suggested by the already known pathways of the glycolytic 

 breakdown of sugars, which lead to PGA as an intermediate. 

 Since the sugar phosphates are important early products of 

 carbon reduction in photosynthesis, it was proposed that 

 they are formed from PGA by a reversal of the glycolytic 

 pathway. Degradation of the radioactive hexoses from short 

 experiments showed that they were labeled in the two center 

 carbon atoms (numbers 3 and 4) just as one would expect if 

 2 molecules of carboxyl-labeled PGA were first reduced to 

 triose and then linked together by the two labeled carbon 

 atoms to give hexose (Figure 4). 



The hexose and triose phosphates may be converted by 

 aldolase or transaldolase and transketolase enzymes to pen- 

 tose and heptose phosphates (Figure 2 and Table 2). Deg- 

 radation of these sugars and comparison of the labeling pat- 

 terns within the molecules showed that this conversion did 

 occur, and in such a way that 5 molecules of triose phos- 



Heptose 

 and pentose 

 phosphates 



Hexose phosphotes 



Triose phosphate 



I 



-C- 



I 

 -CH 



I 



Ao, 



CHs 



I 

 HC-NHg 



I 

 *COOH 



Alanine 



TPNH 

 ATP 



HgC-OPOsH- 



HC-OH 



I 

 *COOH 



3- PGA 



2- PGA 



CHg 

 II 



C-OPO3H- 

 1 

 -«COOH 



PEPA 



Light 



*C02 



c 



I 



c 



I 



*c 



I 



»c 

 c 



I 



c 



•COOH 



I 



CHt 



I 

 HC-NHa 



I 

 ♦COOH 



t . 



♦COOH 



I 



CHe 



I 



C = 



I 

 *COOH 



Asportic 

 acid 



DPNH 



»COOH 



I 



CHg 



I 

 HC-OH 



I 

 ♦COOH 



Malic ocid 



14 



Figure 4. Labeling of compounds with C 

 during early steps in carbon dioxide reduction 

 during photosynthesis with C^*02- 



16 



