labeled glycolate accumulates during photosynthesis with 

 C^*02 when the CO2 pressure is reduced. 



1. Enzyme systems present in chloroplasts can bring 

 about the oxidation of glycolate to glyoxylate with oxygen 

 and the reduction of glyoxylate to glycolate with DPNH 

 (57). If some steady-state relation between these two acids 

 exists, it might well be shifted toward more glycolate at low 

 CO2 pressures by the increase in the ratio of DPNH/DPN + 

 that would result from the decreased utilization of TPNH 

 for the carbon reduction cycle. Moreover, the oxidation of 

 glycolate by O2 must in fact be limited in rate during photo- 

 synthesis, or glycolate would not be seen at all. Possibly gly- 

 colate is more effectively oxidized by some intermediate hy- 

 droxy 1 or peroxide involved in the liberation of oxygen 

 following the splitting of water during the primary act in 

 photosynthesis. If so, such an intermediate oxidant may de- 

 crease in concentration at low CO2 pressure because of re- 

 combination with primary reductant that would build up, 

 again as a result of decreased utilization by the carbon re- 

 duction cycle. A decrease in the oxidant concentration would 

 reduce the oxidation of glycolate. 



2. Low CO2 pressure might result in higher pH inside 

 the chloroplasts. The phosphoketolase reaction, leading to 

 acetyl CoA and involving the removal of OH" from gly- 

 colaldehyde-ThPP, might be blocked, and the oxidation of 

 the glycolaldehyde-ThPP to glycolyl CoA might be favored. 



3. If glycolyl CoA is formed and is a biosynthetic inter- 

 mediate, the reactions in which it is used might require CO2 

 analogous to the conversion of acetyl CoA to malonyl CoA 

 in fatty acid biosynthesis. Low CO2 pressure could thus lead 

 to an increased concentration of glycolyl CoA and permit its 

 more rapid hydrolysis to glycolate. 



Tanner and co-workers (58,59) have recently proposed 

 a direct route from CO2 to glycolic acid during photosyn- 

 thesis. According to his scheme, CO2 is reduced by TPNH 

 and MnCl~ to the radical CHO-. Two of these CHO- radi- 

 cals are then condensed to give glyoxal, thence glycolic acid. 



44 



