Inactivation of maize phosphoenolpyruvate carboxylase by urea.

Phosphoenolpyruvate carboxylase purified from leaves of maize (Zea mays, L.) is sensitive to the presence of urea. Exposure to 2.5 m urea for 30 min completely inactivates the enzyme, whereas for a concentration of 1.5 m urea, about 1 h is required. Malate appears to have no effect on inactivation by urea of phosphoenolpyruvate carboxylase. However, the presence of 20 mm phosphoenolpyruvate or 20 mm glucose-6-phosphate prevents significant inactivation by 1.5 m urea for at least 1 h. The inactivation by urea is reversible by dilution. The inhibition by urea and the protective effects of phosphoenolpyruvate and glucose-6-phosphate are associated with changes in aggregation state.