Modulation in vivo by nitrate salts of the activity and properties of phosphoenolpyruvate carboxylase in leaves of Alternanthera pungens (C < inf > 4 < /inf > plant) and A. sessilis (C < inf > 3 < /inf > species)

Abstract

Feeding K + or Na + nitrate salts in vivo enhanced the activity of phosphoenolpyruvate carboxylase (PEPC) in the leaf extracts of Alternanthera pungens (C 4 plant) and A. sessilis (C 3 species). The increase was more pronounced in A. pungens than in A. sessilis. Chloride salts increased the PEPC activity only marginally. However, the sulfate salts were either not effective or inhibitory. Feeding nitrate modulated the regulatory properties of PEPC in A. pungens, resulting in increased K I (malate) and decreased K A (glucose-6-P). The sensitivity of PEPC to malate, which gives a measure of phosphorylation status of the enzyme, indicated that feeding leaves with NO 3- enhanced the phosphorylation status of the enzyme. The reduction in PEPC activity due to cycloheximide treatment suggested that increased synthesis of PEPC protein kinase may be one of the reasons for the enhancement in PEPC activity, after the nitrate feeding. We suggest that nitrate salts could be used as a tool to modulate and analyze the properties of PEPC in C 3 and C 4 plants.