602 G. SCHMIDT 



all nucleosidases are phosphorylases, but that hydrolytic enzymes for the 

 cleavage of the bonds between purine or pyrimidine bases and ribose do 

 exist. The cleavage of nucleosides is, therefore, another example of the ex- 

 istence of dual mechanisms for the enzymic cleavage of glycosidic bonds. 



1. Nucleoside Phosphorylases 

 a. Purine Nucleoside Phosphorylases 



(1) Animal Enzymes. The first thorough studies of these enzymes were 

 carried out on purified rat and calf liver extracts by Kalckar,!^* who ob- 

 tained active preparations from the supernatant solutions obtained by 

 high-speed centrifugation (16,000 r.p.m.). The extracts were purified by 

 ammonium sulfate fractionation. The fraction which precipitated between 

 0.4 and 0.6 saturation was further purified by isoelectric precipitation at 

 pH 6. The enzyme activity was recovered in the supernatant. 



An enzyme of very similar specificity was purified by Heppel and Hil- 

 moe^^^ from yeast autolysates by ammonium sulfate fractionation and sub- 

 sequent adsorption on aged calcium phosphate gel. A 19-fold purification 

 was achieved but considerable losses occurred during the procedure. 



Specificity: liver enzyme. Originally Kalckar found that only ribo- and 

 deoxyriboguanosine and -hypoxanthine were split by purine nucleoside 

 phosphorylase of rat liver, and that adenosine, xanthosine, and the pyrimi- 

 dine nucleosides were resistant. Cardini and his associates^^^ as well as 

 Friedkin^^^'i^^ found later, however, that uridine and xanthosine were also 

 split by highly concentrated liver enzyme. The phosphorolysis of uridine 

 by highly concentrated solutions of purine nucleoside phosphorylase is 

 most likely caused by contamination with thymidine phosphorylase (see 

 below). Isoguanosine and adenine thiomethylriboside are not split by the 

 enzyme.^"" 



Highly purified purine nucleoside phosphorylase was recently obtained 

 by Korn and Buchanan^oi .202 from aqueous extracts of acetone-dried beef 

 liver, by alcohol and ammonium sulfate fractionation and by adsorption on 

 silica gel. The final preparation, which was approximately 200 times as 

 active as the crude aqueous extract, was capable of converting adenine to 

 inosine in the presence of rib ose-1 -phosphate. 202 It is as yet undecided 



i9« L. A. Heppel and R. J. Hilmoe, J. Biol. Chem. 198, 683 (1952). 



1" C. E. Cardini, A. C. Paladini, R. Caputto, and L. F. Leloir, Acta Physiol. Lati- 



noamer. 1, 57 (1950). 

 188 M. Friedkin, J. Am. Chem.. Soc. 74, 112 (1952). 

 199 M. Friedkin, Federation Proc. 11, 216 (1952). 



20° M. L. Schaedel, M. J. Waldvogel, and F. Schlenk, /. Biol. Chem. 171, 135 (1947). 

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

 202 E. D. Korn, F. C. Charalampous, and J. M. Buchanan, J. Am. Chem. Soc. 75, 



3610 (1953). 



