Development of NAD(P)H: and NADH:Nitrate Reductase Activities in Soybean Cotyledons

The cotyledons of soybean begin to develop photosynthetic capacity shortly after emergence. The cotyledons develop nitrate reductase (NR) activity in parallel with an increase in chlorophyll and a decrease in protein. In crude extracts of 5- to 8-day-old cotyledons, NR activity is greatest with NADH as electron donor. In extracts of older cotyledons, NR activity is greatest with NADPH. Blue-Sepharose was used to purify and separate the NR activities into two fractions. When the blue-Sepharose was eluted with NADPH, NR activity was obtained which was most active with NADPH as electron donor. Assays of the NADPH-eluted NR with different concentrations of nitrate revealed that the highest activity was obtained in 80 millimolar KNO₃. Thus, this fraction has properties similar to the low nitrate affinity NAD(P)H:NR of soybean leaves. When 5- to 8-day-old cotyledons were extracted and purified, further elution of the blue-Sepharose with KNO₃, subsequent to the NADPH elution, yielded an NR fraction most active with NADH. Assays of this fraction with different nitrate concentrations revealed that this NR had a higher nitrate affinity and was similar to the NADH:NR of soybean leaves. The KNO₃-eluted NR fraction which was purified from the extracts of 9- to 14-day-old cotyledons, was most active with NADPH. The analysis of these fractions prepared from the extracts of older cotyledons indicated that residual NAD(P)H:NR contaminated the NADH:NR. Despite this complication, the pattern of development of the purified NR fractions was consistent with the changes observed in the crude extract NR activities. It was concluded that NADH:NR was most active in young cotyledons and that as the cotyledons aged the NAD(P)H:NR became more active.     

Synthesis of Nitrate Reductase in Chlorella: II. EVIDENCE FOR SYNTHESIS IN AMMONIA-GROWN CELLS

Antiserum was prepared against nitrate reductase (EC 1.6.6.1) purified to homogeneity from Chlorella vulgaris Beijerinck. Both crude antiserum and anti-nitrate reductase antibodies prepared from it were used as re-agents to study the synthesis of nitrate reductase. Cell extracts from cultures which were grown with ammonia salts as the sole source of nitrogen contained almost no active enzyme. These extracts did contain material which binds to antibody and is thus immunologically related to purified nitrate reductase. The presence of this cross reacting material in cell extracts was detected by the ability of these extracts to (a) lower the titer of antisera; (b) form a biphasic precipitin curve with purified antibody; and (c) increase the peak height of a standard amount of purified nitrate reductase in rocket immunoelectrophoresis assay. These results suggest that ammonia-grown cells contain nitrate reductase precursor protein.     

Purification and Kinetics of Higher Plant NADH:Nitrate Reductase

Squash cotyledon (Cucurbita pepo L.) NADH:nitrate reductase (NR) was purified 150-fold with 50% recovery by a single step procedure based on the affinity of the NR for blue-Sepharose. Blue-Sepharose, which is prepared by direct coupling of Cibacron blue to Sepharose, appears to bind squash NR at the NADH site. The NR can be purified in 2 to 3 hours to a specific activity of 2 μmol of NADH oxidized/minute • milligram of protein. Corn (Zea mays L.) leaf NR was also purified to a specific activity of 6.9 μmol of NADH oxidized/minute • milligram of protein using a blue-Sepharose affinity step. The blue-Sepharose method offers the advantages of a rapid purification of plant NR to a high specific activity with reasonable recovery of total activity.

The kinetic mechanism of higher plant NR was investigated using these highly purified squash and corn NR preparations. Based on initial velocity and product inhibition studies utilizing both enzymes, a two-site ping-pong mechanism is proposed for NR. This kinetic mechanism incorporates the concept of the reduced NR transferring electrons from the NADH site to a physically separated nitrate site.

     

Immunochemical characterization and localization of nitrate reductase in norflurazon-treated soybean cotyledons

Immunochemical procedures were used to characterize and localize NADH:nitrate reductase (NR; EC 1.6.6.1) in cotyledons of norflurazon-treated soybeans [ Glycine max (L.) Merr. cv. 'Hill']. Antiserum prepared to NR isolated from Chlorella strongly reacted against NR from norflurazon-treated cotyledons. This serum inhibited the NR activity in crude extracts of norflurazon-treated soybean cotyledons by 98% even at a 1:2000 dilution of crude serum. Pre-immune serum had no effect on the activity. These data indicate that there are similar antigenic determinants at the active site of both Chlorella and norflurazon-treated soybean NR. Whole cotyledons were homogenized in lithium dodecyl sulfate-containing buffer, electrophoretically separated and blotted to nitrocellulose. When the blots were reacted with the anti-NR serum only a single protein (M_r= 98 kdalton) was visualized. Immunofluorescence studies on fixed tissue sections revealed intense fluorescence in the cytoplasm. Weaker reactions were associated with organelles tentatively identified as plastids. Pre-immune serum controls were completely unstained using immunocytochemical procedures.     

Regulation of Corn Leaf Nitrate Reductase : I. Immunochemical Methods for Analysis of the Enzyme's Protein Component

NADH:nitrate reductase was extracted from corn leaves (Zea mays L. W64A × W182E) and purified on blue Sepharose. After the nitrate reductase was further purified by polyacrylamide gel electrophoresis, it was used to immunize mice and a rabbit. Western blots of crude leaf extracts were used to demonstrate monospecificity of the mouse ascitic fluids and the rabbit antiserum. The electrophoretic properties of purified corn and squash NADH:nitrate reductases in both native and denatured states were shown to be similar using western blotting with mouse ascitic fluid. The corn leaf enzyme has a 115,000 polypeptide subunit like that of squash. Western blots could detect 3 to 10 nanograms of nitrate reductase protein. But the detection of proteolytic degradation products using western blotting was inconsistent and remains to be established. An enzyme-linked immunosorbent assay (ELISA) was developed for quantifying nitrate reductase protein in the crude extracts of corn leaves. Using a standard curve based on nitrate reductase activity, the ELISA for corn nitrate reductase could detect 0.5 to 10 nanograms of nitrate reductase protein and was adequately sensitive for quantitative analysis of nitrate reductase in crude extracts of leaves even when activity levels were very low. When the ELISA was used to compare the nitrate reductase protein content of corn roots and leaves, these tissues were estimated to contain 0.24 to 0.5 and 4 to 5 micrograms nitrate reductase protein/gram root and leaf, respectively.     

Properties of Bromphenol Blue as an Electron Donor for Higher Plant NADH: Nitrate Reductase

Bromphenol blue, which was reduced with dithionite, was found to support nitrate reduction catalyzed by squash NADH:nitrate reductase at a rate about 5 times greater than NADH with freshly prepared enzyme and 10 times or more with enzyme having been frozen and thawed. Kinetic analysis of bromphenol blue as a substrate for squash nitrate reductase yielded apparent K_m values of 60 micromolar for bromphenol blue at 10 millimolar nitrate and 500 micromolar for nitrate at 0.2 millimolar bromphenol blue. With the same preparation of enzyme the apparent K_m values were 9 micromolar for NADH at 10 millimolar nitrate and 50 micromolar nitrate at 0.1 millimolar NADH. Bromphenol blue was found to be a noncompetitive inhibitor versus NADH with a K_i of 0.3 millimolar. When squash NADH:nitrate reductase activity was inactivated with p-hydroxymercuribenzoate or denatured by heating at 40°C, the bromphenol blue nitrate reductase activity was not lost. These results were taken to indicate that bromphenol blue and NADH donated electrons to nitrate reductase at different sites. When monoclonal antibodies prepared against corn and squash nitrate reductases were used to inhibit the nitrate reductase activities supported by NADH, bromphenol blue, and methyl viologen, differential inhibition was found which tended to indicate that the three electron donors were interacting with the enzyme at different sites. One monoclonal antibody prepared against squash nitrate reductase inhibited all three activities of both corn and squash nitrate reductase. It appears this antibody may bind to a highly conserved antigenic site in the nitrate binding region of the enzyme.     

Inactivation of Sunflower NADH:Nitrate Reductase by White Light-Activated Rose Bengal

Chemical modification of purified nitrate reductase (NR) from sunflower leaves by white light-irradiated rose bengal was studied. NADH:NR activity was inhibited by light-activated rose bengal in both a concentration- and time-dependent manner. MV:NR activity was less sensitive to inhibition than NADH:NR activity, especially when the enzyme was preincubated with NADH. Preincubation of the enzyme with FAD protected inhibition of NADH:NR activity but not the MV:NR activity. These results suggest that sunflower NR contains sensitive histidine residue which interacts with reduced FAD during catalytic electron transfer. Most importantly, NADH-reduced NR was more sensitive to the irradiated dye, indicating that conformation of the oxidized and reduced enzyme forms were different.     

Regulation of Corn Leaf Nitrate Reductase : II. Synthesis and Turnover of the Enzyme's Activity and Protein

The appearance and disappearance of NADH:nitrate reductase (NR) in the leaves of corn (Zea mays L. W64A × W182E) were studied using activity assays, an enzyme-linked immunosorbent assay (ELISA) and western blotting. N-starved, etiolated corn plants were treated with nutrients containing either 35 millimolar NH₄-nitrate or K-nitrate and immediately thereafter given light. The curve for enhancement of NR activity had three phases: 1 hour lag, 5 hour rapid increase, and steady state. The pattern for NR protein, as measured with the ELISA, also had three phases, but the increase was more rapid and the steady state was established earlier. To differentiate the effects of N nutrition from those of light, N-starved etiolated plants were given N nutrients 4 hours before light. During the dark pretreatment, NR activity and protein increased. When the light was turned on the NR activity and protein increased very rapidly without a lag. Western blots of polyacrylamide gels of native and denatured crude extracts showed that NADH:NR polypeptide was absent prior to treatment with N nutrients, but appeared after nitrate was given in dark or light. A low level of NR activity was found in N-starved, etiolated plants and it was shown by western blotting to be an NR form with a different electrophoretic mobility in nondenaturing gels. Since this minor NR form was not influenced by either nitrate or light, it was designated a constitutive NR. Dark decay of NR activity and protein was also studied. After the plants which had been in light with N nutrients for 24 hours were transferred to dark, the NR activity dropped by 30% within 1 hour, but the NR protein did not decrease. This inactivation of NR was further supported by returning the plants to the light after 1.5 hours of dark and finding the activity restored without change in NR protein. After the initial activity drop, a parallel decrease in NR activity and protein was observed, which was likely due to irreversible degradation by proteolysis.     

Immunochemical Characterization of Nitrate Reductase Forms from Wild-Type (cv Williams) and nr(1) Mutant Soybean

Soybean (Glycine max L. Merr.) leaves contain two forms of nitrate reductase (NR)—NAD(P)H:NR and NADH:NR. Wild-type (cv Williams), nr₁ mutant and an unrelated cultivar (Prize) were grown with either no N source or with nitrate. Crude extracts were assayed for NR activities and the enzyme forms were purified on blue Sepharose. Analyses were done by polyacrylamide gel electrophoresis and `Western blotting' using antibodies specific for NR. NAD(P)H:NR was identified as the constitutive NR present in wild-type and Prize, but was absent from the mutant. All three soybean lines contained nitrate-inducible NADH:NR with highest activity at pH 7.5. The results showed that NAD(P)H:NR and constitutive NR were one in the same and confirmed the presence of NADH:NR with pH 7.5 optimum.     

Sunflower Nitrate Reductase: Purification and Characterization

A single isoform, NADH: nitrate reductase (NR), was purified 500 folds from sunflower leaves by affinity chromatography on Blue Sepharose CL-6B. Purified NR had a pH optima of 7.25 and a molecular weight of 210 kD. In SDS-PAGE, two bands of 47 and 56 kD were obtained. NADH: ferric citrate reductase activity was copurified with NR with a specific activity of 2. The Vmax of NADH: ferric citrate reductase was 8.69 units mg^-1 protein and the apparent Km for ferric citrate was 0.435 mM.     

Monoclonal antibody-based immunoaffinity chromatography for purifying corn and squash NADH: nitrate reductases. Evidence for an interchain disulfide bond in nitrate reductase

NADH: nitrate reductase (EC 1.6.6.1) (NR) is present in small amounts in plant tissues and its polypeptide in inherently labile. Consequently, NR is difficult to purify. We have generated 20 monoclonal antibodies (McAb) for corn and squash NR and selected two for use in immunoaffinity chromatography. Squash McAb CM 15(11) and corn McAb ZM 2(69)9, which both bind corn and squash NR, were covalently coupled to Sepharose and used for purification of NR with elution of the purified enzyme by a pH 11 buffer. Although this procedure yielded highly purified NR, its activity was diminished by the pH 11 treatment. When corn leaf crude extract was applied to McAb CM 15(11)-Sepharose, NR bound and could be eluted in homogeneous form by its substrate, NADH. Corn leaf NR prepared by substrate elution retained a high level of NADH: NR activity. Immunoaffinity-purified corn and squash NR were shown to have an interchain disulfide bond as well as a reactive thiol group. These results are discussed in relation to the recently obtained sequences of NR clones and suggestions made for site-directed mutagenesis experiments to aid in identifying the cysteine residues of NR associated with these features of the enzyme.     

Biochemical Characterization of Soybean Mutants Lacking Constitutive NADH:Nitrate Reductase

Two nitrate reductase (NR) mutants were selected for low nitrate reductase (LNR) activity by in vivo NR microassays of M₂ seedlings derived from nitrosomethylurea-mutagenized soybean (Glycine max [L.] Merr. cv Williams) seeds. The mutants (LNR-5 and LNR-6) appeared to have normal nitrate-inducible NR activity. Both mutants, however, showed decreased NR activity in vivo and in vitro compared with the wild-type. In vitro FMNH₂-dependent nitrate reduction and Cyt c reductase activity of nitrate-grown plants, and nitrogenous gas evolution during in vivo NR assays of urea-grown plants, were also decreased in the mutants. The latter observation was due to insufficient generation of nitrite substrate, rather than some inherent difference in enzyme between mutant and wild-type plants. When grown on urea, crude extracts of LNR-5 and LNR-6 lines had similar NADPH:NR activities to that of the wild type, but both mutants had very little NADH:NR activity, relative to the wild type. Blue Sepharose columns loaded with NR extract of urea-grown mutants and sequentially eluted with NADPH and NADH yielded a NADPH:NR peak only, while the wild-type yielded both NADPH: and NADH:NR peaks. Activity profiles confirmed the lack of constitutive NADH:NR in the mutants throughout development. The results provide additional support to our claim that wild-type soybean contains three NR isozymes, namely, constitutive NADPH:NR (c₁NR), constitutive NADH:NR (c₂NR), and nitrate-inducible NR (iNR).     

Characterization of Nitrate Reductase from Corn Leaves (Zea mays cv W64A x W182E) : Two Molecular Forms of the Enzyme

The primary leaves from corn seedlings grown for 6 days were harvested, frozen with liquid N₂ and extracted in a Tris buffer (pH 8.5, 250 millimolar) containing 1 millimolar dithiothreitol, 10 millimolar cysteine, 1 millimolar EDTA, 20 micromolar flavin adenine dinucleotide and 10% (v/v) glycerol. Nitrate reductase (NR) in the crude extract was stable for several days at 0°C and for several months at −80°C. The enzyme was purified using (NH₄)₂SO₄ fractionation, brushite-hydroxyl-apatite chromatography and blue-sepharose affinity chromatography. The enzyme was eluted from the blue-sepharose column with a linear gradient of NADH (0-100 micromolar) or with 0.3 molar KNO₃. About 10% of the original activity was recovered with NADH (NADH-NR). It had a specific activity of about 60 to 70 units (micromoles NO₂⁻ per minute per milligram protein). A sequential elution with NADH followed by KNO₃ (0.3 molar) or KCl (0.3 molar) yielded 2 peaks. Rechromatography of each peak gave two peaks again. These results indicate that we are dealing with two forms of the same enzyme rather than two different NR proteins. The two NRs had different molecular weights as judged by chromatography on Toyopearl. The NADH-NR was more sensitive than the NO₃⁻-NR to antibody prepared against barley leaf NR. In Ouchterlony assays a single precipitin line, with completely fused boundaries, was observed.     

Nitrate reductase inactivator from Spirodela polyrhiza (L.) Schleiden

NADH:nitrate reductase (EC 1.6.6.1) activity in the crude extract from Spirodela polyrhiza was relatively labile in vitro. Inclusion of polyvinylpolypyrrolidone into the extraction medium had only a slight effect on the stability of the enzyme, whereas addition of 3 % casein, azocasein, or other proteins to the extraction medium greatly increased the nitrate reductase (NR) activity. Various protease inhibitors were tested for their ability to prevent the loss of NR activity in vitro. Iodoacetate and para-chloromercuric benzoate, the thiol-protease inhibitors, as well as pepstatin, the aspartic-protease inhibitor had no effect on stability of the nitrate reductase. EDTA had a slight stimulatory effect, whereas 5 mM o-phenantroline, another inhibitor of the metallo-proteases increased the activity of nitrate reductase. The highest enzyme activity was found in the presence of phenylmethylsulphonyl fluoride and di-isopropyl phosphorofluoridate both being serine-protease inhibitors. The protease-like inactivator was separated from Spirodela polyrhiza by ammonium sulfate fractionation and acid treatment (pH 4.0). After centrifugation the protein of inactivator in supernatant adjusted to pH 7.5 was removed. When this fraction was examined by electrophoresis in polyacrylamide which copolymerized with edestin, the protein of the nitrate reductase inactivator remained at the cathode. Fractions containing a protein of inactivator degraded casein to products soluble in trichloroacetic acid. Inhibition of the inactivator proteolytic activity by phenylmethylsulphonyl fluoride and di-isopropyl phosphorofluoridate but not by other reagents (thiol- and metallo-protease inhibitors) suggested the involvement of a serine residue at its active site. The inactivator fraction from Spirodela polyrhiza resulted in a loss of the nitrate reductase activity in crude extracts from both cucumber and corn seedlings. A biochemical nature a protein of the nitrate reductase inactivator from S. polyrhiza is discussed.     

Purification of Squash NADH:Nitrate Reductase by Zinc Chelate Affinity Chromatography

NADH:nitrate reductase (EC 1.6.6.1) was isolated and purified from the green cotyledons of 5-day-old squash seedlings (Cucurbita maxima L.). The 10-hour purification procedure consisted of two steps: direct application of crude enzyme to blue Sepharose and specific elution with NADH followed by direct application of this effluent to a Zn²⁺ column with elution by decreasing the pH of the phosphate buffer from 7.0 to 6.2. The high specific activity (100 micromoles per minute per milligram protein) and high recovery (15-25%) of electrophoretically homogeneous nitrate reductase show that the enzyme was not damaged by exposure to the bound zinc. With this procedure, homogeneous nitrate reductase can be obtained in yields of 0.5 milligram per kilogram cotyledons.     

Temperature dependence of nitrate reductase in the psychrophilic unicellular alga Koliella antarctica and the mesophilic alga Chlorella sorokiniana

Temperature responses of nitrate reductase (NR) were studied in the psychrophilic unicellular alga, Koliella antarctica, and in the mesophilic species, Chlorella sorokiniana. Enzymes from both species were purified to near homogeneity by Blue Sepharose (Pharmacia, Uppsala, Sweden) affinity chromatography and high-resolution anion-exchange chromatography (MonoQ; Pharmacia; Uppsala, Sweden). Both enzymes have a subunit molecular mass of 100 kDa, and K. antarctica NR has a native molecular mass of 367 kDa. NR from K. antarctica used both NADPH and NADH, whereas NR from C. sorokiniana used NADH only. Both NRs used reduced methyl viologen (MVH) or benzyl viologen (BVH). In crude extracts, maximal NADH and MVH-dependent activities of cryophilic NR were found at 15 and 35 degrees C, respectively, and retained 77 and 62% of maximal activity, respectively, at 10 degrees C. Maximal NADH and MVH-dependent activities of mesophilic NR, however, were found at 25 and 45 degrees C, respectively, with only 33 and 23% of maximal activities being retained at 10 degrees C. In presence of 2 microM flavin adenine dinucleotide (FAD), activities of cryophilic NADH:NR and mesophilic NADH:NR were stable up to 25 and 35 degrees C, respectively. Arrhenius plots constructed with cryophilic and mesophilic MVH:NR rate constants, in both presence or absence of FAD, showed break points at 15 and 25 degrees C, respectively. Essentially, similar results were obtained for purified enzymes and for activities measured in crude extracts. Factors by which the rate increases by raising temperature 10 degrees C (Q10) and apparent activation energy (E(a)) values for NADH and MVH activities measured in enzyme preparations without added FAD differed slightly from those measured with FAD. Overall thermal features of the NADH and MVH activities of the cryophilic NR, including optimal temperatures, heat inactivation (with/without added FAD) and break-point temperature in Arrhenius plots, are all shifted by about 10 degrees C towards lower temperatures than those of the mesophilic enzyme. Transfer of electrons from NADH to nitrate occurs via all three redox centres within NR molecule, whereas transfer from MVH requires Mo-pterin prosthetic group only; therefore, our results strongly suggest that structural modification(s) for cold adaptation affect thermodynamic properties of each of the functional domains within NR holoenzyme in equal measure.     

Activation of Thalassiosira pseudonana nadh: Nitrate reductase

NADH: nitrate reductase (NR) has been isolated in both active and inactive states, and both could be purified using blue-Sepharose. The state of activation of the enzyme depended on the presence or absence of agents such as cysteine or EDTA during the assay. When NR was assayed, the addition of activator before NADH led to maximum activity. Therefore, the reduced NR appeared to be inactivated during the assay in the absence of activator. Inactivation may have occurred via a mechanism similar to the inactivation of lipoamide dehydrogenase by trace metals, such as CU²⁺. The activation of NR by cysteine or EDTA was interpreted as protection of the reduced enzyme due to chelation of trace metals in the assay solution by the activators.     

Synthesis and Degradation of Nitrate Reductase during the Cell Cycle of Chlorella Sorokiniana

Studies on the diurnal variations of nitrate reductase (NR) activity during the life cycle of synchronized Chlorella sorokiniana cells grown with a 7:5 light-dark cycle showed that the NADH:NR activity, as well as the NR partial activities NADH:cytochrome c reductase and reduced methyl viologen:NR, closely paralleled the appearance and disappearance of NR protein as shown by sodium dodecyl sulfate gel electrophoresis and immunoblots. Results of pulse-labeling experiments with [³⁵S]methionine further confirmed that diurnal variations of the enzyme activities can be entirely accounted for by the concomitant synthesis and degradation of the NR protein.     

Substrates regulate the phosphorylation status of nitrate reductase

The effect of substrates on the phosphorylation status of nitrate reductase (NR; EC 1.6.6.1) was studied. The enzyme was obtained from the first leaf of 7-day-old oat (Avena sativa L. cv. Suregrain) plants, grown in the light. When desalted crude extracts were incubated with ATP, NR was strongly phosphorylated, as evidenced by the inhibition of the enzyme's activity in the presence of Mg²⁺. NR sensitivity to Mg²⁺ remained unchanged when 10 mM nitrate was added to crude extracts after ATP. Addition of nitrate before or simultaneously with ATP slightly decreased Mg²⁺ inhibition of NR, which was strongly diminished in the presence of 10 mM NO₃^?+ 100 µM NADH. Incubation with NADH alone did not affect the enzyme's susceptibility to Mg²⁺ inhibition. When ammonium sulfate was added to crude extracts, NR was recovered in a 0-40% saturation fraction (F1). After incubation of F1 with ATP, the sensitivity of the enzyme to Mg²⁺ inhibition remained low, but it strongly increased after mixing F1 with a 45-60% saturation fraction (F2) suggesting that also in oats an additional factor (inactivating protein, IP), which probably binds to phospho-NR, would be required to keep the phosphorylated enzyme inactive in a +Mg²⁺ medium. Addition of 10 mM NO₃^?+ 100 µM NADH together with desalted F2 did not prevent Mg²⁺ inhibition suggesting that NO₃^? did not interfere with IP binding to phospho-NR. Again, incubation of F1 with both substrates during in vitro phosphorylation kept the enzyme active after adding F2, even in the presence of Mg²⁺, After in vitro phosphorylation, NR in crude extract was hardly reactivated when incubated alone or in the presence of 10 mM NO₃^? at 30°C. On the other hand, a strong and very rapid reactivation was found when the extract was incubated with both nitrate and NADH. Microcystine, an inhibitor of types 1 and 2A phosphoprotein phosphatases, inhibited the reactivation of phospho-NR induced by the substrates. The results presented here show that the substrates could prevent NR phosphorylation and induce the enzyme's dephosphorylation, but they were effective only after their binding to the NR protein. Thereby, they seemed to affect the NR protein itself and not the phosphatase- or the kinase-proteins. It has been reported that nitrate binding to the enzyme's active site induces conformational changes in the NR protein. We propose that this conformational change would prevent NR phosphorylation, by converting the enzyme into a form in which the site recognized by the protein kinase is no longer accessible, and, simultaneously, stimulate NR dephophorylation by allowing the specific phosphatases to recognize NR.     

Nitrate Reductases from Wild-Type and nr(1)-Mutant Soybean (Glycine max [L.] Merr.) Leaves : I. Purification, Kinetics, and Physical Properties

NADH:nitrate reductase (EC 1.6.6.1) and NAD(P)H:nitrate reductase (EC 1.6.6.2) were purified from wild-type soybean (Glycine max [L.] Merr., cv Williams) and nr₁-mutant soybean plants. Purification included Blue Sepharose- and hydroxylapatite-column chromatography using acetone powders from fully expanded unifoliolate leaves as the enzyme source.

Two forms of constitutive nitrate reductase were sequentially eluted with NADPH and NADH from Blue Sepharose loaded with extract from wild-type plants grown on urea as sole nitrogen source. The form eluted with NADPH was designated c₁NR, and the form eluted with NADH was designated c₂NR. Nitrate-grown nr₁ mutant soybean plants yielded a NADH:nitrate reductase (designated iNR) when Blue Sepharose columns were eluted with NADH; NADPH failed to elute any NR form from Blue Sepharose loaded with this extract. Both c₁NR and c₂NR had similar pH optima of 6.5, sedimentation behavior (s_20,w of 5.5-6.0), and electrophoretic mobility. However, c₁NR was more active with NADPH than with NADH, while c₂NR preferred NADH as electron donor. Apparent Michaelis constants for nitrate were 5 millimolar (c₁NR) and 0.19 millimolar (c₂NR). The iNR from the mutant had a pH optimum of 7.5, s_20,w of 7.6, and was less mobile on polyacrylamide gels than c₁NR and c₂NR. The iNR preferred NADH over NADPH and had an apparent Michaelis constant of 0.13 millimolar for nitrate.

Thus, wild-type soybean contains two forms of constitutive nitrate reductase, both differing in their physical properties from nitrate reductases common in higher plants. The inducible nitrate reductase form present in soybeans, however, appears to be similar to most substrateinduced nitrate reductases found in higher plants.