thence aconitic acid, thence isocitric acid. Proceeding along 

 the Kreb's cycle, the next two steps are oxidation to oxalo- 

 succinic acid, followed by oxidation and decarboxylation 

 to give a-ketoglutaric acid. Finally, the reductive amination 

 would give glutamic acid. This pathway may be followed in 

 Chlorella pyrenoidosa in the synthesis of glutamic acid, par- 

 ticularly when the light is turned off. We suspect that it is 

 not the principal pathway during photosynthesis for two rea- 

 sons, one experimental and one theoretical. Experimentally, 

 the rates of labeling of the intermediate compounds such as 

 citric acid and ketoglutaric acid are too slow to permit them 

 to serve as precursors to the more rapidly labeled reservoir 

 of glutamic acid. Theoretically, the pathway is objectionable 

 to us as a photosynthetic route because it involves two oxida- 

 tions and a decarboxylation. 



How else might glutamic acid be formed during photo- 

 synthesis? The availability of three-carbon and two-carbon 

 compounds suggests the possibility of a simple condensation. 

 Barker and co-workers (35-37) found an enzymic pathway in 

 certain microorganisms leading from glutamic acid to py- 

 ruvic acid and acetate via citramalate, mesaconic acid, and 

 /3-methylaspartate. The reverse of this pathway might operate 

 during photosynthesis also. However, we have been unable 

 so far to find significant amounts of radiocarbon in either /?- 

 methylaspartic acid or mesaconic acid in Chlorella which 

 were synthesizing glutamic acid from C^'*02. Moreover, a gen- 

 eral energy-conserving principle would suggest that PEPA 

 and not free pyruvic acid should be the three-carbon com- 

 pound that combines with the two-carbon fragment. As we 

 shall see in the discussion for the synthesis of aromatic rings, 

 it has been proposed that PEPA can condense with an alde- 

 hyde, erythrose phosphate, to give (eventually) phosphoshi- 

 kimic acid (38). Perhaps a similar reaction between PEPA 

 and glyoxylic acid could lead to a product such as y-hydroxy- 

 glutamic acid, which could be subsequently converted to 

 glutamic acid. Dekker (39) has reported the presence of an 

 enzyme in rat liver that converts y-hydroxyglutamic acid to 



35 



