 5'-phosphate per mol of subunit and exhibited maxima at about 360 and 415 nm in absorption and circular dichroism spectra. The intensities of the two bands were dependent on the buffer pH; at neutral or slightly alkaline pH, where the enzyme showed its maximum activity, the absorption peak at 360 nm was prominent. The enzyme was specific for L-aspartate and L-cysteine sulfinate as amino donors and alpha-ketoglutarate as an amino acceptor; the KmS were determined to be 3.0 mM for L-aspartate and 2.6 mM for alpha-ketoglutarate. The enzyme was most active at 70 degrees C and had a higher thermostability than the enzyme from Escherichia coli. The N-terminal amino acid sequence (24 residues) did not show any similarity with the sequences of mammalian and E. coli enzymes, but several residues were identical with those of the thermoacidophilic archaebacterial enzyme recently reported.     

Cloning and characterization of monofunctional catalase from photosynthetic bacterium Rhodospirillum rubrum S1

In this study, an approx. 2.5-kb gene fragment including the catalase gene from Rhodospirillum rubrum S1 was cloned and characterized. The determination of the complete nucleotide sequence revealed that the cloned DNA fragment was organized into three open reading frames, designated as ORF1, catalase, and ORF3 in that order. The catalase gene consisted of 1,455 nucleotides and 484 amino acids, including the initiation and stop codons, and was located 326 bp upstream in the opposite direction of ORF1. The catalase was overproduced in Escherichia coli UM255, a catalase-deficient mutant, and then purified for the biochemical characterization of the enzyme. The purified catalase had an estimated molecular mass of 189 kDa, consisting of four identical subunits of 61 kDa. The enzyme exhibited activity over a broad pH range from pH 5.0 to pH 11.0 and temperature range from 20 degrees C to 60 degrees C. The catalase activity was inhibited by 3-amino-1,2,4-triazole, cyanide, azide, and hydroxylamine. The enzyme's K(m) value and V(max) of the catalase for H2O2 were 21.8 mM and 39,960 U/mg, respectively. Spectrophotometric analysis revealed that the ratio of A406 to A280 for the catalase was 0.97, indicating the presence of a ferric component. The absorption spectrum of catalase-4 exhibited a Soret band at 406 nm, which is typical of a heme-containing catalase. Treatment of the enzyme with dithionite did not alter the spectral shape and revealed no peroxidase activity. The combined results of the gene sequence and biochemical characterization proved that the catalase cloned from strain S1in this study was a typical monofunctional catalase, which differed from the other types of catalases found in strain S1.     

Existence of aa 3-Type Ubiquinol Oxidase as a Terminal Oxidase in Sulfite Oxidation of Acidithiobacillus thiooxidans

It was found that Acidithiobacillus thiooxidans has sulfite:ubiquinone oxidoreductase and ubiquinol oxidase activities in the cells. Ubiquinol oxidase was purified from plasma membranes of strain NB1-3 in a nearly homogeneous state. A purified enzyme showed absorption peaks at 419 and 595 nm in the oxidized form and at 442 and 605 nm in the reduced form. Pyridine ferrohaemochrome prepared from the enzyme showed an α-peak characteristic of haem a at 587 nm, indicating that the enzyme contains haem a as a component. The CO difference spectrum of ubiquinol oxidase showed two peaks at 428 nm and 595 nm, and a trough at 446 nm, suggesting the existence of an aa ₃-type cytochrome in the enzyme. Ubiquinol oxidase was composed of three subunits with apparent molecular masses of 57 kDa, 34 kDa, and 23 kDa. The optimum pH and temperature for ubiquinol oxidation were pH 6.0 and 30 °C. The activity was completely inhibited by sodium cyanide at 1.0 mM. In contrast, the activity was inhibited weakly by antimycin A₁ and myxothiazol, which are inhibitors of mitochondrial bc ₁ complex. Quinone analog 2-heptyl-4-hydoroxyquinoline N-oxide (HOQNO) strongly inhibited ubiquinol oxidase activity. Nickel and tungstate (0.1 mM), which are used as a bacteriostatic agent for A. thiooxidans-dependent concrete corrosion, inhibited ubiquinol oxidase activity 100 and 70% respectively.     

Procerain,a stable cysteine protease from the latex of Calotropis procera

A protease was purified to homogeneity from the latex of medicinal plant Calotropis procera (Family-Asclepiadaceae). The molecular mass and isoelectric point of the enzyme are 28.8 kDa and 9.32, respectively. Hydrolysis of azoalbumin by the enzyme was optimal in the range of pH 7.0-9.0 and temperature 55-60 degree C. The enzyme hydrolyses denatured natural substrates like casein, azoalbumin, and azocasein with high specific activity. Proteolytic and amidolytic activities of the enzyme were activated by thiol protease activators and inhibited by thiol protease inhibitors, indicating the enzyme to be a cysteine protease. The enzyme named as procerain, cleaves N-succinyl-Ala-Ala-Ala-p-nitroanilide but not -Ala-Ala-p-nitroanilide, -Ala p-nitroanilide and N-d-Benzoyl--Arg-p-nitroanilide and appears to be peptide length dependent. The extinction coefficient (epsilon 1% 280 nm) of the enzyme was 24.9 and it had no detectable carbohydrate moiety. Procerain contains eight tryptophan, 20 tyrosine and seven cysteine residues forming three disulfide bridges, and the remaining one being free. Procerain retains full activity over a broad range of pH 3.0-12.0 and temperatures up to 70 degree C, besides being stable at very high concentrations of chemical denaturants and organic solvents. Polyclonal antibodies against procerain do not cross-react with other related proteases. Procerain unlike most of the plant cysteine proteases has blocked N-terminal residue.     

Serine hydroxymethyltransferase from spinach leaf mitochondria. Purification and characterization

A mitochondrial serine hydroxymethyltransferase (EC 2.1.2.1) has for the first time been purified close to homogeneity from a photosynthetically active tissue, spinach ( Spinacea oleracea L. cv Viking II) leaves. The specific activity of the enzyme was 7.8 μmol (mg protein)⁻¹ min⁻¹ using L-serine as substrate. The enzyme was stable for at least 8 weeks at 4°C in the presence of folate. The pH optimum was at pH 8.5 where the enzyme had a K_m for L-serine of 0.9 m M . Carboxymethoxylamine was a strong competitive inhibitor with a K₁ of 1.4 μM. An absorption spectrum taken of the enzyme in the presence of glycine and tetrahydrofolate showed a peak at 492 nm, probably originating from a substrate-enzyme complex. The molecular weight obtained by gel filtration was 209 kDa. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis of the purified enzyme showed that the apparent molecular weight of the subunit was 53 kDa, indicating four subunits.     

Staphylococcal acid phosphatase: extensive purification and characterization of the loosely bound enzyme

Acid phosphatase of Staphylococcus aureus PS55 was eluted from the surface of these cells with 1.0 m KCl at pH 8.5 by gentle agitation at 25 C and was purified 44-fold (51% recovery) by two cycles of dialysis and gel filtration. The eluted enzyme which had a 280/260 (nm) absorbancy ratio of 0.71 required at least 0.5 m salt solution for solubilization; however, most of the purified product which had a 280/260 (nm) absorbancy ratio of 1.72 was soluble in dilute buffer solution [0.01 m tris(hydroxymethyl)aminomethane chloride, pH 8.5]. Purified acid phosphatase appeared homogeneous according to the criteria of gel filtration, starch-block electrophoresis, and analytical ultracentrifugation. In a starch block, migration was toward the cathode at pH 8.0. Maximal activity occurred at pH 5.2 to 5.3 and salt concentration had little effect on phosphatase activity up to 1.0 m KCl or NaCl. Progressive loss of enzymatic acitivity occurred at higher salt concentrations. Molecular weight of purified acid phosphatase was estimated to be 58,000.     

Purification and properties of an aryl-alcohol dehydrogenase from the white-rot fungus Phanerochaete chrysosporium

An intracellular aryl-alcohol dehydrogenase (previously referred to as aryl-aldehyde reductase) was purified from the white-rot fungus Phanerochaete chrysosporium. The enzyme reduced veratraldehyde to veratryl alcohol using NADPH as a cofactor. Other aromatic benzaldehydes were also reduced, but not aromatic ketones. Methoxy-substituted rings were better substrates than hydroxylated ones. The enzyme was also able to reduce a dimeric aldehyde (4-benzyloxy-3-methoxybenzaldehyde). The highest reduction rate was measured when 3,5-dimethoxybenzaldehyde was used as a substrate. On SDS/PAGE the purified enzyme showed one major band with a molecular mass of 47 kDa, whereas gel filtration suggested a molecular mass of 280 kDa. Polyclonal antibodies raised against the gel purified 47-kDa protein were able to immunoprecipitate the aryl-alcohol dehydrogenase indicating that its activity possibly resides entirely in this protein fragment. The pI of the enzyme was 5.2 and it was most active at pH 6.1. The aryl-alcohol dehydrogenase was partially inhibited by typical oxidoreductase inhibitors.     

Purification and characterization of ferredoxin–nitrite reductase from the eukaryotic microalga Monoraphidium braunii

Nitrite reductase (NiR; EC 1.7.7.1) from the eukaryotic microalga Monoraphidium braunii has been purified to electrophoretic homogeneity, resulting in a preparation with a specific activity of 3574 nkat mg^–1 and a purification factor of 2553-fold. The enzyme is a single polypeptide chain with a molecular mass of 63 kDa, and absorption maxima at 690, 573, 385 and 280 nm. Kinetic data indicate K_m values of 0.7 mM for nitrite, 10 μM for M. braunii ferredoxin (Fd) and 0.26 mM for methyl viologen. The enzyme showed an optimum pH of 7.5 in 100 mM Tris–HCl buffer and an optimum temperature of 40 °C. NiR activity was inhibited by the sulfhydryl reagent p-hydroxymercuribenzoate and the chelating reagent KCN. Immunological studies revealed the presence of common antigenic determinants, at the Fd-binding domain, in NiR and glutamate synthase (EC 1.4.7.1) from M. braunii.