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Articles

Relationship between the Level of Adenine Nucleotides and the Carboxylation Activity of Illuminated Isolated Spinach Chloroplasts

A Study with Antimycin A

^a Physiologie Cellulaire Végétale, Centre National de la Recherche Scientifique, Université Paris-Sud, 91405 Orsay, France

The changes in the levels of intact spinach (Spinacia oleracea L.) chloroplast adenine nucleotides during the time course of light-dependent CO₂ fixation were determined with respect to the effect of antimycin A. This study demonstrated that antimycin A lowered the rate of ATP formation during the induction period of carboxylation. While the steady state levels of ATP and the energy-charge value also decreased in the presence of antimycin, the concomitant increase of the CO₂ fixation activities insured higher ATP turnover rates. Changes in the labeling of CO₂ fixation products during the lag phase suggested a stepwise activation of the Calvin cycle, with fructose 1,6-diphosphate, and ribulose 5-phosphate kinase being activated before ribulose 1,5-diphosphate carboxylase. The possible mechanisms of the enhancement of CO₂ fixation activity by antimycin A in relation to its action on photophosphorylation during the lag phase are discussed.