tion, decarboxylation occurs to give acetaldehyde-ThPP. 

 This compound reacts with oxidized lipoic acid to give acetyl 

 dihydrolipoic acid, which in turn reacts with CoA to give 

 dihydrolipoic acid and acetyl CoA (53-56). 



Wilson and Calvin (19), following their observation of 

 glycolate accumulation at low CO2 pressure, suggested that 

 the glycolyl moiety transferred by transketolase is the source 

 of glycolic acid. We should now like to suggest specifically 

 that the glycolaldehyde-ThPP compound formed in the first 

 step of the transketolase or phosphoketolase reactions may 

 undergo oxidation to give glycolyl CoA and, eventually, 

 glycolate. This oxidation need not follow a pathway exactly 

 analogous to the oxidation of acetaldehyde-ThPP, but we 

 have shown it so in Figure 7. 



As mentioned earlier, during photosynthesis glycolate 

 can be converted to the alpha and beta carbon atoms of PGA 

 via carbon atoms 1 and 2 of the pentose in the carbon reduc- 

 tion cycle. Thus it appears that the pathway from pentose 

 phosphate to glycolate and glyceraldehyde phosphate should 

 be reversible. The incorporation of glycolate via such a path- 

 way would require an energy input, probably in the form of 

 an activation by ATP. Finally, some state in the incorpora- 

 tion pathway should involve equilibration with a symmetric 

 intermediate because administration of glycolate-2-C^* to 

 photosynthesizing plants leads to PGA labeled equally in 

 the alpha and beta carbon atoms. We have indicated one 

 such symmetric compoimd and there may be other possi- 

 bilities. 



The formation of glycolyl CoA and reduced lipoic acid 

 as shown in Figure 7 are hypothetical. If glycolyl CoA were 

 formed, it seems likely that it would be an important inter- 

 mediate in paths as yet unknown. In any event, if there is 

 any conversion of carbon atoms number I and 2 of ketose 

 to glycolic acid during photosynthesis, then an oxidation of 

 the glycolyl fragment is required so that some cofactor, al- 

 though not necessarily lipoic acid, must be reduced. 



Let us now attempt to explain the observation that 



43 



