643 



J. A. Bassham 



O 78IO 12 14 16 18 20 22 24 26 2 8 30 32 34 36 36 40 



Time (minutes) with ''^COg 



Fig. 3. Effects of 8-methyl lipolc acid on labeling of ccmpounds with ^^C 

 during photosynthesis with -"-^COp . A Sucrose, O f ructose-1 , 6-dlphosphate , 

 B sedohe ptulose-l,7-diphosphate, □ glucose "diphosphate", •PGA. 



that this is by no means the case, since PGA drops instead of increasing and 

 since rlbulose diphosphate does not drop Irrmedlately to zero concentration. The 

 extremely rapid drop in PGA concentration requires either that the carboxylation 

 reaction leading to the formation of PGA be inhibited or that the reduction of 

 PGA be greatly accelerated. The latter possibility is not borne out by the 

 transient behavior of other carbon fixation products. Although there Is a rise 

 in the fructose and sedoheptulose diphosphates, there is a corresponding drop 

 In their monophosphates, which would not be expected frcan an acceleration of 

 PGA reduction. 



Thus we are left with a dramatic inhibition of reactions leading to PGA, but 

 without a conconittant Increase in the level of rlbulose diphosphate which we 

 would expect if the carboxylation of rlbulose diphosphate to give PGA were 

 stopped. Perhaps in addition to the inhibition of the carboxylation of rlbulose 

 diphosphate to give PGA, there is a subsequent inhibition of the formation of 

 ATP. This would prevent the level of rlbulose diphosphate from rising. 



Another possibility Is that in the in vivo system phosphate groups frcm the 

 no. 1 carbon atoms of fructose and sedoheptulose diphosphates are transferred 

 to rlbulose-5-phosphate to make ribulose-l,5-dlphosphate. This would account 

 for the apparent inhibition of the conversion of the hexose and heptose diphos- 

 phates to their monophosphates. Whatever the precise explanation of the various 

 effects of the disulfide compounds may prove to be. It is clear that these 

 substances Interfere suddenly and dramatically with the carbon reduction cycle. 



