404 R. M. S. SMELLIE 



c. Studies with C^^- and C^^-labeled Glycine 



Since the carbon of formate gives rise to carbon atoms 2 and 8 in the 

 purine ring while the carbons of glycine are incorporated mainly into posi- 

 tions 4 and 5, the renewal of either of these pairs of carbon atoms must 

 involve complete rupture of the ring structure. The pattern of incorpora- 

 tion of C'^- or C'^-labeled glycine should therefore be similar to that ob- 

 taining with formate. 



Payne et al}°'^^ have examined the incorporation of glycine-2-C'^ into 

 the DNA of different tissues of normal mice and of mice with mammary 

 carcinomata. Once again in normal animals the DNA of spleen and intestine 

 exhibits a much greater uptake of isotope than that of liver, the relative 

 values being about 10, 12, and 1. The presence of a neoplasm leads to greater 

 uptake of isotope by the DNA purines of liver and spleen, although the 

 effect is not so marked as was found with formate. 



Several authors^" ■''■^^'^^ have measured the relative incorporation of C^^ 

 from glycine into the PNA and DNA of different tissues, using glycine 

 labeled in the carboxyl or methylene groups. The results of some of these 

 analyses are shown in Table V of Chapter 25. As with formate and phos- 

 phorus, the PNA renewal at any one time is appreciably greater than that 

 of DNA, and as might be expected there is reasonably good correlation 

 between formate and glycine as precursors. 



The kinetics of incorporation of glycine-2-C^^ and P^^ into the PNA and 

 DNA of rat liver and tumor have been examined by Tyner et a/.'^" They 

 found that the specific activities of the P^^ and C'^ in the purine nucleotides 

 of DNA were similar, whereas with the PNA the specific activities of the 

 P^2 in the purine nucleotides was appreciably higher than the specific ac- 

 tivity of the C^^ in the same nucleotides. 



d. The Replacement of the Purine Nitrogen 



The metabolism of the nitrogen atoms of the purines of PNA and DNA 

 may be studied with the aid of N^^-labeled ammonia or glycine as pre- 

 cursors of the nitrogen in the ring. Unfortunately, the techniques for the 

 estimation of stable isotopes are less sensitive than those for radioactive 

 isotopes, and it is therefore necessary to administer larger doses of the 

 precursor and to isolate larger quantities of material for assay. The dosage 

 of glycine and ammonium salts which is commonly used is so high that it 

 may have a marked effect on the body pools of these substances. Neverthe- 



" D. Elwyn and D. B. Sprinson, J. Am. Chem. Soc. 72, 3317 (1950). 



«2 G. A. LePage and C. Heidelberger, J. Biol. Chem. 188, 593 (1951). 



" C. Heidelberger and G. A. LePage, Proc. Soc. Exptl. Biol. Med. 76, 464 (1951). 



" R. Abrams, Arch. Biochem. and Biophys. 33, 436 (1951). 



«5 E. P. Tyner, C. Heidelberger, and G. A. LePage, Cancer Research 12, 158 (1952). 



