This seems entirely reasonable when one considers that PGA 

 is both the immediate precursor in these reactions and the 

 primary product of carbon reduction during photosynthesis. 

 These amino acids could then supply ammonia via transami- 

 nase reactions for the synthesis of many other amino acids. 

 Holm-Hansen et al. (33) have demonstrated the presence of 

 a transaminase activity in spinach chloroplasts, which is very 

 effective in the transfer of amino groups from unlabeled ala- 

 nine to C^^-labeled pyruvic acid. 



The three-carbon precursors to these amino acids are in 

 rapid equilibrium with PGA. PEPA becomes C^^-saturated 

 during photosynthesis in C^^02 in Chlorella almost as soon 

 as PGA itself. The proposed phosphoenoloxalacetate prob- 

 ably does not exist except in enzyme complexes. Thus, by 

 the time the PGA is C^*-saturated, these amino acids are 

 being labeled as rapidly as if they were formed directly from 



It has been suggested that glutamic acid is formed dur- 

 ing photosynthesis by a carboxylation of y-aminobutyric acid 

 (34). Judging by our studies with Chlorella pyrenoidosa dur- 

 ing steady-state photosynthesis with C^''02, this reaction ap- 

 parently does not constitute a source of glutamate, since y- 

 aminobutyric acid does not become labeled, even by the time 

 the glutamic acid is 50 per cent saturated with C^* and long 

 after the rate of labeling of glutamic acid has passed its maxi- 

 mum. Clearly, a compound cannot be a precursor in a steady- 

 state system unless it is itself continuously regenerated. If 

 the reaction does occur at all, the glutamic acid so formed 

 could only be a shuttle for CO2, regenerating unlabeled y- 

 aminobutyric acid. Even so, such a carboxylation reaction 

 does not account for more than about I per cent of the car- 

 bon fixed in our studies of steady-state CO2 fixation by Chlo- 

 rella. 



One possible route from PGA to glutamic acid would 

 begin with conversion of PGA to PEPA, followed by car- 

 boxylation of PEPA to give oxalacetic acid. Condensation 

 of oxalacetic acid with acetyl CoA would give citric acid, 



34 



