112 PETER REICHARD 



Reichard : In the case of G — yes, and in the case of U — no, and for A we have 

 no evidence. 



Hess : What is the reducing system involved or have you any experience about 

 the type of system you would expect ? I ask because a reducing system can also act 

 as a controlling mechanism upon the synthesis of DXPP from XDP. Certainly, the 

 reducing equivalents in growing cells are available in high concentrations, i.e. 

 speaking for the TPNH level, which in a given control range could well be a 

 critical metabolite. 



Reichard : I think you might be quite right but I do not know very much about 

 the events during the reduction. We know that it is a complicated series of reactions 

 and not a single reaction. We also know that we can demonstrate a stimulation of 

 the dialysed or ion exchange-treated enzyme preparation by TPNH. Now you 

 could draw the tentative conclusion that TPNH is the source of hydrogen during 

 the reduction, but on the other hand it might be an indirect effect. I don't know 

 yet but we are just starting to purify the enzymes. 



