294 Discussion 



Slater : Prof. Chance, what do you mean by reversed effect? 



Chance : We can titrate the steady state of pyridine nucleotide back 

 and forth between reduced and oxidized, by acid and base. It is well 

 known that ammonia and amines penetrate the yeast cell and are only 

 slowly pumped out. 



Slater : Then it should be possible to study this effect of ammonia at 

 constant pH. 



Chance : We did study this at constant pH, but you can get rid of it 

 by lowering the pH. 



Krebs: Dr. Magasanik, were the ammonia concentrations, to which 

 you referred, really comparable in level to those which one might 

 expect in the cell? 



Magasanik : I think so. Dr. Lin worked on this as far as the glycerol 

 dehydrogenase is concerned. We have published the figures on inosinic 

 acid dehydrogenase (Magasanik, B., Moyed, S. H., and Gehring, L. B. 

 (1957). J. hiol. Chem.., 226, 339). 



Lynen: Dr. Holzer, did you measure the time-course of the dis- 

 appearance of ammonia? 



Holzer: It is a straight line for a long time. The disappearance of 

 a-ketoglutarate explains about 60 to 90 per cent of the disappearance of 

 ammonium ions in the first seconds. The remaining 10 to 40 per cent 

 of the ammonium ions are converted perhaps to carbamyl phosphate, or 

 participate in other ammonium-fixing reactions. 



Krebs : These are likely to be other ammonium-binding reactions, but 

 I take it that you consider this to be the most important one, and that 

 the other amino acids are all formed by transamination. 



