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  • 1
    Electronic Resource
    Electronic Resource
    Studies of diurnal variations of nitrate reductase activity in barley leaves using various assay methods (1983)
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 57 (1983), S. 0 
    Details
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: The in vitro and various modifications of the in vivo assay for nitrate reductase have been compared in order to elucidate their usefulness in studies of diurnal variations of enzyme activity in barley leaves (Hordeum vulgare L. cv. Herta). Generally, activity was low in the morning and increased rapidly during the first hours of the photoperiod. In the in vivo assay the leaf tissue was vacuum-infiltrated, whereafter either N2 was bubbled through the assay buffer (anaerobic assay), or no N2 was used (aerobic assay). Activity was 2–25 times higher in the anaerobic than in the aerobic assay. Anaerobiosis enhanced activity most during the dark period when the nitrate reductase level was low. Aerobic in vivo activity usually showed a more rapid decrease towards the end of the light period than did anaerobic activity. Addition of glucose and/or nitrate to the in vivo assay buffer usually stimulated activity more in the aerobic than in the anaerobic assay. In the morning, at the end of the dark period, these additives stimulated activity by 20–400% depending on growth and assay conditions. Later in the day stimulation was usually less, and even a slight inhibition was observed when only nitrate (0.1 M) was added. The effect of these additives on the activity patterns determined was to dampen the oscillations. The additives were therefore not advantageous when testing diurnal variations. However, when the plants were grown under relatively poor light conditions it was necessary to add nitrate and glucose to the aerobic in vivo assay buffer since activity was otherwise too low to be measured. The in vitro assay gave about 5 times higher activity than the anaerobic in vivo assay. During the last part of the dark period in vivo activity (without glucose and KNO3 in the assay buffer) decreased while in vitro activity remained constant.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1399-3054.1983.tb02300.x
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  • 2
    Electronic Resource
    Electronic Resource
    Rhythms in magnesium ion inhibition and hysteretic properties of nitrate reductase in the CAM plant Bryophyllum fedtschenkoi (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 98 (1996), S. 0 
    Details
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Nitrate reductase (NR, EC 1.6.6.1) was tested in crude extracts of leaves from Bryophyllum fedtschenkoi plants growing under alternating light/darkness as well as in excised leaves kept in continuous light or darkness. In most extracts NR activity was inhibited 20–80% by 5 mM Mg2+ A light or darkness shift (30 min darkness) during the first part of the photoperiod gave an increase in the Mg2+ inhibition and a decrease in NR activity. Magnesium ion inhibition of NR also showed diurnal variations. Strongest inhibition was found in extracts made during the latter part of the photoperiod and start of the dark period. Pre-incubation of crude extracts with ATP increased Mg2+ inhibition, indicating that phosphorylation of NR is involved in regulation of NR in Crassulacean acid metabolism (CAM) plants. In continuous light an increase in Mg2+ inhibition occurred after 20 h and 40 h, indicating a rhythm in the phosphorylation of NR. A delay in the production of nitrite in the assay (hysteresis) was generally seen in extracts susceptible to Mg2+ inhibition. The rhythms related to NR activity showed the same period length (20 h) as the rhythm in CO2 exchange. However, in contrast to the rhythm in CO2 exchange, NR rhythms were strongly damped in continuous light. In constant darkness the rhythms were even more damped. The results show that post-translational modification of CAM NR is influenced by light/darkness and by an endogenous rhythm.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1399-3054.1996.tb00685.x
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  • 3
    Electronic Resource
    Electronic Resource
    Light/dark regulation of higher plant nitrate reductase related to hysteresis and calcium/magnesium inhibition (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 91 (1994), S. 0 
    Details
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Examination of nitrate reductase (NR, EC 1.6.6.1) activity in crude extracts made from squash leaves before and after a light/dark transition, indicates the existence of two different forms of nitrate reductase; a ‘light form’ with a pH optimum of 7.8 that is not inhibited by calcium or magnesium, and a ‘dark form’ with a pH optimum of 7.6 that is strongly inhibited by calcium or magnesium. The same properties also characterise purified NR. The ‘light and dark forms’ of NR correspond to the two kinetically different forms of purified NR showing (1) linear product formation and (2) delayed product formation, i.e. hysteretic behaviour.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1399-3054.1994.tb00434.x
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  • 4
    Electronic Resource
    Electronic Resource
    Circadian rhythmicity of nitrate reductase activity in barley leaves (1984)
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 61 (1984), S. 0 
    Details
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Nitrate reductase (EC 1.6.6.1) activity showed circadian rhythmicity in the first leaf of 8–11 days old barley (Hordeum vulgare L. cv. Herta) plants. Circadian rhythms were found using both the in vitro and in vivo method for testing the enzyme activity. When the light intensity was reduced from 65 to 20 W m−2, the amplitude was smaller and the oscillations were damped sooner. In continuous darkness nitrate reductase activity decreased in a two step process. Three different light qualities were tested which all gave the same results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1399-3054.1984.tb05900.x
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  • 5
    Electronic Resource
    Electronic Resource
    Diurnal variations of nitrate reductase activity and stability in barley leaves (1983)
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 58 (1983), S. 0 
    Details
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Diurnal variations of nitrate reductase (NR) activity and stability have been studied in leaves of barley seedlings (Hordeum vulgare L. cv. Herta) grown in an 8 h light/16 h darkness regime. Stability (decay) of NR was tested both in the extracts and in the plants. In the morning, when the plants were transferred to light, NR activity increased rapidly during the first hour and then remained constant. After the photoperiod, activity decreased rapidly during the first hour of darkness and then remained fairly constant during the rest of the dark period. The high NR activity during the photoperiod was associated with low NR stability both in the extracts and in the plants. On the other hand the low NR activity during the dark period was associated with high stability in the extracts and in the plants.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1399-3054.1983.tb04166.x
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  • 6
    Electronic Resource
    Electronic Resource
    A minimal model of light-induced circadian rhythms of nitrate reductase activity in leaves of barley (1984)
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 62 (1984), S. 0 
    Details
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Observed circadian rhythms of nitrate reductase (NR) (EC 1.6.6.1) activity in leaves of barley (Hordeum vulgare L. cv. Herta) under continuous light conditions are described by a simple kinetic model. The oscillatory mechanism has been decomposed into the negative and positive feedback loops which are necessary according to present theories of chemical oscillating systems. Our results indicate that the decrease of NR activity in darkness can be considered as a reversible unimolecular conversion of the active form of NR into an inactive form, forming a negative stabilizing feedback loop. The light-induced increase of NR activity is related to a positive destabilizing feedback loop. In our treatment this process is represented as an autocatalytical reaction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1399-3054.1984.tb02804.x
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  • 7
    Electronic Resource
    Electronic Resource
    Light regulation of nitrate reductase in green leaves of higher plants (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 90 (1994), S. 0 
    Details
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Light stimulates de nova synthesis, as well as activation of higher plant nitrate reductase at the protein level. In green leaves, the light effects are mediated through photosynthetically active light by products of the Calvin cycle. A daily, light-induced increase in nitrate reduclase (NR) mRNA. protein, and activity is observed in various higher plants. Glucose or sucrose can replace light in eliciting the increase in nitrate reductase mRNA accumulation, and sonic metabolite from nitrate assimilation exerts a negative feedback on transcription of the nitrate reductase genes. The positive feedforward induced by sugars, and the negative feedback from nitrogen compound(s) apparently result in the circadian rhythms of NR mRNA observed in various plants. Nitrate reductase shows hysteretic behaviour, converting between a low and a high activity form in response to NAD(P)H. The low activity form is inhibited by calcium/ magnesium, as opposed to the high activity form that is not inhibited. Examination of nitrate reductase activity in crude extracts made from plants before and after a light-dark transition suggests the existence of two different forms of nitrate reductase: a‘light form’with a pH optimum of 7.8 in potassium phosphate buffer that is not inhibited by calcium or magnesium, and a “dark form’with li pH optimum of 7.5 that is strongly inhibited by calcium or magnesium. It is postulated that a transient lack of NADH as a result of decreased illumination can convert nitrate reductase into the less active form in situ.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1399-3054.1994.tb08822.x
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  • 8
    Electronic Resource
    Electronic Resource
    Diurnal variations of nitrite reductase, glutamine synthetase, glutamate synthase, alanine aminotransferase and aspartate aminotransferase in barley leaves (1984)
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 61 (1984), S. 0 
    Details
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Diurnal variations of in vitro activity of 5 enzymes of nitrogen metabolism were studied. Barley (Hordeum vulgare L. cv. Herta) seedlings were grown in 8 h short days, in daylight or under fluorescent lamps. During, the photoperiod nitrite reductase (EC 1.7.7.1) increased by an average of 18% in daylight and 10% under fluorescent lamps. Glutamine synthetase (EC 6.3.1.2) activity increased by 14 and 10%, respectively. The increase in enzyme activity reflected the overall increase in soluble proteins which was 8% in daylight and 3% under fluorescent lamps. Alanine aminotransferase (EC 2.6.1.2) increased by 82% in daylight and 37% under fluorescent lamps. Desalting of the extracts did not alter the enzyme activity and thus supported the assumption that changes in extractable enzyme activity are due to changes in the amount of (active) enzyme protein. Glutamate synthase (EC 1.4.7.1) activity did not show regular diurnal variations, and aspartate aminotransferase (EC 2.6.1.1) activity was almost constant.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1399-3054.1984.tb05899.x
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  • 9
    Electronic Resource
    Electronic Resource
    Regulation of nitrate reductase and phosphoenolpyruvate carboxylase activities in barley leaf protoplasts (1996)
    Springer
    Planta 200 (1996), S. 181-185 
    Details
    ISSN: 1432-2048
    Keywords: Acid loading ; Hordeum ; Light activation ; Nitrate reductase ; Phosphoenolpyruvate carboxylase ; Protoplasts
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Barley leaf protoplasts were incubated in light or darkness in the presence of various inhibitors, metabolites or weak acids/bases. Nitrate reductase (NR) and phosphoenolpyruvate carboxylase (PEPCase) were rapidly extracted from the protoplasts and assayed under sub-optimal conditions, i.e. in the presence of Mg2+ and malate, respectively. Under these conditions changes in activities are thought to reflect changes in the phosphorylation states of the enzymes. The NR was activated by illumination to 90% of its maximal activity within 10 min. Photosynthetic electron transport appeared necessary for light activation of NR since activation was inhibited by the photosynthetic electron-transport inhibitor 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU), and, additionally, an electron acceptor (HCO 3 - ) was required. The PEPCase was also activated by light. However, this activation was not prevented by DCMU or lack of HCO 3 - . Loading of protoplasts in the dark with a weak acid resulted in activation of both NR and PEPCase. For NR, full activation was completed within 5 min, whereas for PEPCase a slower, modest activation continued for at least 40 min. Incubation of protoplasts with a weak base also gave activation of PEPCase, but not of NR. On the contrary, base loading counteracted light activation of NR. Since several treatments tested resulted in the modulation of either NR or PEPCase activity, but not both, signal transduction cascades leading to changes in activities appear to be very different for the two enzymes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00208307
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  • 10
    Electronic Resource
    Electronic Resource
    Light regulation of phosphoenolpyruvate carboxylase in barley mesophyll protoplasts is modulated by protein synthesis and calcium, and not necessarily correlated with phosphoenolpyruvate carboxylase kinase activity (1996)
    Springer
    Planta 200 (1996), S. 174-180 
    Details
    ISSN: 1432-2048
    Keywords: Barley ; Hordeum ; Kinase ; Phosphoenol ; pyruvate carboxylase ; Protoplasts
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The regulation of phosphoenolpyruvate carboxylase (PEPCase, EC. 4.1.1.31) and PEPCase kinase was investigated using barley (Hordeum vulgare L.) mesophyll protoplasts. Incubation of protoplasts in the light resulted in a reduction in the sensitivity of PEPCase to the inhibitor L-malate; PEPCase from protoplasts incubated in the light for 1 h was inhibited 48±2% by 2mM malate, whereas the enzyme from protoplasts incubated for 1 h in the dark was inhibited by 67±2%. Light-induced reduction of sensitivity of PEPCase to malate was decreased by cycloheximide (CHM), indicating the involvement of protein synthesis. The PEPCase kinase in protoplasts increased with time after isolation in darkness, and increased still further following light treatment. The increase in kinase activity in the light was sensitive to CHM. When protoplasts were illuminated in the presence of EGTA and the calcium ionophore A23187 to reduce intracellular Ca2+, the reduction in the senstivity of PEPCase to malate was enhanced, though no more PEPCase kinase activity was detected than in protoplasts illuminated in the absence of EGTA and A23187. Incubation with 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) had no effect on the light-induced reduction of sensitivity of PEPCase to malate inhibition or on light-activation of PEPCase kinase. These results indicate that there is a constitutive PEPCase kinase activity in C3 leaf tissue, that there is another kinase which is light-activated in a CHMsensitive way, that the sensitivity of PEPCase to its inhibitor may not always be correlated with apparent PEPCase kinase actvity, and that PEPCase and PEPCase kinase are regulated in a different manner in C3 protoplasts than in C4 protoplasts or leaf tissue.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00208306
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