ENZYMES ATTACKING NUCLEIC ACIDS 605 



Paege and Schlenk^^^ demonstrated the presence of potent uracil ribo- 

 nucleoside phosphorylases in aciueous cell-free extracts of E. cold, Aerohacter 

 aerogenes, and Micrococcus hjsodcicticus. Cytidine was inert toward these 

 enzymes. 



Direct evidence for the existence of specific pyrimidine nucleoside phos- 

 phorylases Avas subsequently furnished by the purification of two enzyme 

 fractions whose properties will be briefly described below. 



(1) Animal Thymidine Phosphonjlase. The reversible phosphorolytic 

 cleavage of deoxythymidine was first observed in kidney extracts by Man- 

 son and Lampen.'-^''"''' Friedkin-"<='<i obtained from horse liver a highly pu- 

 rified thymidine nucleoside phosphorylase. 



Specificity. Friedkin's enzyme catalyzes the reversible exchange of the 

 phosphoryl group of deoxyribose-1 -phosphate and- — at slower rates — of 

 ribose-1-phosphate with thymine or uracil. 2-Thiouracil, 5-aminouracil, 

 5-iodouracil, and 5-bromouracil are likewise substrates of the enzyme. 

 2-Deoxy thymidine was isolated by Friedkin and Roberts ;^^^'='i thiouracil 

 riboside and deoxyriboside were isolated by Strominger and Friedkin^^'^ 

 from digests of the respective substrate mixtures with thymidine phos- 

 phorylase. 



Cytosine and orotic acid, as well as adenosine, guanosine, and hypo- 

 xanthosine, are inert in the enzyme system. 



Equilibrium. Since thymine inhibits the reaction by an unknown mech- 

 anism, a well-defined efjuilibrium constant has not been obtained. At suf- 

 ficiently high concentrations of deoxyribose-1-phosphate 80 to 90% thymine 

 formation was observed. 



yH optimum. The pH optimum for the enzymic arsenolysis catalyzed by 

 thymidine phosphorylase is between 5.7 and G. The activity decreases rap- 

 idly at the acid, slowly at the alkaline, side of the optimum and has a 

 plateau at 03% of the optimal activity between pH 8.1 and 8.8. 



Heat inactivation. The activity decreases by 9% during 8 miiuites' heat- 

 ing at 50°, by 44 % at 00°, and by 97 % at 70°. 



(2) Bacterial Uridine Phosphorylase. A specific uridine phosphorylase was 

 purified from E. coli E-2G (Iowa State College Laboratory culture) by Paege 

 and Schlenk'-"' by ammonium sulfate fractionation, adsorption on alumi- 

 num hydroxide, and elution with sodium phosphate. 



Specificity. The enzyme catalyzes only the reversible phosphorolysis of 



2" L..M. Paege and F. Schlenk, Arch. Biochem. 28, 348 (1950). 



2'>» L. A. Manson and J. O. Lampen, Federation Proc. 8, 224 (1949). 



2'"^ L. A. Manson, Thesis, Washington University, St. Louis, 1949. 



2'"= M. Friedkin and D. Roberts, J. Biol. Chern. 207, 245 (1954). 



*"<iM. Friedkin and D. Roberts, J. Biol. Chern. 207, 257 (1954). 



2"«D. B. Strominger and M. Friedkin, J. Biol. Chern. 208, 663 (1954). 



"" L. M. Paege and F. Schlenk, Arch. Biochem. and Biophys. 40, 57 (1952). 



