PEP Carboxylases from Two C4Species ofPanicumwith Markedly Different Susceptibilities to Cold Inactivation

摘要

Phosphoenolpyruvate carboxylase (PEPC) (EC 4.1.1.31) assayed in extracts ofPanicum maximumJacq. loses up to 50%of its activity after incubation for 60 minutes at 0°C while the enzyme fromP. miliaceumL. is completely stable under these conditions. Following dilution at room temperature the enzyme fromP. maximumis labile, while that fromP. miliaceumis stable. TheP. maximumenzyme 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, Pidikinase from maize, provide no protection ofP. maximumPEPC against either cold or dilution. The chaotropic ion SCN-causes partial inactivation of the enzyme fromP. miliaceumin 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