Transport of Phosphoenolpyruvate by Chloroplasts from Mesembryanthemum crystallinum L. Exhibiting Crassulacean Acid Metabolism.

Chloroplasts from CAM-Mesembryanthemum crystallinum can transport phosphoenolpyruvate (PEP) across the envelope. The initial velocities of PEP uptake in the dark at 4 degrees C exhibited saturation kinetics with increasing external PEP concentration. PEP uptake had a V(max) of 6.46 (+/-0.05) micromoles per milligram chlorophyll per hour and an apparent K(mpep) of 0.148 (+/-0.004) millimolar. The uptake was competitively inhibited by Pi (apparent K(i) = 0.19 millimolar), by glycerate 3-phosphate (apparent K(i) = 0.13 millimolar), and by dihydroxyacetone phosphate, but malate and pyruvate were without effect. The chloroplasts were able to synthesize PEP when presented with pyruvate. PEP synthesis was light dependent. The prolonged synthesis and export of PEP from the chloroplasts required the presence of Pi or glycerate 3-phosphate in the external medium. It is suggested that the transport of pyruvate and PEP across the chloroplasts envelope is required during the gluconeogenic conversion of carbon from malate to storage carbohydrate in the light.