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F. SCHLENK 



TABLE I 

 Per Cent Renewal of Nucleic Acid Purines in the Rat from Purine 



Nucleosides'^ 



" Nucleosides administered at 0.2 mM. per kg. of body weight per day. 



TABLE II 



Per Cent Renewal of Nucleic Acid Pyrimidines in the Rat from Pyrimidine 



Nucleosides and Nucleotides^' • '^ 



" Nucleotides administered at 0.4 mM. per kg. of body weight per day, nucleosides at 0.34 mM. per kg 

 per day. 



ribonucleic acid purines; inosine is inferior to adenosine; the isotope of 

 2 , 6-diaminopurine riboside, guanosine, and crotonoside recurs only in gua- 

 nosine. No significant incorporation of the isotope into deoxyribonucleic 

 acid was observed. Table II shows the results of experiments with pyrimi- 

 dine nucleosides^^ and nucleotides.^^ The label of each compound is found 

 preferentially in the corresponding base, but there is also extensive conver- 

 sion into the other pyrimidines. Reichard and Estborn^® have carried out 

 similar experiments with labeled deoxynucleosides. In rats the label of 

 deoxycytidine was found in cytosine and thymine of deoxynucleic acid, but 

 not in the pyrimidines of ribonucleic acid. Thymidine was recovered only 

 in the thymine of the deoxynucleic acid. This indicates that the methyla- 



'« P. Reichard and B. Estborn, J. Biol. Chem. 188, 839 (1951). 



