Equilibration of Label in Malate during Dark Fixation of CO(2) in Kalanchoë fedtschenkoi

In vitro studies of dark (14)CO(2) fixation with isolated cell aggregates of Kalanchoë fedtschenkoi showed that malate synthesized after 20 sec is predominantly (85 to 92%) labeled at carbon 4, while after 20 min only 65 to 69% of the radioactivity was located in this position. The intramolecular labeling pattern of malate could not be changed by supplementing the cells with carboxylation reaction substrates such as ribulose diphosphate or phosphoenolpyruvate. The kinetic decline of label at carbon 4 of malate occurs independently of CO(2) fixation, since 4-(14)C-labeled aspartate fed to the cells gave rise to malate labeled 62% at carbon 4 after 20 min. Furthermore, the cells were capable of converting fed malate to fumarate. It is concluded that synthesis of malate during dark CO(2) fixation is accomplished by a single carboxylation step via phosphoenolpyruvate carboxylase and labeling patterns observed in malate are a consequence of the action of fumarase.