Biological Assimilation and Dissimilation of Nitrogen 541 



Intermediary steps and balance of the ornithine cycle of urea synthesis 



AT-Acetyl-glutamate 



1. NHs + CO2 + ATP ^ Carbamyl phosphate + ADP; 



2. Carbamyl phosphate + Ornithine-^ Citrulhne + H3PO4; 



3. Citrulline + Aspartate + ATP ^ Argininosuccinate + ADP + H3PO4; 



4. Argininosuccinate -^ Arginine + Fumarate ; 



5. Arginine + H2O -> Ornithine + Urea; 



Balance (1+2 + 3+4 + 5) 



COOH NH2 COOH 



I / I 



NH3 + CO2 + CHNH2 + 2ATP -> CO + CH + 2ADP + 2H3PO4 



I \ II 



CH2 NH2 CH 



I I 



COOH COOH 



homogenate. By making use of specific enzyme inhibitors for the suppression 

 of various intermediary reactions, I. Klyuge was able to show that the nitrogen 

 of L-amino acids and of glycine must first be transferred to L-aspartate by way 

 of two-stage transamination {via glutamate), and the nitrogen of ammonia by 

 transreamination, i.e. likewise via glutamate. This is followed by the reaction of 

 aspartate with citrulline to yield arginine {via argininosuccinate) and further, 

 urea. 



Estimations of the disappearance of added substrates from the homogenate 

 have shown [26] that in rat liver tissue the dissimilation of natural amino acids, 

 e.g., glycine, L-lysine, and even L-alanine, proceeds at a sufficiently rapid rate 

 only when the equilibria of the reversible transamination reactions are shifted 

 unidirectionally by being coupled with the irreversible reaction between 

 aspartate and citrulline, so that the amino nitrogen is trapped and channelled 

 towards the formation of urea. 



These studies have provided direct experimental evidence in support of our 

 contention (Braunshtein, [4, 27]), that in mammals about one half of the cata- 

 bolic nitrogen is converted in the liver to urea by repeated transfer reactions, 

 via the dicarboxylic amino acids, without preliminary deamination; at present, 

 this view is rapidly gaining general acceptance. 



As regards the ammonia which supplies the other moiety of urea nitrogen in 

 the first step of the ornithine cycle, its major part must be of extrahepatic origin: 

 it is evidently formed diffusely in various organs by the slow deamination 

 (chiefly indirect) of amino acids, and is transferred to liver with the blood, mainly 

 as the amide group of serum glutamine. Under physiological conditions this 

 glutamine may constitute the principal source of both nitrogen atoms of urea. 



From the evolutionary standpoint it is noteworthy that in the mammals, 

 which have shifted from ammonioteUc to ureotelic nitrogen catabohsm, the 

 tissues are almost completely devoid of L-amino acid oxidases, while these 

 enzymes are very active in micro-organisms and lower animals, which excrete 

 ammonia. 



Concomitantly, the conversion of ornithine via citrulline to arginine, i.e., the 

 phylogenetically very ancient path of biosynthesis of an amino acid of major 



