Discussion 185 



the glycolytic pathway. We discovered this fructose-6-phosphate 

 phosphoketolase enzyme in the course of unsuccessful attempts to 

 demonstrate phosphorylation linked to the oxidation of TPNH. 



Dickens: This enzyme has not been described other than in micro- 

 organisms? 



Racker : We looked for it in animal tissues but could not find it. 



Greville: Is there any evidence, at least in animal tissues, that the 

 pentose phosphate pathway forms a cycle, i.e. that it goes from glucose- 

 6-phosphate through the pentose phosphates and back to glucose- 

 6 -phosphate? 



Dickens : Yes, I think so. This is quite clear from the work of Marks 

 and others (Hiatt, 1957, 1958, loc. cit.; Feigelson and Marks, 1957, loc. 

 cit.; Marks, P. A., and Feigelson, P. (1957). J. biol. Chem., 226, 1001). 

 Labelling of the glycogen glucose and RNA-ribose molecules, derived 

 particularly from glucose labelled in the 2-position, indicates quite 

 clearly that one first-stage reaction is decarboxylation at C-1 of the 

 glucose, and a build-up to hexose. The labelling of the carbons fits the 

 participation of this pathway very well. That is in the intact animal. 

 An extensive non-oxidative transketolase-transaldolase interconversion 

 of hexose and pentose also occurs. 



Holzer: You mentioned the discrepancy between our results and 

 those of Clock and McLean concerning the determination of DPNH and 

 DPN in diabetic animals. This might be due to our animals having too 

 much alloxan. We did not attempt to clarify this because, in the 

 meantime, together with Prof. Lynen we carried out experiments on 

 yeast cells (Holzer, H., Holzer, E., and Schultz, G. (1955). Biochem. Z., 

 326, 385; Holzer, H., Schultz, G., and Lynen, F. (1956). Biochem. Z., 

 328, 252) showing that the concentrations of free DPNH and DPN 

 (which are responsible for the thermodynamic situation) are different 

 by a factor of 100 from those measured with the methods used hitherto. 

 In these experiments we determined the concentrations of acetaldehyde 

 and alcohol, and calculated from these data, by means of the equilibrium 

 constant of the alcohol dehydrogenase reaction, the DPNH /DPN ratio. 

 It would be of interest to carry out the same type of experiments with 

 animal tissues, since it is likely that in animal tissues there exists an 

 equilibrium between pyruvate, lactate, DPNH and DPN, catalysed 

 by lactic dehydrogenase which is very active in animal tissues. From 

 the analyses of lactate and pyruvate the ratio of free DPNH to DPN 

 might be calculated (cf. Biicher, Th., and Klingenberg, M. (1958). 

 Angew. Chem., 70, 552). 



Dickens : That would be extremely valuable and certainly needs to be 

 done. 



Racker: Prof. Dickens mentioned that the animals had too little 

 alloxan, and Prof. Holzer mentioned that they had too much alloxan — 

 what is the right amount? 



Dickens : If you give them an enormous dose, they are in a moribund 

 condition, not only from diabetes, but probably because of other non- 

 specific toxic effects of alloxan. 



Holzer: Ours had 700 to 1000 mg. per cent of glucose in the blood. 



