39. ANTIMETABOLITES AND NUCLEIC ACID METABOLISM 475 



tive during the increased time required for complete reaction. Preliminary 

 evidence for a direct interaction between azaserine and the enzyme had been 

 reported earlier 142, 143 ; thus, washing of Ehrlich ascites carcinoma cells pre- 

 viously exposed in vivo to azaserine did not restore their normal capacity 

 for the synthesis of purines. Experiments in vitro with these cells also dem- 

 onstrated that a finite time was required before inhibition was established. 

 These diazo compounds appear to react with the enzyme to form a stable 

 complex which cannot be dissociated by glut amine. The correctness of this 

 view has been shown by the isolation of two peptides containing azaserine 

 residues, after reaction of the highly purified enzyme with 0-(diazoacetyl- 

 2-C 14 )-L-serine, and subsequent digestion of the product with trypsin 144 ; 

 analogous covalent bonding did not result when crystalline bovine serum 

 albumin was treated with azaserine-C 14 . Identification of the radioactive 

 peptides should provide valuable information concerning the structure of 

 the active center of this enzyme. 



In general, the most rapidly proliferating tissues, such as tumor and in- 

 testine, experience the most profound inhibition of purine synthesis by 

 azaserine or DON, while liver is relatively immune to their action. 145 This 

 may be attributable in part to deamination of the azaserine, known to occur 

 in rat liver. 146 ' 147 A number of sublines of transplantable tumors of mice 

 have been selected for resistance to azaserine or DON. One of these, a re- 

 sistant strain of plasma cell neoplasm 70429, appeared abruptly during 

 chronic treatment with azaserine of mice bearing this tumor. 148 The situation 

 resembled the appearance of full resistance to streptomycin, which may 

 occur in one step in certain bacterial strains. In chemotherapeutic studies 

 in children with acute leukemia, 149 and also in studies with E. coli, li0 re- 

 fractoriness to these agents has developed so promptly as again to suggest 

 a one-step mechanism; however, an example of what might be considered 

 stepwise increases in the degree of resistance, similar to those noted with 

 penicillin, also has been recorded. 151 In a cell-free preparation from an aza- 

 serine-resistant strain of the plasma cell neoplasm (70429), the incorpora- 

 tion of glycine into acid-soluble purines was inhibited by azaserine to the 



142 G. A. LePage, J. Greenlees, and J. F. Fernandes, Ann. N. Y. Acad. Sci. 63, 999 

 (1956). 



143 J. Greenlees and G. A. LePage, Cancer Research 16, 808 (1956). 



144 R. L. Herrmann, R. A. Day, and J. M. Buchanan, Abstr. 135th Meeting Am. Chem. 

 Soc, Boston, Mass., p. 45C (1959). 



145 E. C. Moore and G. A. LePage, Cancer Research 17, 804 (1957). 



146 H. C. Reilly, Proc. Am. Assoc. Cancer Research 2, 41 (1955). 



147 G. A. LePage and A. C. Sartorelli, Texas Repts. Biol, and Med. 15, 169 (1957). 



148 M. Potter and L. W. Law, J. Natl. Cancer Inst. 18, 413 (1957). 



149 G. S. Tarnowski and C. C. Stock, Cancer Research 17, 1033 (1957). 



150 R. E. Maxwell and V. S. Nickel, Antibiotics & Chemotherapy 7, 81 (1957). 



151 A. C. Sartorelli and G. A. LePage, Cancer Research 18, 457 (1958). 



