224 Essays in Biochemistry 



O O COOH 



H 2 N— C— O— P(OH) 2 + H 2 N— CH -> 



I 

 HC— COOH 



H 



Carbamyl 



phosphate 



H HC— COOH 



I 

 H 



Carbamyl 

 aspartic acid 



Considerable interest attaches to this reaction, for carbamyl aspartic 

 acid (ureidosuccinic acid) can function as a precursor of pyrimidines, 

 through orotic acid, according to scheme 4. 



Nil, COOH 



I " I 

 0=C CH 2 



I I 



UN CH 



I 

 COOH 



Ureidosuccinic 

 acid 



NH— C=0 



I I 



0=C CH 9 



I I 



HN CH 



I 

 COOH 



Dihydroorotic 

 acid 



NH— C=0 



I I 



0=C CH 



HN C 



COOH 



Orotic 

 acid 



NH— C=0 



0=C CH 



I II 

 HN CH 



. I 

 ribose — PO4 



Uridylic 

 acid 



Scheme 4 



There is ample evidence for the mammalian and bacterial incorpora- 

 tion of orotic acid into the pyrimidines of nucleic acid, 24-26 and evi- 

 dence has been obtained for the steps represented in scheme 4. 27 ' 28 

 According to this pathway, the cyclic ureido configuration, as well as 

 the carbon skeleton incorporated in the pyrimidine structure, will have 

 originated in carbamyl phosphate and aspartic acid. 



