BIOSYNTHESIS OF NUCLEIC ACIDS 



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—EL 



A IG DAP G 



Purines 



Fig. 6. Utilization of purines and purine derivatives by L. casei. 



The incorporation of equimolecular amounts of supplements added to a medium 

 containing O.Co mjug. of folic acid per ml. The left column of each pair represents the 

 renewal of the adenine, the right the renewal of the quanine. 



A, adenine; G, guanine; iG, isoguanine; DAP, 2,6-diaminopurine; and an added 

 S refers to the riboside (InS, inosine), and an added T to the ribotide. 



there is a strong preferential utilization of preformed purines with a conse- 

 quent suppression of the synthesis de novo.^^-^^ In experiments where only 

 the purine moieties were labeled, the relative capabilities for utilization of a 

 series of purine derivatives in competition with the synthesis de novo were 

 compared.^^'^*''*" In some instances the synthesis de novo was also measured 

 directly in replicate experiments with labeled formate or glycine.^" '^^ 



In Fig. 6 the pairs of columns represent the adenine and guanine of the 

 PNA, and the extent of conversion of the supplement into each purine is 

 indicated. Under these conditions adenine or guanine (also hypoxanthine or 

 xanthine) furnishes about 80% of the PNA purines. 2 , 6-Diaminopurine 

 alone inhibits all growth in this medium^^^ but when present with adenine 

 or with guanine it is utiUzed as readily as are those purines,^" and the open 

 columns in Fig. 6 are to indicate that abiUty to utilize diaminopurine. 



The purine moieties of the nucleosides are considerably less extensively 

 utilized^^ than are the free purines. Adenosine was converted preferentially 

 into PNA adenine, and guanosine and diaminopurine riboside were con- 

 verted preferentially into PNA guanine, facts which are in accord with an 

 assumption that they are initially transformed by substitution or exchange 

 reactions into the respective "active" derivatives on the pathways leading 

 to polynucleotide purines (Fig. 5). Inosine was less efficiently utihzed than 

 the above three, and the incorporation into the two PNA purines was es- 

 sentially equal, which is in harmony with the expectation that it is incor- 

 porated prior to the bifurcation of the pathways. The facts that 2,6-di- 

 aminopurine riboside is utilized under conditions where its parent purine 



>»i G. B. Elion and G. H. Hitchings, J. Biol. Chem. 187, 511 (1950). 



