Nitrogen Transfer in Biosynthetic Mechanisms 223 



Synthetic Mechanisms Associated with Arginine Formation 



As familiarity with these mechanisms increases, it appears that the 

 stepwise manner in which "active" groupings are built up and trans- 

 ferred not only adapts them to the utilization of a common supply 

 of energy but also allows their participation in the synthesis of many 

 compounds that possess the ureide, amidine, or guanidine structure. 



Arginine Synthesis in General 



The interest in arginine synthesis has in the past been primarily 

 confined to the ornithine cycle. It is perhaps as a result of gaps in 

 our knowledge of nitrogen metabolism that the significance of these 

 mechanisms in providing arginine for cellular protein has been some- 

 what neglected. The important implications of the isotope experi- 

 ments carried out by Schoenheimer and his group with respect to the 

 rates of protein synthesis and the origin of the incorporated amino 

 acids are too well known to require discussion here. 21 The incorpora- 

 tion of N 15 derived from NH 3 or amino acids into the amidine group 

 of tissue arginine is in accord with the operation of an ornithine cycle. 

 Evidence of this type, moreover, obtained by "trapping" arginine in 

 tissue proteins, offers the additional demonstration that the same 

 arginine- forming mechanisms are drawn upon to supply the needs of 

 protein synthesis. 



Many unicellular organisms that lack arginase are able to synthesize 

 arginine, and, wherever it has been investigated, citrulline invariably 

 appears to lie in the pathway of arginine synthesis from ornithine. 

 Specific enzymes catalyzing each of the individual reactions have al- 

 ready been detected in a number of these organisms, thus providing 

 more detailed evidence that the same group of arginine- forming mecha- 

 nisms which operate in the mammalian liver are widely distributed in 

 nature. 



Carhamyl Group Transfer 



Just as the carbamyl group of compound X or carbamyl phosphate 

 can be transferred to ornithine, there is evidence that this group can 

 also be transferred to aspartic acid to form carbamyl aspartic acid, 

 as shown in reaction 5. Here, as in scheme 3, compound X has been 

 omitted for convenience. The reaction occurs in mammalian liver 

 preparations with compound X, and in bacteria with carbamyl phos- 

 phate. 15 - 22 - 23 



