Formation of 3-phosphoglyceric acid by carbon dioxide fixation with spinach leaf enzymes.

The work of Calvin and his coworkers (1,2) has established that 3-phosphoglyceric acid’ is an early product of photosynthesis formed by a “dark” carboxylation reaction. Studies of Fager (3) with spinach chloroplasts have also demonstrated PGA formation as the result of a reaction of CO?: with an unidentified acceptor. Horecker and Weissbach were the first investigators to demonstrate the occurrence of a carboxylation reaction in soluble preparations of spinach leaf leading to the formation of carboxyllabeled PGA, in the presence of Cl402 and ribosed-phosphate, or to that of predominantly P-labeled PGA in the presence of CO2 and ribosed-phosphate-1-C14. Attempts in this laboratory to determine whether soluble spinach leaf preparations would bring about a reversal of the Horecker reaction, i.e. a decarboxylation or oxidative decarboxylation of PGA, in the presence of possible “glycolaldehyde” acceptors such as triose phosphate and hydrogen acceptors such as DPN+ or TPN+, gave negative results; there was no net production of CO2, or exchange of Cl402 with PGA, under a variety of experimental conditions. In the course of these experiments it was noted that, in t’he presence of ribose-5-phosphate, the spinach preparations fixed