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

Abstract

Chloroplasts from CAM-Mesembryanthemum crystallinum can transport phosphoenolpyruvate (PEP) across the envelope. The initial velocities of PEP uptake in the dark at 4.degree.C exhibied saturation kinetics with increasing external PEP uptake had a Vmax of 6.46 (.+-. 0.05) micromoles per milligram chlorophyll per hour and an apparent KmPEP of 0.148 (.+-. 0.004) millimolar. The uptake was competitively inhibited by Pi (apparent Ki = 0.19 millimolar), by glycerate 3-phosphate (apparent Ki = 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 for the presence of Pi or glycerate 3-phosphate in the external medium. It is suggested that the transport of pyruvte and PEP across the chloroplasts envelope is required during the gluconeogenic conversion of carbon from malate to storge carbohydrate in the light.