338 F. SCHLENK 



fer. Friedkin and co-workers"^ have observed rapid utilization of thymidine- 

 C" for DNA synthesis in embryonated hens' eggs. Thymine-C'^ was not 

 utilized and no appreciable activity could be found in the ribonucleic acid 

 fraction. 



Lieberman and Romberg"^ consider the possibihty that dihydroorotic 

 acid rather than orotic acid may be the precursor of nucleic acid pyrimidines 

 (see Chapter 23, Addendum). 



Nucleosides and Nucleotides as Nucleic Acid Precursors 



The following recent reports on utilization of labeled compounds augment 

 the metabolic picture: Roll and Weinfeld"* have obtained adenylic and 

 guanylic acids a and b which were uniformly labeled with C'^, N^^, and P^-. 

 These were injected into rats; after one day, the nucleic acid fragments were 

 isolated. There was no specific utilization of the phosphorus of any of the 

 administered nucleotides. Ribose of the adenylic acids was incorporated to 

 about 80%, which strongly suggests that the major pathway of utiUzation 

 does not involve a rupture of the ribosidic linkage. In contrast hereto, the 

 incorporation of the ribose of the guanylic acids was only 20 per cent of that 

 of the base. 



It would seem that dephosphorylation of adenylic acid under these con- 

 ditions does not militate against its role as an intermediate of polynucleo- 

 tide synthesis. Loss of the phosphoric acid group may be incidental during 

 transport to the tissues; it may facilitate penetration into the cells. 



New data on pyrimidine nucleotide, phosphate-P^^, and orotic acid in- 

 corporation have appeared. "'•^^'' An extensive examination of pyrimidine- 

 deficient mutants produced by ultraviolet irradiation of Aerobacter aero- 

 genes has been reported by Nelson and Shapiro. ^^' No mutants were found 

 which responded to pyrimidine nucleosides or nucleotides only, but not to 

 the bases. On the other hand, nucleosides and nucleotides always served 

 equally well as did the bases. 



Further work on 4-amino-5-imidazolecarboxamide has confirmed the 

 occurrence of its riboside and ribotide.'" Involvement of thymidine in the 

 utilization of this base is suggested.^" 



"6 M. Friedkin, D. Tilson, and D. Roberts, Federation Proc. 13, 214 (1954). 

 '" I. Lieberman and A. Romberg, Biochim. et Biophys. Acta 12, 223 (1953). 

 i« P. M. Roll and H. Weinfeld, Federation Proc. 13, 282 (1954); P. M. Roll, H. Wein- 



feld, and G. B. Brown, Biochim. et Biophys. Acta 13, 141 (1954). 

 1" L. L. Weed and D. W. Wilson, J. Biol. Chem. 202, 745 (1953). 

 16" K. Moldave and C. Heidelberger, /. Am. Chem. Soc. 76, 679 (1954). 

 1" E. V. Nelson and S. K. Shapiro, J. Bacterial. 67, 692 (1954). 

 i« J. S. Gots, Nature 172, 256 (1953). 

 1" J. M. Weaver and W. Shive, J. Am. Chem. Soc. 75, 4628 (1953). 



