BIOSYNTHESIS OF NUCLEOSIDES AND NUCLEOTIDES 333 



CH,-NH. CH.-NH, CH.-NHCHO 



I R>bose-PO, ^ ' "HCOOH" ' 



cooH »► o=c ^ y^ 



+ ..NH," HN-Ribose-5-PO.H2 N-Ribose-S-POjH^ 







II 



. 2'NH" H.N C N "HCOOH" ^ _ , , . 



+ ^ ^"' > II II *- Inosine-5 -phosphate 



Ribose-S-POjH. 

 Fig. 7. Working scheme of nucleotide biosynthesis.'" 



ited cultures of E. coli by Stetten and Fox/"' it was identified by Shive 

 and his associates.""-"' Greenberg"" came to the conclusion that the 

 carboxamide as such is not an intermediate in the formation of inosinic 

 acid, nor does inosine have to be considered as an obligatory precursor. 

 There is indication that the carboxamide ribotide is the immediate precursor 

 of inosinic acid. Schulman and Buchanan studied the incorporation of 

 formate-C'^ into this compound, which yields carbon atom 2 of the base."^ 

 In presence of inosinic acid a high rate of exchange of the carbon atom 2 

 of the purine with the carbon of labeled formate was found. Hypoxanthine 

 and inosine were inert. "^ 



The concept of Greenberg'"^ that the free purines may not be the products 

 of direct assembly of their structural units, but instead may be formed by 

 combination of ahphatic nitrogenous compounds with ribose phosphate, can 

 be formulated as in Fig. 7.^° '^"^ There is some indication that the purine nu- 

 cleus is assembled to the carboxamide stage while in linkage with pentose, 

 because Greenberg"^ was able to show that the carboxamide riboside rather 

 than the carboxamide itself is produced by young cultures of sulfadiazine- 

 inhibited E. coli. Earlier investigators missed this compound because their 

 conditions of isolation inadvertently caused hydrolysis. On the other hand, 

 instead of being an intermediate, the carboxamide riboside and carbox- 

 amide itself may be merely split products of inosinic acid."' Buchanan and 

 co-workers"''^* observed that purified beef liver nucleosidase is capable of 

 combining the carboxamide with ribose-1 -phosphate. The carboxamide 

 riboside is readily converted to the ribotide by liver or yeast enzymes in 

 presence of ATP and Mg^."'* 



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



"0 W. Shive, W. W. Ackermann, M. Gordon, M. E. Getzendanner, and R. E. Eakin, 



J. Am. Chem. Soc. 68, 725 (1947). 

 '" J. M. Ravel, R. E. Eakin, and W. Shive, J. Biol. Chem. 172, 67 (1948). 

 "2 M. P. Schulman and J. M. Buchanan, J. Biol. Chem. 196, 513 (1952). 

 "3 G. R. Greenberg, J. Am. Chem. Soc. 74, 6307 (1952). 

 '1^ G. R. Greenberg, Federation Proc. 12, 211 (1953). 



