14CO2 Labeling Studies of 2-Carboxyarabinitol 1-Phosphate Synthesis

2-Carboxyarabinitol 1-phosphate (CAIP) is involved in the regulation of ribulose 1,5-bisphosphate carboxylase (rubisco) activity in many plants, but the biochemical pathway for its synthesis is unknown. In an attempt to induce synthesis of 14C-CAIP in vivo, intact leaflets of Phaseolus vulgaris were pulse-labeled with 14CO2 and chased with 12CO2 under conditions which result in leaves accumulating unlabeled CAIP. Sugar-phosphates were isolated from leaf extracts by anion exchange chromatography and constituent metabolites were separated by 2D-thin-layer chromatography. No l4C-labeled CAIP was recovered from extracts prepared from leaves experiencing a range of exposure/chase conditions, a range of leaf/plant ages, or from other species differing in their ability to accumulate unlabeled CAIP. Appropriate control experiments indicated no loss of 14C-standard which had been added at the time of killing the leaf. The data suggest that carboxyarabinitol 1-phosphate is not synthesized in vivo as some “misfire” catalysis by rubisco, and that the precursor to its synthesis is far “downstream” of CO2 fixation.