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

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Abstract
CO2-dependent O2 evolution by isolated pea (Pisum sativum cv. ''Massey Gem'') chloroplasts was increased 2-12 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. It was concluded that the adenine nucleotides were acting catalytically. The concentration of ATP required for half-maximum rate of O2 evolution was 16 to 25 .mu.M. The degree to which ATP stimulated O2 evolution depended on the age of pea plants from whcih the chloroplasts were isolated. Spinach (Spinacia oleracea cv. ''True Hybrid 102'') chloroplasts did not show a consistent stimultion 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 .mu.mol ATP hydrolyzed per mg chlorophyll-1 h-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. The ATP-consuming reaction, with a high affinity for ATP, apparently 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. Calculation showed that the adenine nucleotide transporter on the outer chloroplast membranes could operate at a sufficient rate to produce such an effect.

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