Thermal characteristics of C4 photosynthetic enzymes from leaves of three sugarcane species differing in cold sensitivity

Phosphoenolpyruvate carboxylase, NADP-malate dehydrogenase and NADP-malic enzyme in desalted extracts from the leaves of three sugarcane species differing in cold sensitivity were relatively stable at cold temperatures, and their Arrhenius plots appeared as straight lines. Pyruvate,Pi dikinase (PPDK) from the three species was cold-inactivated, and its Arrhenius plots exhibited a clear breakpoint around 10.6 degrees C. Analysis of cold lability of PPDK using deuterium oxide and Triton X-100 showed that the interactions between the subunits possibly involve hydrophobic bonds which would lead to cold lability. There were no apparent differences among the three sugarcane species in the thermal properties of the four C-4 photosynthetic enzymes. The results suggest that the differences in cold sensitivity of sugarcane photosynthesis may not relate to the thermal properties of C-4 photosynthetic enzymes per se.