Evidence for Cyclic Photophosphorylation during 14CO2 Fixation in Intact Chloroplasts

This study examines the effect of antimycin A and nitrite on (14)CO(2) fixation in intact chloroplasts isolated from spinach (Spinacia oleracea L.) leaves. Antimycin A (2 micromolar) strongly inhibited CO(2) fixation but did not appear to inhibit or uncouple linear electron transport in intact chloroplasts. The addition of small quantities (40-100 micromolar) of nitrite or oxaloacetate, but not NH(4)Cl, in the presence of antimycin A restored photosynthesis. Antimycin A inhibition, and the subsequent restoration of photosynthetic activities by nitrite or oxaloacetate, was observed over a wide range of CO(2) concentration, light intensity, and temperature. High O(2) concentration (up to 240 micromolar) did not appear to influence the extent of the inhibition by antimycin A, nor the subsequent restoration of photosynthetic activity by nitrite or oxaloacetate. Studies of O(2) exchanges during photosynthesis in cells and chloroplasts indicated that 2 micromolar antimycin A stimulated O(2) uptake by about 25% while net O(2) evolution was inhibited by 76%. O(2) uptake in chloroplasts in the presence of 2 micromolar antimycin A was 67% of total O(2) evolution. These results suggest that only a small proportion of the O(2) uptake measured was directly linked to ATP generation. The above evidence indicates that cyclic photophosphorylation is the predominant energy-balancing reaction during photosynthesis in intact chloroplasts. On the other hand, pseudocyclic O(2) uptake appears to play only a minimal role.