Inhibition of CO 2 fixation by adenosine 5′-diphosphate and the role of phosphate transport in isolated pea chloroplasts

Abstract

CO 2-dependent oxygen evolution by isolated pea ( Pisum sativum cv Massey Gem) chloroplasts was inhibited by ADP. AMP inhibited oxygen evolution to a greater extent than ADP while no inhibition was observed with ATP. Oxygen evolution with ADP or AMP was more sensitive to phosphate inhibition than in the absence of added adenine nucleotides. Photosynthetic intermediates (ribose 5-phosphate or 3-phosphoglycerate) partially relieved the inhibition by ADP. Pyrophosphate also inhibited oxygen evolution, yet, with PP 1 plus ADP, oxygen evolution was stimulated. Inhibition of the phosphate transporter using p-chloromercuriphenylsulfonic acid did not reverse ADP inhibition but the subsequent reversal of ADP inhibition by PP 1 was prevented. Pyrophosphate relieved inhibition of oxygen evolution by P i both in the presence and in the absence of added adenine nucleotides. Uptake of ADP into pea chloroplasts apparently results in an inhibition of 3-phosphoglycerate kinase, and the subsequent accumulation of 3-phosphoglycerate and decrease in internal phosphate make the chloroplasts highly susceptible to inhibition by phosphate. We propose that PP i enters chloroplasts via the phosphate transporter and is hydrolyzed internally by the chloroplast inorganic pyrophosphatase. The increase in internal phosphate concentration would prevent loss of photosynthetic intermediates from the chloroplast, under these conditions, and thus stimulate oxygen evolution.