METABOLISM OF THE NUCLEIC ACIDS 405 



less, much valuable information concerning the metabolism of the nucleic 

 acids has been obtained using N^^-labeled precursors. 



Experiments of this type, carried out by several groups of workers, show 

 that the metabolic acti\dty of PXA and DNA nitrogen, as gaged by uptake 

 of N'^, follows the same general pattern as that observed with radioactive 

 phosphorus and carbon. For example, the uptake of heavy nitrogen by 

 the PNA and DNA of spleen and intestine is considerably greater than the 

 corresponding value for kidney^^"®^ while in regenerating liver, the incor- 

 poration of glycine-N^^ into the PNA and DNA is very much higher than 

 in normal resting liver.^^-'^'' The ratios of uptake of labeled nitrogen into 

 the PNA and DNA of several tissues, shown in Table V of Chapter 25, 

 make it clear that the isotope content of the PNA is in all cases much 

 greater than that of the DNA. In most tissues, the ratio PNA/DNA is 

 similar to that found with other isotopes, but, in liver, the value tends to 

 be somewhat higher than the corresponding figures for P^- and formate- 

 or glycine-C^^. In general, where it is observed that the PNA/DNA ratio 

 is very high, the actual isotope content of the DNA is extremely low, and 

 even a slight error in analyses would have a marked effect on the relative 

 values. 



3. Labeled Adenine as a Precursor of PNA and DNA Purines 



The ability of intact purines to serve as precursors of the nucleic acids 

 in many organisms has been studied by several groups of workers,®^ •''^"^^ 

 and this subject has been extensively reviewed by Brown et al.,''^ by David- 

 gQj^ 48,80 \^y Christman,^^ and by Brown. ^^ From all this work, it appears 



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