332 F. SCHLENK 



^C=0 C=0 



I »► I *► 



= 0-^ Ribose-N-C 



I H I 



COOH COOH 



H H 



o=c c=o o=c c=o 



HO I *► I I 



^CH Ribose-N^ ^CH 



Ribose-N-C C 



H H H 



Fig. 6. A suggested mechanism for the biosynthesis of uridine. '"^ 



the glycosidic bond by combination with the pentose, occurs first. The for- 

 mer reaction has not been tested in vitro with the nucleosidase presently 

 available, and no enzyme has been found so far which is capable of combin- 

 ing the 1 -phosphate esters of ribose or deoxyribose with orotic acid. The 

 isolation of orotidine by Mitchell and his co-workers'^ will facilitate the 

 study of this problem. In Chapter 23 it has been pointed out that orotic 

 acid, labeled in many different ways, is readily incorporated into the nu- 

 cleic acid fraction of various types of cells. This favors strongly the assump- 

 tion that orotic acid is a key intermediate in pyrimidine nucleoside and 

 nucleotide formation. 



On much firmer ground is the claim of Greenbergio^-^'^'' that purine nu- 

 cleosides and nucleotides may be formed by combination of the carbohy- 

 drate with purine precursors, followed by completion of the purine system. 

 Experiments of this type have been restricted to inosine and inosinic acid. 

 We owe the most significant contributions to Greenberg and to Buchanan 

 and their co-workers; rapid progress may be anticipated. Figuring most 

 prominently as a purine precursor, in these studies, is 4-amino-5-imidazole- 

 carboxamide:'"* 



H2N C NH 



C CH 



/ \ / 

 H2N N 



This incomplete hypoxanthine was first encountered in sulfadiazine-inhib- 



">« G. R. Greenberg, Federation Proc. 9, 179 (1950). 

 10^ G. R. Greenberg, J. Biol. Chem. 190, 611 (1951). 

 108 A. Windaus and W. Langenbeck, Ber. 56, 683 (1923). 



