490 R. E. HANDSCHUMACHER AND A. D. WELCH 



zymic digestion, indicate marked incorporation of the analog into the termi- 

 nal positions of shorter polynucleotide chains, a circumstance which suggests 

 that the presence of azaguanine in some cases may terminate further addi- 

 tions to the polymer. 255, 260 The form of the analog in the DNA of B. cereus 

 has been established by hydrolysis to the corresponding deoxyribonucleo- 

 sides and isolation of deoxyazaguanosine. 240, 255 In neither the RNA nor 

 the DNA has an azaadenine derivative been detected, even when the growth 

 of a bacterial culture was inhibited by azaadenine; however, azaadenine was 

 converted to derivatives of azaguanine, with subsequent incorporation of 

 these into nucleic acid. 255 Azaguanosine and the corresponding deoxyribo- 

 nucleoside have been prepared enzymically by condensation of azaguanine 

 with ribose-1-phosphate or deoxyribose-1-phosphate, a reaction catalyzed 

 by nucleoside phosphorylases from horse liver. 262 Azaguanosine-5'-phos- 

 phate (as well as azaguanosine 240 ) has been isolated from B. cereus exposed 

 to azaguanine 255, 260 ; and the enzymic synthesis of the ribonucleotide has 

 been reported by direct reaction of azaguanine with pyrophosphorylribose- 

 5-phosphate. 188 Correlations have been noted in both mouse tumors and 

 S. faecalis between resistance to azaguanine and the capacity to form aza- 

 guanine nucleotides in vivo 179 *- m • 183 • 259 ; a reflection of this is seen in the levels 

 of the nucleotide pyrophosphorylase activity in vitro. 173 Differences in cellu- 

 lar permeability in the various strains of mouse leukemias has been elimi- 

 nated as a possible mechanism of resistance. 263 These results support the 

 concept, derived from earlier suggestions, that the analog must be con- 

 verted to a ribonucleotide before it becomes an active inhibitor. 232, 264, 166 

 However, preliminary trials with azadeoxyguanosine as an inhibitor of the 

 growth of Tetrahymena 262 or with the mixed 2'-3'-monophosphates of aza- 

 guanosine as an inhibitor of tumor growth 261 did not indicate enhanced ac- 

 tivity as compared to that of azaguanosine. The partial identification of the 

 di- and triphosphates of azaguanine, which must certainly be formed prior 

 to incorporation into nucleic acids, has been reported for extracts of S. 

 faecalis. 179 Also, these ribonucleotides have been prepared by chemical and 

 enzymic methods; in the latter a preparation from hog kidney was em- 

 ployed. 189 



A considerable limitation of the action of azaguanine in mammalian 

 systems results from its rapid deamination by guanase in vivo 26b or in 

 vitro 266 ' 268 to azaxanthine, biologically a relatively inert compound. 236, 269 



262 M. Friedkin, /. Biol. Chem. 209, 295 (1954). 



263 J. D. Davidson, Proc. Am. Assoc. Cancer Research 2, 290 (1958). 



264 G. B. Elion, S. Singer, and G. H. Hitchings, Federation Proc. 15, (1956). 



265 H. G. Mandel, E. L. Alpen, W. D. Winters, and P. K. Smith, J. Biol. Chem. 193, 

 63 (1951). 



266 A. Roush and K. R. Norris, Arch. Biochem. 29, 124 (1950). 



267 J. Kream and E. Chargaff, J. Am. Chem. Soc. 74, 4274 (1952). 



