Nitrogen Transfer in Biosynthetic Mechanisms 219 



who have conducted extensive investigations of the mammalian enzyme 

 system. 8 Compound X contains phosphate which is bound in an ex- 

 tremely labile form, and the compound readily undergoes spontaneous 

 decomposition to orthophosphate, CO2, and NH 3 . 9 * 10 Recent experience 

 with the high-energy phosphate-coupled degradation of citrulline in 

 bacterial systems, 11-14 and with the behavior of synthetic carbamyl 

 phosphate in both bacterial and mammalian systems described by 

 Lipmann and his collaborators, 15 makes it extremely likely that the 

 carbamyl phosphate structure is formed in reaction lb as a part of 

 compound X. However, we know little of the mode of linkage to the 

 glutamic acid derivative. 



With the aid of ATP, the carbamic acid formed from C0 2 and NH a 

 is transformed to an anhydride of phosphoric and carbamic acids. The 

 energy of the pyrophosphate bond is retained in carbamyl phosphate 

 and can then be utilized in the attachment of the carbamyl group to 

 the terminal nitrogen of ornithine. There are many indications that 

 reactions 1 and 2 are reversible in the mammalian liver. 15,16 



Investigations of bacteria reveal that the same reversible reactions 

 also represent the bacterial mechanisms, possibly without participation 

 of the glutamic acid derivative for no evidence has yet been obtained 

 that this derivative is required in bacteria. It may be seen that the 

 degradation of citrulline to NH 3 , C0 2 , and ornithine proceeds with 

 the generation of ATP by a reversal of reactions 1 and 2. Prompted 

 by a consideration of the chemistry of carbamyl phosphate, it is tempt- 

 ing to visualize that the acid anhydride structure permits the energy 

 to be retained in the carbamyl group in the direction of synthesis or 

 in the phosphate bond in the direction of degradation. Enzymatic 

 cleavage on one side of the anhydride oxygen would yield high-energy 

 phosphate (anhydrophosphate accepted by ADP), and carbamic acid 

 which breaks down to C0 2 and NH a . Cleavage on the other side would 

 result in a carbamyl group (accepted by ornithine) and orthophos- 

 phate. 15 - 17 



NH 2 CO~0— PO(OH) 2 -f- NH 2 R -> NH 2 CO~NHR + H 3 P0 4 

 NH 2 CO— 0~PO(OH) 2 + ADP -» NH2COOH + ADP~PO(OH) 2 



Conversion of Citrulline to Arginine 



ATP is also required in the formation of arginine. The generation 

 of ATP accompanies the oxidative processes of respiration and, indeed, 

 it was the recognition that phosphate-bond energy is the driving force 

 of these reactions that finally permitted them to be made experimen- 

 tally independent of oxidative metabolism. 4,5,7 



