644 



J. A. Bassham 



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7 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 



Time (minutes) with ''kXD2 



Fig. 4. Effects of 8-methyl llpoic acid on labeling of conpounds with l^C 

 during photosynthesis with l^COp . A Glucose-6-phosphate, # sedoheptulose-?- 

 phosphateT O fructose-6-phosphate, Dribulose-l,5-diphosphate. 



The inmediacy of their effects makes it seen plausible to suppose that they are 

 interferring in some way with electron transport by compounds with disulfide 

 groups. 



We have recently carried out experiments under our steady state conditions 

 of photosynthesis with 1^C02 i" which CO2 in air is suddenly replaced by nitro- 

 gen. After about 10 minutes the 1^^002 in air is again added to the photosynthe- 

 sizing algae. The results of these experiments are shown in Figs. 5 and 6. The 

 drop in the level of PGA and the rise and fall in the level of ribulose diphos- 

 phate are similar to those noted by Wilson and Calvin (15). The behavior of the 

 intermediates on addition of 1^002 again is quite interesting. As would be 

 expected, the level of ribulose diphosphate falls while that of PGA rises as 

 the carboxylation reaction resumes. l\fhat is intriguing is the fact that the 

 level of fructose diphosphate rises as rapidly as that of PGA during the first 

 few seconds. This is an additional bit of evidence for the direct conversion of 

 the carboxylation intermediates to sugar phosphate without equilibration 

 through the PGA pool. 



Another interesting point is the fact that fructose diphosphate and sedohep- 

 tulose phosphate rise and then fall after the readdition of the l^^COp. Why do 

 they fall? One could argue that the initial carboxylation reaction is faster 

 than usual because of the high level of ribulose diphosphate, and that once the 

 ribulose diphosphate has been depleted, the carboxylation reaction becomes 

 slower. However, the level of PGA which continues to rise for many minutes be- 

 lies this argument. Perhaps the explanation is to be found in the previously 



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