Persistent circadian rhythms in the phosphorylation state of phosphoenolpyruvate carboxylase from Bryophyllum fedtschenkoi leaves and in its sensitivity to inhibition by malate

Phosphoenolpyruvate carboxylase (EC 4.1.1.31; PEPCase) from Bryophyllum fedtschenkoi leaves has previously been shown to exist in two forms in vivo. During the night the enzyme is phosphorylated and relatively insensitive to feedback inhibition by malate whereas during the day the enzyme is dephosphorylated and more sensitive to inhibition by malate. These properties of PEPCase have now been investigated in leaves maintained under constant conditions of temperature and lighting. When leaves were maintained in continuous darkness and CO₂-free air at 15°C, PEPCase exhibited a persistent circadian rhythm of interconversion between the two forms. There was a good correlation between periods during which the leaves were fixing respiratory CO₂ and periods during which PEPCase was in the form normally observed at night. When leaves were maintained in continuous light and normal air at 15°C, starting at the end of a night or the end of a day, a circadian rhythm of net uptake of CO₂ was observed. Only when these constant conditions were applied at the end of a day was a circadian rhythm of interconversions between the two forms of PEPCase observed and the rhythms of enzyme interconversion and CO₂ uptake did not correlate in phase or period.Abbreviations CAM Crassulacean acid metabolism - FW fresh weight - PEPCase phosphoenolpyruvate carboxylase - RuBPCase ribulose-1,5-bisphosphate carboxylase To whom correspondence should be addressed.