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No light activation and high malate sensitivity of phosphoenolpyruvate carboxylase in guard cell protoplasts of commelina communis l



No light activation and high malate sensitivity of phosphoenolpyruvate carboxylase in guard cell protoplasts of commelina communis l



Journal of Experimental Botany 41(230): 1103-1108



Guard cell protoplasts (GCP) were prepared from leaves of Commelina communis L. and phosphoenolpyruvate carboxylase (PEPc) activity recorded after injection of the protoplasts directly into the assay medium. The GCP were lysed immediately by the presence of Triton X-100 and a lowered osmotic concentration in the assay cuvette enabling PEPc activity to be measured with 'nascent' enzyme. There was no light activation of the enzyme with Km PEP (about 3.7 mol m-3) and Vmax being similar in light- or dark-treated protoplasts. Illumination of the GCP in the presence of CO2-free air and KCl, a treatment which is known to swell GCP, did not change the kinetics. PEPc activity at saturating PEP was very sensitive to malate inhibition, 20 mmol m-3 (the I50 value) inhibiting activity by about 50%. Inhibition was similar in light- or dark-treated protoplasts. Malate inhibition was, however, much less (I50 = 500 mmol m-3) if the enzyme source was a protoplast extract kept in the absence of glycerol. Inclusion of 20% glycerol in the extraction medium maintained the enzyme in the malate-sensitive form as occurred in the in vivo assays. The high apparent Km PEP and the high sensitivity to malate inhibition of GCP PEPc are features unlike those observed with PEPc from leaf tissues of C4 and CAM plants and from GCP extracts. PEPc activity increased slightly in the presence of KCl in the assay medium up to about 10 mol m-3 and thereafter activity slowly declined as KCl concentrations increased further.

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Accession: 007597380

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DOI: 10.1093/jxb/41.9.1103



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