226 KoNRAD BloCh 



fact that at least one step of the tricarboxyHc acid cycle is extremely 

 slow in yeast, namely the conversion of malate into oxaloacetate. A 

 recent paper by Foulkes (1951, Biochem. J., 48, 378) shows that the 

 reactions of citric acid are also slow. 



Thus I regard it as fair to invoke the reactions of the tricarboxylic 

 acid cycle to explain a number of synthetic processes, because the 

 reactions can occur, though I doubt the validity of the assumption 

 that the tricarboxylic acid cycle is the major pathway of respiration 

 in micro-organisms. 



Wood: I would like to raise the question of whether we are on solid 

 ground when we use succinate oxidation as a measure of the occurrence 

 of the Krebs cycle. Do we know the actual intermediates of the Krebs 

 cycle? Perhaps we know the carbon skeleton of each intermediate, 

 but I do not believe we know the structure of the actual intermediates. 

 There have been several experiments, especially in the isotope field, 

 where unlabelled a-ketoglutarate has been added as a pool to trap 

 isotopically labelled a-ketoglutarate that presumably would be formed 

 during the oxidation of a labelled substrate. I question very much 

 whether any definite conclusion can be drawn from the fact that the 

 pool of a-ketoglutarate is not labelled as much as expected, because I 

 am not at all certain that a-ketoglutarate as such is the real intermediate, 

 or in the case of succinate, that succinate as such is the intermediate. 

 If one considers the numerous intermediates of anaerobic glycolysis 

 and how little we knew a few years ago of the true structure of the 

 intermediate compounds of this transformation, one is certainly justified 

 in thinking that the intermediates in the Krebs cycle may be different 

 than we now believe, and that we may not detect or recognize them by 

 the usual methods. 



Krebs: In general terms I agree with what Dr. Wood has said, but 

 in animal tissues the position is somewhat different because the 

 reactivity of all the intermediates of the tricarboxylic cycle can be easily 

 demonstrated in this material. In yeast the evidence is still largely 

 missing, and it is not impossible that the intermediates are not succinate 

 or malate but some closely related substances. The main point is that 

 at present the evidence in support of the view that the tricarboxylic 

 acid cycle is a major respiratory mechanism in yeast should not be 

 regarded as complete. 



