254 Discussion 



more limited than usual in the entry of glucose through all pathways. 

 To what extent might this be responsible for some of the effects which 

 you have seen? 



Potter: It would be worth while to add more hexokinase to the 

 experimental set-up and study the effect. Dr. Racker showed that 

 when phosphate was pushed to a very high level, there was still a 19 per 

 cent Pasteur effect. Without seeing the actual plot of the data one 

 could not judge whether or not there is a certain amount of Pasteur 

 effect which cannot be explained in the classical way. 



Racker : I agree that we should keep in mind the possibility of multiple 

 causes for the Pasteur effect. If this will be shown, we will be faced with 

 the problem of evaluating quantitatively the relative contribution of 

 the various mechanisms. My guess is that this will not be simpler than 

 to quantitate the relative contribution of, e.g. the shunt pathway in 

 glucose metabolism. Nevertheless, Prof. Potter, have you made attempts 

 to determine the concentrations of glucose-6-phosphate and 6-phos- 

 phogluconate in your system and are they high enough to account for 

 the observed inhibition? I ask, because Dr. Wu has determined the 

 intracellular concentration of these two intermediates in ascites cells 

 and they are too low to account for the Pasteur effect — unless we 

 invoke the fashionable concept of compartmentation again. 



Potter: I should like to relate why we got into this in the first place. 

 Our original observations indicated that there was a lack of trans- 

 hydrogenase and TPN cytochrome c reductase in certain tumour tissues. 

 Then, also, the studies by Drs. Clock and McLean (1957, Biochem. J., 65, 

 413) indicated that TPN is very low. Therefore, I think that tumour 

 tissues are precisely the tissues to which this mechanism does not 

 apply; i.e. they may lack this system. That does not imply that those 

 tissues cannot carry out this pathway. In most tissues this pathway 

 may actually result in an increased disappearance of glucose because 

 of a failure to pile up these intermediates which have a blocking effect. 



Experiments carried out many years ago by Dr. LePage and myself 

 (1946, Amer. J. Physiol., 146, 267) showed that glucose-6-phosphate 

 exists in respectable amounts in tissues. Dr. Aisenberg (unpublished 

 data) says that there appears to be an actual increase in glucose- 

 6-phosphate in the presence of mitochondria. 



Racker: The intracellular concentration of glucose-6-phosphate in 

 ascites cells is well below the Ki value for product inhibition of hexokin- 

 ase. We should also keep in mind that an increase in glucose-6-phos- 

 phate concentration per se cannot account for the Pasteur effect. To 

 paraphrase a proverb, "You cannot eat your hexokinase and have it." 

 If hexokinase makes enough glucose-6-phosphate so that it accumulates 

 in respectable amounts, the removal of this compound rather than its 

 formation must be limiting. I have also two mild objections to the 

 proposal of inhibition of hexose isomerase. One is based on the very 

 high cellular contents in hexose isomerase. The second is that we 

 could not explain by this concept a Pasteur effect with fructose as 

 substrate. 



Lynen: I wonder whether the mechanism discussed can be used to 



