637 



J. A. Bassham 



there is a higher label in carbon no. 4 than in carbon no. 3 if the equilibra- 

 tion of label between the two triose phosphates has been incomplete so that 

 the specific activity of the glyceraldehyde phosphate is higher than that of 

 phosphodlhydroxyacetone . 



A transketolase reaction on fructose-6-phosphate, and a subsequent condensa- 

 tion of the resulting tetrose phosphate with dihydroxyacetone phosphate leads 

 to the formation of sedoheptulose diphosphate and of sedoheptulose monophos- 

 phate with the label predominantly in carbons 3,M and 5 with more label in 3 

 and 5 than in 4. Such a labeling pattern was observed when leaves were exposed 

 to 1^C02 during photosynthesis for a second or less (D. This was evidence for 

 the existence of differentially labeled triose phosphate pools and was so 

 recognized at the time (D. 



Another transketolase reaction on sedoheptulose-7-phosphate leads to the 

 fonnation of a ribulose-5-phosphate labeled in carbon atoms 1,2 and 3. The 

 carbon atoms 1 and 2 resulting from the transketolase reactions on fructose-6- 

 phosphate and sedoheptulose-7-phosphate are considered to be in equilibrium with 

 a pool of thiamine pyrophosphate-glycolaldehyde addition ccxnpound. Two-carbon 

 moieties from this pool undergo a reversible reaction with glyceraldehyde 

 phosphate to give molecules of xylulose-5-phosphate (labeled in carbon atom no. 

 3) which in turn is in equilibrium with a ribulose-5-phosphate pool and the 

 ribose-5-phosphate pool mentioned above. Rapid reversible equilibration among 

 these pentose phosphate pools and the thiamine pyrophosphate glycolaldehyde 

 pool results in the feedback of labeling from carbon atoms 1 and 2 of ribose 

 phosphate through ribulose phosphate, xylulose phosphate to glycolaldehyde 

 thiamine pyrophosphate addition compound and thence to the number 1 and 2 

 carbon atoms of fructose-6-phosphate and sedoheptulose-7-phosphate . This is 

 essentially the explanation which we have given previously (6) for the asym- 

 metric labeling of hexose discovered by Kandler and Gibbs (7). 



This rapid reversible equilibration is entirely to be expected, since the 

 free energy changes associated with the transketolase and eperimisation reac- 

 tions under steady state conditions probably are all in the range P^ = +1.5 

 to -1.5 Kcal. This close to equilibration, and with low activation energies for 

 the reactions involved, the ratio between forward and back reactions is given 

 by ps = _RT In f/b where f is the rate of the forward reaction and b is the 

 rate of the back reaction. (5) At 25°C, PS = -1.37 log f/b, so that the 

 back reaction is approximately 10^ of the rate of the forward reaction. 



Other reactions for the glycolaldehyde thiajnine addition canpound are its 

 conversion by phosphoroclastlc splitting to acetyl phosphate, and its oxidation 

 to glycolic acid. Evidence for the acetyl phosphate formation by this path in 

 photosynthesis is lacking. Tl:e stimulation of fonnation of labeled glycolic 

 acid during photosynthesis with 1^C02 at high O2 levels tias been demonstratedvo}. 



The Gibbs effect is often quoted as an argument against the correctness of 

 the PSCR cycle (9). This effect, asyiranetric labeling of hexose following short 

 periods of photosynthesis with 1^002, consists of two parts: carbon atom no. 

 4 is more highly labeled than carbon atom no. 3, and carbon atoms 1 and 2 are 

 more highly labeled than carbon atoms 5 and 6. 



There is a difficulty in the explanation of the greater labeling of carbon 

 atom 4 as compared with carbon atom 3- The pools of the two kinds of triose 

 phosphate would have to be inccxnpletely equilibrated with respect to carbon 

 labeling. By the argument presented above, cne would expect these two types of 



