Nitrogen Transfer in Biosynthetic Mechanisms 225 



The possibility exists that argininosuccinic acid may also lie in the 

 pathway to pyrimidines since hydrolytic cleavage can theoretically 

 occur so as to form ornithine and ureidosuccinic acid. 



Amidine Group Transfer 



Arginine assumes an important role in the formation of guanidino- 

 acetic acid (glycocyamine) and creatine. 29,30 The former compound 

 is synthesized in the kidney and is subsequently converted to creatine 

 by a methylation which occurs in the liver. Guanidinoacetic acid is 

 formed by the interaction of arginine and glycine, according to reac- 

 tion 6. The process represented in this reaction involves the transfer 

 of the amidine group (originally formed in reaction 3) from arginine 

 to glycine. The transfer proves to be reversible. 31 - 32 



H— N H 



\ I 



C— NHR + HN— C— COOH ^± 



/ I I 



H 2 N H H 



Arginine Glycine 



H— N H 



\ I 



C— N— C— COOH + NHoR (6) 



/ I I 

 H 2 N H H 



Guanidino acetic Ornithine 



acid 



Aspartic Acid and Conversion of Inosinic Acid to Adenylic Acid 



The 6-NHo group of adenylic acid is replaced in intact animals more 

 rapidly than the nitrogens of the ring. 33 Although it is known that 

 the deamination of adenylic acid to inosinic acid occurs hydrolytically 

 and irreversibly, the mechanism of amination has been obscure. Both 

 of the nucleotides are active in metabolism as the mono-, di-, and 

 triphosphates, for inosine triphosphate, like ATP, acts as a donor in 

 systems requiring high-energy phosphate. 



Progress in solving this problem has come from the isolation of 

 adenylosuccinic acid, a new nucleotide that may prove to be an inter- 

 mediate in the amination process. The compound is formed, reversibly, 

 from fumaric and adenylic acid, as shown in the second part of reaction 

 7, by an enzyme found in yeast. 34 Structurally, adenylosuccinic acid 

 resembles argininosuccinic acid, for the 6-NH L > group of purines can be 

 looked upon as part of a cyclic amidine configuration. The similarities 



