39. ANTIMETABOLITES AXD NUCLEIC ACID METABOLISM 487 



was not sustained, and little or no effect on the appearance of the C 14 of 

 guanine in the adenine of the nucleic acids was seen, even at early time 

 periods. Similarly, thioguanine did not exert a primary effect on the in- 

 corporation of adenine per se into the purines of nucleic acids. When the 

 incorporation of glycine into the purines of nucleic acids was challenged by 

 pretreatment with thioguanine, incorporation into adenine was suppressed 

 by 75 % and that into guanine by 60 %. This effect was achieved under con- 

 ditions in which no blockade in the incorporation of glycine into proteins 

 was observed. Similar studies have been carried out on the effect of thio- 

 guanine on the incorporation of formate into purines and of methionine 

 into proteins of L-1210 leukemia in vitro. 229 In experiments with glycine-C 14 

 the accumulation of formylglycinamide ribonucleotide, which normally re- 

 sults from azaserine treatment, was sharply reduced by the subsequent 

 administration of thioguanine. 228 It was suggested that this apparent inter- 

 ference at an early stage of the synthesis of purines de novo maybe attribut- 

 able to a competition between thioguanine and normal intermediates for 

 pyrophosphorylribose-5-phosphate. A third possible site of action, revealed 

 in part by the data concerning the incorporation of guanine and glycine, is 

 suggested by experiments in vitro with Ehrlich ascites carcinoma cells. In 

 these, a marked depression of the incorporation of inosinic acid into adenine 

 of the nucleic acids occurred at levels of thioguanine which had a less strik- 

 ing or no effect on the incorporation of adenine, guanine, or even glycine. 228 



In a preliminary study with Bacillus cereus 2 ™ thioguanine caused inhibi- 

 tion of growth at levels which were one-fifth those required for effects on 

 nucleic acid content of the cell. Under these conditions, interference with 

 the formation or function of purine-containing cof actors may be the pri- 

 mary cause of inhibition of growth; however, proportional restriction of 

 growth and synthesis of nucleic acids may occur at these low levels of in- 

 hibitor. 



Thus, it can be seen that in many respects thioguanine resembles mercap- 

 topurine in the types of inhibition established, even with respect to effects 

 involving specific metabolites. This has led to the speculation that mercap- 

 topurine may exert a portion of its effects following conversion to thio- 

 guanine nucleotide. However, precise definition of the mechanisms of action 

 of thioguanine must await enzymic studies with isolated portions of the 

 anabolic pathways involving purine-containing compounds. 



3. AZAGUANINE 



By ring closure of diaminopyrimidines with nitrous acid a group of 8-aza- 

 purines (r-triazolo-[d]-pyrimidines) has been prepared; analogs of the sev- 



229 F. U. Brown and H. G. Mandel, Proc. Assoc. Cancer Research 2, 284 (1958). 



230 H. G. Mandel and R. G. Latimer, Proc. Am. Assoc. Cancer Research 3, 40 (1959). 



