Nitrogen Transfer in Biosynthetic Mechanisms 217 



CO- + NH 3 Ornitkim 



I 1 I 



Aspartic acid 



Urea 



Carbamylphosphate 



Citrulline 



ArgininosuccLnic 



Arginine 



Arginine phosphate 



T 



4-. 



Aspartic 

 acid 



Carbainylaspartic 

 acid 



I' 1 



i 



Carbainylaspartic Guanidino 

 acid acetic acid 



I 

 Pyrimidines 



i 

 Pyrimidines 



Scheme 1 



I 

 Creatine 



Proteins 

 — Glycine 



Creatine 

 phosphate 



cycle shown in italics. Before pursuing the various relationships, it is 

 well to examine the reaction mechanisms through which they are inter- 

 linked. 



In 1932, Krebs found that the respiring liver slice can form large 

 amounts of urea from C0 2 and NH 3 in the presence of a small amount 

 of ornithine. As an explanation of his observations, he proposed 1 the 

 three-step cyclic mechanism shown in scheme 2. 



0=C 



/ 

 \ 



NH, 



NH 3 



I 



Step 2 



-> H*N— C 



/ 



NH 



C0 2 + NH 3 



NH 



I 

 R 



Step 1 



NH 2 <r~ 



R 



Step 3 



NH 



0=C 



R 

 -H 2 

 NH 2 



NH, 



Scheme 2 (RNH 2 = ornithine) 



After successive additions of C0 2 and NH 3 to ornithine and citrulline, 

 to form arginine, urea is split off by an arginase-catalyzed hydrolysis. 

 The liberated ornithine again participates in the cycle. The entire 

 process has come to be known as the ornithine cycle. Krebs observed 

 that urea-synthesizing activity disappeared when the cells were dis- 

 rupted or when respiration was prevented. Permeability barriers, the 

 unrecognized presence of essential substrates, and the inability to inter- 

 rupt the cycle, owing to the complex dependence on respiration, im- 

 peded further exploration with intact cells. 



In the past few years, as an outgrowth of the basic observations, it 

 has been possible to obtain synthesis in broken-cell suspensions, to 



