Inhibition of the photosynthetic activities of isolated spinach chloroplasts by phosphonate compounds

The effects of three closely related phosphonate compounds on several photosynthetic activities of isolated chloroplasts were investigated. Phosphonoformic and phosphonopropionic acid were found to inhibit both CO₂ fixation and the reduction of 3-phosphoglyceric acid, with CO₂ fixation being more sensitive. In contrast, phosphonoacetic acid was only slightly inhibitory. The lack of inhibition appeared to be due to its inability to enter the stroma via the phosphate translocator. Measurements of changes in stromal metabolite levels following the inhibition of CO₂ fixation by either phosphonoformic or phosphonopropionic acid indicated that the activity of ribulose bisphosphate carboxylase/oxygenase was reduced. Studies with the isolated enzyme confirmed that both of these compounds were effective competitive inhibitors of the carboxylase activity of the enzyme.