A study of the in-vitro regulation of phosphoenolpyruvate carboxylase from the epidermis of Commelina communis by malate and glucose-6-phosphate

A comparison of the kinetic properties of phosphoenolpyruvate carboxylase from guard-cell and mesophyll-cell protoplasts of Commelina communis

Some kinetic properties of partially purified phosphoenolpyruvate carboxylase (PEPCase) from guard-cell and mesophyll-cell protoplasts of Commelina communis are described. The PEPCase activity inherent to each cell type was determined and the apparent K m (phosphoenolpyruvate) and K i (malate) were compared. Malate sensitivity was much higher (K i malate 0.4 mol m(-3)) in the extract of guard-cell protoplasts than in that of mesophyllcell protoplasts (K i malate 4.2 mol m(-3)). The stimulation of activity by glucose-6-phosphate in the presence of malate ('deinhibition') was also investigated in extracts from both cell types and was found to be similar to previously reported results with epidermal tissue. The effect of contamination of an extract of guard-cell protoplasts with mesophyll-cell protoplasts was measured in the presence and absence of malate. It was found that a small amount to mesophyll-cell contaminant appears to desensitize the malate inhibition of PEPCase from guard-cell protoplasts. It is concluded that experiments which use epidermal tissue to study guardcell PEPCase may give misleading information as a consequence of mesophyll contamination.

Properties of phosphoenolpyruvate carboxylase in desalted extracts from isolated guard-cell protoplasts

Properties of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) obtained from isolated guard-cell protoplasts of Vicia faba L. were determined following rapidly desalting of the extract on a Sephadex G 25 column. The activity of PEP carboxylase was measured as a function of PEP and malate concentration, pH and K(+) concentration within 2-3 min after homogenization of the guard-cell protoplasts. The activity of this enzyme was stimulated by PEP concentrations of 0.1 to 0.75 mM and by K(+) ions (12 mM), but inhibited by PEP concentrations above 1 mM and by malate. Changes in the Km(PEP) and Vmax values with increasing malate concentrations (2.5 and 5 mM) indicate that the malate level, varying in relation to the physiological state of guard cells, plays an important role in regulating the properties of phosphoenolpyruvate carboxylase.