BIOSYNTHESIS OF PURINES AND PYRIMIDINES 291 



TABLE VIII 



Comparison of Radioactive Inosine and Hypoxanthine in Inosinic Acid 



Synthesis by a Purified Enzyme System'^" 



Radioactivity incorporated 

 into inosinic acid, 

 Radioactive substrate Nonradioactive additions counts/min. 



Inosine Hypoxanthine, ATP 132 



Hypoxanthine Inosine, ATP 233 



The enzyme system involved in the synthesis of inosinic acid from hy- 

 poxanthine in pigeon hver has been purified and fractionated by Wilhams 

 and Buchanan^^'^^* and by Kom and Buchanan.''^ As was pointed out by 

 Buchanan/^ the study of this reaction should yield very valuable general 

 information on ribotide formation, even though the introduction of ribose 

 and phosphate during the biosynthesis of purines takes place before the 

 purine ring is closed. The reaction which required ribose-5'-phosphate + 

 ATP was catalyzed by at least two enzymes which could be purified by 

 ethanol fractionation. It was sho^^^l^^ that none of the enzymes was identical 

 with Kalckar's^^ nucleoside phosphorylase. A comparison between hypoxan- 

 thine-C^* and inosine-C" with regard to transformation to inosinic acid 

 showed that inosine was not an intermediate in the conversion of hypoxan- 

 thine to its nucleotide (Table VIII). The above-mentioned results of 

 Greenberg®^ were explained by postulating the existence of two different 

 independent reactions, one leading from hypoxanthine to inosine and the 

 other from hypoxanthine to inosinic acid. 



An interesting observation has been made by Buchanan and Schulman®^ 

 on the participation of the citrovorum factor in the synthesis of inosinic 

 acid. The addition of this factor to pigeon liver extracts enhances the de novo 

 synthesis of inosinic acid from glycine. In a system in which nonlabeled 

 inosinic acid had been added at the beginning of the experiment and bicar- 

 bonate had been omitted, the citrovorum factor stimulated the incorpora- 

 tion of labeled glycine only shghtly but markedly increased the incorpora- 

 tion of formate-C'^ into position 2 of inosinic acid. A possible scheme for 

 this "enzymic exchange" of formate with position 2 of inosinic acid, as de- 

 picted by Buchanan and Schulman, is given in Fig. 6. This problem is also 

 discussed in Chapter 24. 



71 W. J. Williams and J. M. Buchanan, Federation Proc. 11, 311 (1952). 



"* W. J. Williams and J. AI. Buchanan, J. Biol. Chem. 203, 583 (1953). 



" E. D. Korn and J. M. Buchanan, Federation Proc. 12, 233 (1953). 



7' J. M. Buchanan, in "Phosphorus Metabolism" (McElroy and Glass, eds.). Vol. 2, 



p. 406. Johns Hopkins Press, Baltimore, 1952. 

 7* H. M. Kalckar, J. Biol. Chem. 167, 429 (1947). 



