316 



Fig. 3. Equilibrium between enzymic synthesis and splitting of uridine," accord- 

 ing to reaction (7). Purified enzyme from Escherichia coli was used with a substrate 

 concentration of 4.4 /zM. per ml.; synthesis of uridine, -O — ^O-; splitting of uridine, 



-• — •-. 



A highly purified enzyme was obtained from E. colif^ it shows specificity 

 for uridine :^^ 



Uridine + phosphate ^ uracil + ribose-1 -phosphate 



(7) 



Cytidine, purine ribosides, and thymine deoxyriboside are not split, and 

 orotic acid does not react with ribose-1 -phosphate in the presence of this 

 enzyme. The study of this reaction was facilitated by a modified orcinol 

 test which permits one to distinguish uracil- and cytosine-bound ribose from 

 other ribosides.^" The equilibrium attained in reaction (7) is illustrated in 

 Fig. 3. 



For the metabolism of cytidine, a connection with uridine by amination 

 or deamination is well established. The recent discovery of orotidine^'^ sug- 

 gests a special enzyme,^-'* bringing about the first step in the utilization 

 of orotic acid which has been observed in many types of cells. 



5. Pyrimidine Nucleoside Hydrolases 



The discovery of phosphorolytic cleavage of nucleosides^ '^ suggested the 

 ubiquity of this mechanism in nucleoside metabolism. The first exception 



'0 L. M. Paege and F. Schlenk, Arch. Biochem. 28, 348 (1950). 



" L. M. Paege and F. Schlenk, Arch. Biochem. and Biophys. 40, 42 (1952). 



« A. M. Michelson, W. Drell, and H. K. Mitchell, Proc. Natl. Acad. Sci. U. S. 37, 



396 (1951). 

 ^2" I. Lieberman, A. Romberg, E. S. Simms, and S. R. Kornberg, Federation Proc. 



13, 252 (1954) . 



