Equilibration of Label in Malate during Dark Fixation of CO2 in Kalanchoë fedtschenkoi

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Abstract
In vitro studies of dark 14CO2 fixation with isolated cell aggregates of Kalanchoe fedtschenkoi showed that malate synthesized after 20 s was predominantly (85-92%) labeled with C 4, while after 20 min only 65-69% 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 and phosphoenolpyruvate. The kinetic decline of label at C 4 of malate occurs independently of CO2 fixation, since 4-14C-labeled aspartate fed to the cells gave rise to malate labeled 62% at C 4 after 20 min. The cells were capable of converting fed malate to fumarate. Synthesis of malate during dark CO2 fixation apparently is accomplished by a single carboxylation step via phosphoenolpyruvate carboxylase and labeling patterns observed in malate are a consequence of the action of fumarase.