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Stimulation of Carbon Dioxide Fixation in Isolated Pea Chloroplasts by Catalytic Amounts of Adenine Nucleotides ¹

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

Carbon dioxide-dependent O₂ evolution by isolated pea (Pisum sativum var. Massey Gem) chloroplasts was increased two to 12 times by the addition of ATP. O₂ evolution was also stimulated by ADP and to a lesser extent by AMP. The ATP effects were not due to broken chloroplasts present in the preparations nor was ATP acting as a phosphate source. We concluded that the adenine nucleotides were acting catalytically. The concentration of ATP required for half-maximum rate of O₂ evolution was 16 to 25 µM. The degree to which ATP stimulated O₂ evolution depended on the age of pea plants from which the chloroplasts were isolated. Spinach (Spinacia oleracea var. True Hybrid 102) chloroplasts did not show a consistent stimulation of O₂ evolution by adenine nucleotides.

The adenine nucleotide content of pea chloroplasts was not lower than that of spinach chloroplasts, but pea chloroplasts which showed a large stimulation of O₂ evolution by ATP contained an ATP-hydrolyzing reaction with rates of 10 to 50 µmol ATP hydrolyzed mg chlorophyll^–1 hour^–1. The rate of the ATP-consuming reaction was much lower in spinach chloroplasts and in chloroplasts from older pea plants which did not show large stimulation of O₂ evolution by ATP. We propose that the ATP-consuming reaction, with a high affinity for ATP, decreased the effective size of the ATP pool available for CO₂ fixation. Added adenine nucleotides could be transported into the chloroplasts increasing the concentration of internal nucleotides. Calculations showed that the adenine nucleotide transporter on the outer chloroplast membranes could operate at a sufficient rate to produce such an effect.