PEP Carboxylases from Two C₄ Species of Panicum with Markedly Different Susceptibilities to Cold Inactivation

Phosphoenolpyruvate carboxylase (PEPC) (EC 4.1.1.31) assayed in extracts of Panicum maximum Jacq. loses up to 50% of its activity after incubation for 60 minutes at 0°C while the enzyme from P. miliaceum L. is completely stable under these conditions. Following dilution at room temperature the enzyme from P. maximum is labile, while that from P. miliaceum is stable. The P. maximum enzyme can be largely stabilized against dilution and against cold-inactivation by D2O which stabilizes hydrophobic bonds and the compatible solutes proline, betaine and trimethylamine-N-oxide. Mineral ions, previously demonstrated to be protective against cold inactivation of pyruvate, Pi dikinase from maize, provide no protection of P. maximum PEPC against either cold or dilution. The chaotropic ion SCN- causes partial inactivation of the enzyme from P. miliaceum in the cold. The possible interrelationship between inactivation by dilution and inactivation by cold is discussed. The enzyme from both species, when assayed without preincubation at low temperature, exhibits similar, slightly curvilinear Arrhenius plots; and no differences were found between the two species in the temperature dependence of photosynthesis.