320 JOHN M. BUCHANAN 



enylic acid from inosinic acid is catalyzed by two enzymes and proceeds via 

 the intermediate adenylosuccinic acid. 77 The reaction sequence is in several 

 respects analogous to other reactions in which the a-amino nitrogen of 

 aspartic acid is transferred to form an amide [Eqs. (15 and 1G)]. Both Eqs. 



Inosinic acid + aspartic acid + GTP ?=* 



(15) 

 adenylosuccinic acid -f- GDP + orthophosphate 



Adenylosuccinic acid <^ adenylic acid + fumaric acid (16) 



(15) and (16) are reversible. The involvement of guanosine compounds in 

 Eq. (15) has been definitely proven in the enzyme system isolated from E. 

 coli. n A similar function for guanosine compounds has also been demon- 

 strated for the system in bone marrow but here there is a cooperating func- 

 tion of adenosine triphosphate which is not at present completely under- 

 stood. 76 When inosinic acid is labeled in position G with O 18 , O 18 is found in 

 the orthophosphate formed. There are many similarities between Eq. (9), 

 the synthesis of 5-amino-4-imidazole-A r -succinocarboxamide ribonucleotide, 

 and Eq. (15). The major difference is the preference of the nucleoside tri- 

 phosphate, which by the hydrolysis of a phosphoanhydride bond provides 

 the energy for the synthesis of the carbon to nitrogen bond. 



The splitting of adenylosuccinic acid to adenylic acid and fumaric acid 

 is in close analogy to the splitting of 5-amino-4-imidazole-./V-succinocar- 

 boxamide ribonucleotide to 5-amino-4-imidazolecarboxamide ribonucleotide 

 and fumaric acid. In fact, present evidence indicates that adenylosuccinase 

 is responsible for both functions. 5 ' 2 Upon purifying the enzyme over 200-fold, 

 activites toward both substrates are maintained in a constant ratio during 

 the various steps of purification. Moreover, in a certain class of mutant of 

 Neurospora crassa loss of activity for one substrate is always accompanied 

 by loss of activity for the other. Similar observations have been made with 

 mutants of E. coli and Salmonella typhimnrium. 79 This is thus an unusual 

 circumstance in which one genetic event results in the loss of enzymic ac- 

 tivities for two nonsequential steps in a biosynthetic pathway. 



VI. Synthesis of Guanylic Acid from Inosinic Acid 



The first step in the formation of guanylic acid from inosinic acid (Fig. 3) 

 involves the oxidation of inosinic acid to xanthylic acid by DPN [Eq. (17)]. 



Inosinic acid 4- DPN + H 2 <=± xanthylic acid + DPNH + H + (17) 



The enzyme responsible for this reaction has been called inosine-5'-phos- 

 phate dehydrogenase. The further animation of xanthylic acid to guanylic 



77 C. E. Carter and L. H. Cohen, J. Biol. Chem. 222, 17 (1956). 



78 I. Lieberman, J. Am. Chem. Soc. 78, 251 (1956). 



79 J. S. Gots and E. G. Gollub, Proc. Natl. Acad. Sci. U. S. 43, 826 (1957). 



