316 JOHN M. BUCHANAN 



adenine nucleotides cannot be replaced in the reaction by the other purine 

 and pyrimidine nucleotides. 



During its further conversion to inosinic acid, 5-amino-4-imidazole-iV r - 

 succinocarboxamide ribonucleotide is cleaved to fumaric acid and 5-amino- 

 4-imidazolecarboxamide ribonucleotide [Eq. (10)]. 52 This reaction has an 

 equilibrium constant of 2.3 X 10~ 3 . The enzyme responsible for Eq. (10) 



5-Amino-4-imidazole-jV-succinocarboxamide ribonucleotide <=± 



(10) 

 5-amino-4-imidazolecarboxamide ribonucleotide + fumaric acid 



is believed to be identical with adenylosuccinase for reasons which will be 

 presented in a later section. 



5-Amino-4-imidazolecarboxamide ribonucleotide has an absorption max- 

 imum at 267 m/x and reacts with the Bratton and Marshall reagents to yield 

 a purple-colored product which absorbs maximally at 540 m/z. In contrast 

 to some of the heterocyclic amines mentioned previously, treatment of the 

 carboxamide ribonucleotide briefly at 100° in acid solution does not affect 

 the color yield obtained in the Bratton and Marshall procedure. 



5-Amino-4-imidazolecarboxamide which accumulates with its ribosyl and 

 phosphoribosyl derivatives in cultures of Escherichia eoli poisoned with 

 sulfonamides, was first isolated as the free base by Stetten and Fox 53 and 

 soon thereafter identified by Shive and his co-workers. 64 By more refined 

 methods it was possible to isolate the ribonucleoside and in small quantities 

 the ribonucleotide from these cultures. 55 The ribonucleotide may also be 

 formed enzymically by phosphorylation of the ribonucleoside 56 or by re- 

 action of the base with 5-phosphoribosylpyrophosphate. 39 



The final steps in the synthesis of inosinic acid de novo b7 ' 60 involve the 

 formylation of 5-amino-4-imidazolecarboxamide ribonucleotide by iV 10 -for- 



A rie -Formyl-FH 4 -f- 5-amino-4-imidazolecarboxamide ribonucleotide — * 



(11) 

 5-formamido-4-imidazolecarboxamide ribonucleotide + FH 4 



myltetrahydrofolic acid according to Eq. (11). The formamide compound 

 does not accumulate since the transformylase responsible for Eq. (11) also 



52 R. W. Miller, L. N. Lukens, and J. M. Buchanan, J. Am. Chem. Soc. 79, 1513 (1957). 



53 M. R. Stetten and C. L. Fox, Jr., J. Biol. Chem. 161, 333 (1945). 



54 W. Shive, W. W. Ackermann, M. Gordon, M. E. Getzendaner, and R. E. Eakin, 

 /. Am. Chem. Soc. 69, 725 (1947). 



56 G. R. Greenberg and E. L. Spilman, J. Biol. Chem. 219, 411 (1956). 



56 G. R. Greenberg, J. Biol. Chem. 219, 423 (1956). 



57 G. R. Greenberg, Federation Proc. 13, 745 (1954). 



58 G. R. Greenberg, L. Jaenicke, and M. Silverman, Biochim. et Biophys. Acta 17, 

 589 (1955). 



59 J. G. Flaks, M. J. Erwin, and J. M. Buchanan, J. Biol. Chem. 229, 603 (1957). 



60 L. Warren, J. G. Flaks, and J. M. Buchanan, J. Biol. Chem. 229, 627 (1957). 



