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Pyrophosphate Inhibition of Carbon Dioxide Fixation in Isolated Pea Chloroplasts by Uptake in Exchange for Endogenous Adenine Nucleotides ¹

^a Department of Botany, University of Adelaide, Adelaide 5001, South Australia

Carbon dioxide-dependent O₂ evolution by isolated pea (Pisum sativum) chloroplasts was inhibited by inorganic pyrophosphate (PPi). Oxygen evolution was also inhibited by high concentrations of orthophosphate (Pi) and the inhibition was relieved by 3-phosphoglycerate. In contrast, the inhibition by PPi was not relieved by 3-phosphoglycerate, indicating that hydrolysis of PPi and accumulation of inhibitory concentrations of Pi were not occurring. In agreement with this suggestion, the percentage of ¹⁴C-labeled products diffusing out of the chloroplasts was increased by Pi but not by PPi. The inhibition of O₂ evolution by PPi was reversed by ATP. The concentration of PPi required for 50% inhibition was 1.2 to 1.4 mM and the subsequent stimulation by ATP was half-maximal at 16 to 25 µM. Carbon dioxide-dependent O₂ evolution by spinach chloroplasts, or chloroplasts isolated from older pea plants, was not significantly inhibited by PPi.

Chloroplasts were preloaded with ¹⁴C-ATP and release of the labeled nucleotides was measured to assess the activity of adenine nucleotide transport across the inner chloroplast envelope membrane. A rapid exchange was promoted by the addition of exogenous ATP. Addition of PPi also resulted in a release of endogenous nucleotides. We suggest that PPi inhibits CO₂ fixation by entering the chloroplast in exchange for endogenous adenine nucleotides via the transporter on the inner envelope membrane. The subsequent depletion of the internal adenine nucleotide pool would result in decreased CO₂ fixation due to insufficient ATP. Addition of ATP to PPi-inhibited chloroplasts apparently results in uptake of catalytic amounts of ATP and restoration of the internal adenine nucleotide pool thus relieving the inhibition of CO₂ fixation.

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