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

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Abstract
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.

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