Nitrogen Transfer in Biosynthetic Mechanisms 229 



a- 



+ DPN 

 a - Keto acids -*^ s"+~ Glutamic acid — ^ ^~ Oxalacetic acid * Malic acid 



Y ±H> 



Amino acids — ' ^— a - Ketoglutaric <* >^- Aspartic acid 

 acid 



±H 2 



Aspartic acid + Citrulline —+- Argininosuccinic — *- Arginine + Fumaric acid 



acid 



Urea + Ornithine 

 Scheme 5 



and the conversion of fumaric acid to malic acid. No utilizable energy 

 is made available in the removal of urea. For the remaining two 

 reactions, the DPN-linked oxidation of malic acid to oxalacetic 

 acid permits three high-energy phosphate bonds (~ph) to be gained 

 through phosphorylation coupled to the reoxidation of DPNH, and the 

 synthesis of argininosuccinic acid utilizes l~ph. The net gain of the 

 entire process will be +2-— .ph. 



It is of interest to compare this with the net gain obtained when 

 another nitrogen atom comes to ornithine from the amino acid pool 

 by way of NH 3 . Assume, for purposes of discussion, that the route 

 of deamination is the most favorable energetically, and that NH 3 is 

 liberated by reactions 9 and 10, which allow energy to be gained from 

 the reoxidation of DPNH. The detachment of nitrogen is then fol- 

 lowed by the generation of 3~ph, and the carbamyl group attachment 

 is associated with the utilization of 1^-ph. The two pathways are 

 thus approximately equal in terms of cell economy, for each incor- 

 porates a DPN-linked step. 



As far as we know, the energy liberated in flavoprotein catalyzed 

 reactions, which link directly to oxygen, is not made available in a 

 utilizable form. If oxidative liberation of NH 3 , according to reaction 

 8, were to precede the ornithine step, it would have the effect of lower- 

 ing the energy gain from +2 to — l~ph. 



Nitrogen Equilibration 



In the early investigations of nucleic acid metabolism with N 15 - 

 labeled nitrogen, the two ring nitrogens of pyrimidines and all but one 

 of the four ring nitrogens of purines appeared to come from NH 3 . It 

 now seems highly probable that half of the pyrimidine nitrogen comes 

 from aspartic acid, and evidence is accumulating that positions 1, 3. 

 and 9 of purines are derived from aspartic or glutamic acids and from 

 the amide group of glutamine. 37 The free amino group of adenylic 



