Evidence for Cyclic Photophosphorylation during 14CO2 Fixation in Intact Chloroplasts

Abstract

The effect of antimycin A and nitrite on 14CO2 fixation in intact chloroplasts isolated from spinach (Spinacia oleracea L.) leaves was examined. Antimycin A (2 .mu.M) strongly inhibited CO2 fixation but apparently did not inhibit or uncouple linear electron transport in intact chloroplasts. The addition of small quantities (40-100 .mu.M) of nitrite or oxaloacetate, but not NH4Cl, 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 CO2 concentration, light intensity and temperature. High O2 concentration (up to 240 .mu.M) apparently did not influence the extent of the inhibition of antimycin A, nor the subsequent restoration of photosynthetic activity by nitrite or oxaloacetate. Studies of O2 exchanges during photosynthesis in cells and chloroplasts indicated that 2 .mu.M antimycin A stimulated O2 uptake by about 25% while net O2 evolution was inhibited by 76%. O2 uptake in chloroplasts in the presence of 2 .mu.M antimycin A was 67% of total O2 evolution. Only a small proportion of the O2 uptake measured was apparently directly linked to ATP generation. Cyclic photophosphorylation is the predominant energy-balancing reaction during photosynthesis in intact chloroplasts. Pseudocyclic O2 uptake apparently plays only a minimal role.