300 



PETER REICHARD 



HN-^ CH 



I II 



= C^ .C-COOH 



H 



Orotic acid 



+ riboi^e 



HX' 



■^CH 



- COz 



= C. ^C-COOH 



R 



+ NH3 



NH2 



N 



<^^ 



"CH 



- CO2 



I II 



= C. XH 



I 

 R 



Uridine 



t 

 - NH3 + NH3 



T 

 NH2 



"CH 



.=#*' 



= C^ C-COOH 



I 

 R 



0-C. .CH 



I 

 R 



Cytidine 



Fig. 10. Possible reactions occurring in the biosynthesis of nucleosides from 

 orotic acid. '2^ 



ard.^^ It was found that N^Mabeled orotic acid was extensively incorporated 

 into the pyrimidines of polynucleotides in different organs while the purines 

 were not labeled (Table IX). Except for a deamination of cytosine no deg- 

 radations were carried out to prove the utilization of the whole molecule, 

 but the low dilution of the N'^ seemed to justify the conclusion that this 

 was what had occurred. Later experiments by Weed and Wilson^^^ and 

 others with orotic acids labeled in positions 2 and 6 have confirmed the 

 direct utilization. There is a significant incorporation into the pyrimidines 

 of DNA as well as into those of PNA.^^ When DNA/PNA ratios for the 

 incorporations of orotic acid and glycine were compared, however, it was 

 found that the ratio based on glycine incorporation was considerably higher. 

 It is interesting to compare this finding with the corresponding purine ratios 

 for adenine and glycine; in both cases the ratio based on the incorporation 

 of the preformed large molecule was much lower than that based on the 

 de novo synthesis from glycine. 



The reaction sequence represented in Fig. 10 has been tentatively put 

 forward. ^22 Since neither uracil nor cytosine were polynucleotide pyrimidine 

 precursors, it was thought that the attachment of ribose (or ribose phos- 

 phate) must precede decarboxylation and that orotic acid riboside (or 

 ribotide) should be an intermediate in the formation of uridine (or uridine 

 phosphate) from orotic acid. 



The incorporation of labeled orotic acid into pyrimidines from E. coli B,^^^ 



'" L. L. Weed and D. W. Wilson, J. Biol. Chem. 189, 435 (1951). 

 12* L. L. Weed and S. S. Cohen, /. Biol. Chem. 192, 693 (1951). 



