The addition of chymostatin (100 mg m'^) to the extraction and desalting media for phosphoenolpyruvate carboxylase (EC 4.1.1.31) from Zea mays L. and Setaria verticillata (L.) Beauv. (C4-plants) affected profoundly the kinetics of the enzymic activity, the rate curve became considerably more sigmoid than that of the control and, therefore, the activity at low phosphoenolpyruvate levels was much reduced. Also, the sensitivity towards malate inhibition was substantially increased in both the day and night forms of the enzyme. The above effects were observed at pH 7.0, but not at pH 8.0. Glucose-6-phosphate (5 mM in extraction and desalting media) partly counteracted the effect of chymostatin on malate sensitivity. Hence chymostatin affected directly or indirectly the enzymic dimer/tetramer equilibrium in favour of the dimer.The addition of chymostatin (100 mg m'^) to the extraction and desalting media for phosphoenolpyruvate carboxylase (EC 4.1.1.31) from Zea mays L. and Setaria verticillata (L.) Beauv. (C4-plants) affected profoundly the kinetics of the enzymic activity, the rate curve became considerably more sigmoid than that of the control and, therefore, the activity at low phosphoenolpyruvate levels was much reduced. Also, the sensitivity towards malate inhibition was substantially increased in both the day and night forms of the enzyme. The above effects were observed at pH 7.0, but not at pH 8.0. Glucose-6-phosphate (5 mM in extraction and desalting media) partly counteracted the effect of chymostatin on malate sensitivity. Hence chymostatin affected directly or indirectly the enzymic dimer/tetramer equilibrium in favour of the dimer.
Among various C4 plants we found a wide range in the level of inactivation of phosphoenolpyruvate carboxylase (PEPC) at low temperature (0 °C). The activity of the 2-fold diluted enzyme in crude leaf extracts after 60 min incubation (compared to zero time incubation) at pH 7.5, remained above 87 % at low temperatures for the species Setaria verticillata, Portulaca oleracea, and Saccharum officinarum, and between 11 and 17 % in the species Cynodon dactylon and Atriplex halimus. The enzyme exhibited intermediate levels of inactivation (42 to 58 %) for the species Amaranthus sp., Zea mays, Salsola kali, and Digitaria sanguinalis. The enzyme activity for S. verticillata was unaffected between pH 5.7 and 8.4 during incubation at room and low temperatures. Under similar conditions, the activity of the enzyme from C. dactylon was stable between pH 5.7 and 7.0 and decreased at pH above 7.0, but for Z. mays it was enhanced between pH 5.7 and 6.8 and decreased at pH above 7.0. and G. Zervoudakis ... [et al.].