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Reversal of 3-(3,4-Dichlorophenyl)-1,1-Dimethylurea Inhibition of Carbon Dioxide Fixation in Spinach Chloroplasts and Protoplasts by Dicarboxylic Acids ¹

^a Department of Biochemistry and Microbiology, Rutgers University, New Brunswick, New Jersey 08903

3-(3,4-Dichlorophenyl)-1,1-dimethylurea (DCMU) inhibition of ¹⁴CO₂ fixation in isolated intact spinach (Spinacia oleracea L.) chloroplasts was reversed (by about 34%) by L-malate but not by oxaloacetate (OAA). However, OAA reversed the DCMU inhibition in spinach protoplasts indicating an extrachloroplastic enzyme requirement. Extrachloroplastic OAA reduction was coupled with external dihydroxyacetone phosphate (DHAP) oxidation, and the malate formed from such coupling might then enter the chloroplasts. Evidence was presented using ruptured protoplasts that the export of recently formed 3-phosphoglyceric acid (PGA) out of chloroplasts in exchange for external DHAP was reversed by excess OAA. The PGA/DHAP shuttle across the chloroplast envelope was found to be regulated by the external concentrations of DHAP and OAA.