402 R. M. S. SMELLIE 



mammary gland and mammary carcinoma is appreciably higher than in 

 the normal gland. '^ In contrast to DNA, the phosphorus uptake by the PNA 

 of mouse liver does not appear to be greatly affected by the presence of a 

 neoplasm in other tissues. ^^'^^ 



Recently, Munro et al.^^ have studied the effects of dietary changes on 

 the PNA content of rat livers and also on the renewal of the PNA phos- 

 phorus. They found that, when the animals were fed a protein-containing 

 diet, an increase in energy intake of 1000 kcal. caused a rise of 20 to 30% 

 in the PNA content of the liver, but that there was no significant change in 

 the proportion of phosphorus atoms replaced in a given time. On the other 

 hand, when the animals were fed a protein-free diet, an increase in energy 

 intake of 1000 kcal. caused only a slight rise (5 to 10 %) in the PNA con- 

 tent of the liver, but the rate of replacement of phosphorus atoms was 

 augmented by about 25 %. It appears therefore that at each level of protein 

 intake the total number of phosphorus atoms incorporated into the PNA is 

 increased by raising the energy intake, in one case by an increase in the 

 amount of PNA per liver without a change in the percentage of phosphorus 

 atoms incorporated in a given time, and in the other case by an increase 

 in the incorporation rate with a smaller change in the amount of PNA per 

 liver. 



III. The Metabolism of the Purine and Pyrimidine Bases 

 of PNA and DNA 



1. General 



While much useful information concerning the metabolism of the nucleic 

 acids has been obtained from studies with labeled phosphorus, it is always 

 possible, although unlikely in view of current views on nucleic acid struc- 

 ture, that the phosphorus moiety behaves differently from the main skeleton 

 of the molecule. The metabolism of the bases of the nucleic acids is therefore 

 of considerable importance. Recent reviews of this topic are by Christman," 

 Franke,^'* and Brown. ^* 



2. Small-Molecule Precursors of the Nucleic Acid Purines (See 



ALSO Chapter 25). 



a. The Source of the Individual Atoms of the Purines 



From isotopic studies discussed in Chapter 23, it has been concluded 

 that carbon atoms 2 and 8 of the purine ring are derived from formate or 



" H. N. Munro, D. J. Naismith, and T. W. Wikramanayake, Biochem. J. 54, 198 



(1953). 

 " A. A. Christman, Physiol. Revs. 32, 303 (1952). 

 ^^ W. Franke, Z. Vitamin-Hormon-u. Fermentforsch. 5, 279 (1953). 

 " G. B. Brown, Ann. Rev. Biochem. 22, 141 (1953). 



