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  • 1
    Electronic Resource
    Electronic Resource
    Intact mesophyll protoplasts from Zea mays as a source of phosphoenolpyruvate carboxylase unaffected by extraction: Advantages and limitations (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 80 (1990), S. 0 
    Details
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Isolated intact mesophyll protoplasts from Zea mays L. were used as an enzyme source for studying properties of phosphoenolpyruvate (PEP) carboxylase (EC 4.1 1 31) just after release from cells into the reaction medium. After the injection of protoplasts into the assay mixture, an initial lag of activity was observed, mainly due to the time necessary for complete disruption of protoplasts by the osmotic shock. The final specific activity obtained was ca 18 μmol mg-1 of liberated protein min-1, a value comparable to that usually achieved after arduous purification. Under the assay conditions employed, the chloroplasts were not disrupted and the retention of their proteins, together with the use of purified mesophyll protoplasts, were obviously the reasons for the high specific activity obtained. The activity and properties of phosphoenolpyruvate carboxylase stored in isolated protoplasts were stable for at least 24 h at 5°C. The main difference between the protoplast-derived and the routinely extracted enzyme was the sensitivity to malate inhibition, which was partially lost in the extracted phosphoenolpyruvate carboxylase; no difference was found in the Km(PEP). The stress imposed by the protoplast isolation procedure diminished the sensitivity of the enzyme to malate inhibition, so that it can be inferred that the real malate sensitivity of pbosphocnolpyruvale carboxylase is even greater and that it is grossly underestimated with routinely extracted enzyme.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1399-3054.1990.tb05685.x
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  • 2
    Electronic Resource
    Electronic Resource
    Effects of glycerol on the in vitro stability and regulatory activation/inactivation of pyruvate, orthophosphate dikinase of Zea mays L. (1990)
    Springer
    Photosynthesis research 26 (1990), S. 9-17 
    Details
    ISSN: 1573-5079
    Keywords: activation/inactivation ; glycerol ; pyruvate ; orthophosphate dikinase ; Zea mays L.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Glycerol stabilizes the activity of pyruvate, orthophosphate dikinase extracted from darkened or illuminated maize leaves. It serves as a better protectant of activity than dithiothreitol for the active day-form and the glycerol concentration needed for full protection is inversely related to the level of protein. The night-form of the enzyme is also protected by glycerol not only against inactivation, but also against partial reactivation in storage. Glycerol does not prevent the Pi-dependent activation nor the ADP-dependent inactivation of pyruvate, orthophosphate dikinase, but the rates of both processes are substantially decreased. The ability of the inactive night-form for Pi-dependent activation is also sustained by glycerol for at least 2 h at 20°C, apparently through stabilization of the labile regulatory protein.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00048972
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  • 3
    Electronic Resource
    Electronic Resource
    Artifacts in the assay of maize leaf phosphoenolpyruvate carboxylase activity due to its instability (1988)
    Springer
    Photosynthesis research 18 (1988), S. 317-325 
    Details
    ISSN: 1573-5079
    Keywords: enzyme stability ; maize ; phosphoenolpyruvate carboxylase
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract When the assay of maize leaf phosphoenolpyruvate carboxylase (EC 4.1.1.31) activity is started with phosphoenolpyruvate, much lower reaction rates are obtained as compared to the enzyme-initiated reaction. The difference is due to the lability of the dilute enzyme in the absence of its substrate and is increased with incubation time in the absence of substrate or stabilizers. The activation of the enzyme by glucose-6-phosphate is overestimated with the substrate-initiated assay since a part of the apparent activation is due to stabilization of the enzymic activity by this effector during the minus-substrate preincubation. In contrast, the inhibitory effect of malate is underestimated when the reaction is started with the substrate. The enzyme-initiated assay is recommended provided that the necessary corrections for apparent activity in the absence of substrate and for inactivation during the assay at low substrate levels are made.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00034836
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  • 4
    Electronic Resource
    Electronic Resource
    Effects of temperature and photosynthetic inhibitors on light activation of C4-phosphoenolpyruvate carboxylase (1988)
    Springer
    Photosynthesis research 16 (1988), S. 233-242 
    Details
    ISSN: 1573-5079
    Keywords: light activation ; PEPCase ; photosynthetic inhibitors ; Setaria verticillata (L.) Beauv. ; temperature
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Phosphoenolpyruvate carboxylase from leaves of the C4 plant Setaria verticillata (L.) Beauv. is activated by light; day levels of activity are reached after 30 minutes of illumination. Photoactivation is prevented by inhibitors of photosynthetic electron flow or of photophosphorylation and by D,L-glyceraldehyde, which inhibits the reductive pentose phosphate pathway. Although the extractable activity in the dark is not affected by temperature the photoactivation is prevented when both illumination and extraction are done under low temperature (5 C). High temperature (30 C) during either illumination or extraction is needed for activation. Once the enzyme is photoactivated at 30 C, a transfer of the leaves to 5 C does not abolish the extra activity. The results suggest that both unimpaired electron flow and photophosphorylation are prerequisites for the activation of phosphoenolpyruvate carboxylase. Low temperature apparently suppresses either the transport to the cytoplasm of a photosynthetic intermediate or the activating reaction itself. The inclusion of phosphoenolpyruvate in the extraction medium increases the night activity. On the basis of the available information, it is suggested that phosphoenolpyruvate could be the activator in vivo. In that case, the activation of phosphoenolpyruvate carboxylase would depend on internal CO2 level and prior photoactivation of both pyruvate, orthophosphate, dikinase and NADP malate dehydrogenase.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00028842
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  • 5
    Electronic Resource
    Electronic Resource
    Assaying for pyruvate, orthophosphate dikinase activity: Necessary precautions with phosphoenolpyruvate carboxylase as coupling enzyme (1990)
    Springer
    Photosynthesis research 24 (1990), S. 183-188 
    Details
    ISSN: 1573-5079
    Keywords: assaying technique ; glucose-6-phosphate ; phosphoenolpyruvate carboxylase ; pyruvate ; Pi (orthophosphate) dikinase
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Phosphoenolpyruvate carboxylase (EC 4.1.1.31), used as a coupling enzyme in the assay of the pyruvate, orthophosphate dikinase (EC 2.7.9.1) forward reaction, is a serious limiting factor for the overall rate when added at a level of 0.2–0.3 unit/ml of assay medium. Nonlimiting assay conditions are obtained by either increasing the level of the coupling enzyme to 3 units/ml or adding 6mM glucose-6-phosphate as an activator/stabilizer of phosphoenolpyruvate carboxylase.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00032598
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