184 Discussion 



irreversible, although the equilibrium between glucose and fructose 

 should, in fact, favour glucose to some extent. Dr. Hers believes that 

 the factor which actually drives the reaction towards fructose is the 

 difference in potential between the two coenzymes. Thus, the reaction 

 may be said to derive its energy from a coupled electron transfer 

 between TPNH and DPN. 



A number of systems have now been described which appear to be 

 coupled to the transfer of electrons between TPNH and DPN. A very 

 interesting one is that shown by Prof. Dickens, going from pyruvate to 

 phosphopyruvate via malate and oxaloacetate. The importance of TPNH 

 and DPN in this system has been stressed before by our Chairman. 

 Another one is the L-xylulose to D-xylulose transformation, which goes 

 via xylitol — this has been described by Touster and co-workers (Touster, 

 O., Reynolds, V. H., and Hutcheson, R. M. (1956). J. biol. Chem., 221, 

 697). This view, if correct, would give a special significance to the 

 transhydrogenase as being some kind of uncoupling enzyme for these 

 phenomena which depend on the transfer of electrons between TPNH 

 and DPN for their supply of energy. 



Dr. Hers has also found that he can increase the activity of the 

 pentose phosphate cycle as measured by the production of ^^COg from 

 [l-i*C]-labelled glucose in liver slices, by adding substrates of aldose 

 reductase, the enzyme which will reduce various aldose compounds 

 with the aid of TPNH. Adding substrates of this enzyme to liver slices 

 will increase the operation of the pentose phosphate cycle several fold. 

 Again this is an indication that the reoxidation of TPNH is somehow 

 inhibited in these slices and can be stepped up by adding a substrate 

 which will act as an electron acceptor from TPNH. 



Dickens: The glucose-sorbitol-fructose system is, of course, very 

 restricted in that it occurs in prostate and in the seminal vesicles. A 

 number of other systems which can similarly effect a transhydrogenase 

 type of activity, and which occur more widely, are mentioned in the 

 printed account, but time did not permit their inclusion when presenting 

 this paper. 



Rocker: In Acetobacter xylinum there is actually a reaction which 

 yields an energy-rich phosphate associated with the shunt pathway 

 without being directly linked to TPNH oxidation. I am referring to the 

 phosphorolytic cleavage of fructose-6-phosphate to acetyl phosphate 

 and erythrose-4-phosphate. The enzyme which catalyses the reaction 

 has been named fructose-6-phosphate phosphoketolase. The erythrose- 

 4-phosphate which arises from this reaction can cycle back via the 

 pentose phosphate cycle to fructose-6-phosphate. By repetition of the 

 phosphorolytic cleavage of fructose-6-phosphate and recycling of the 

 erythrose-4-phosphate, one can visualize that the entire hexose molecule 

 is cleaved to 3 moles of acetyl phosphate. Since these micro-organisms 

 contain an active acetokinase, they can convert acetyl phosphate + 

 ADP -► acetate and ATP. Thus, the overall reaction yields 3 moles of 

 ATP per mole of fructose-6-phosphate or, if one subtracts one ATP 

 required for phosphorylation of glucose, the overall yield is two moles of 

 ATP per mole of glucose, which is identical with the energy yield of 



