606 ^- SCHMIDT 



uracil riboside. Adenosine, guanosine, hypoxanthine, ribocytidine, and de- 

 oxy thymidine are inert toward the enzyme. 



Equilibrium constant. The equilibrium constant of 0.62 is less in favor of 

 the synthetic direction than that of purine nucleoside phosphorylases. 



pH Optimum. The optimal pH is near pH 7.2. 



Assaij. The activity of the enzyme can be measured either by colori- 

 metric pentose determination (pyrimidine ribosides yield only very little 

 pigment in comparison with equimolar amounts of ribose-1 -phosphate or 

 free ribose) or by ultraviolet spectrophotometry. 



2. Nucleoside Hydrolases 

 a. Purine Nucleoside Hydrolase of Yeast 



Heppel and Hiknoe^^^ found in 1952 that autolysates of bakers' yeast 

 were capable of splitting nucleosides in absence of phosphate or arsenate. 

 They succeeded in separating the nucleoside hydrolase from the nucleoside 

 phosphorylase by ammonium sulfate fractionation and in achieving further 

 purification of the hydrolase by subsequent adsorption on calcium phos- 

 phate gel. The presence of a purine nucleoside hydrolase in potatoes was 

 likewise demonstrated by Heppel and Hilmoe. 



Specificitij. The purified nucleoside hydrolase splits adenosine, guanosine, 

 hypoxanthine, xanthosine, nicotinamide nucleoside, and some synthetic 

 nucleosides such as 2,6-diaminopurine-9-ribofuranoside. Amongst the 

 latter are o-ribopyranosyl adenine, D-glucopyranosyl adenine, D-arabo- 

 furanosyl adenine, D-ribofuranosyl adenine, which were substituted at the 

 2-carbon, and D-ribofuranosyl nucleosides of some synthetic purines. 



Mechanism. The enzyme catalyzes the reaction: 



hypoxanthosine + water = D-ribose -|- hypoxanthine 



No exchange reaction was detected in the presence of isotopically labelled 

 free purine in the reaction mixture. 



The pH optimum of the purine nucleoside hydrolase is somewhat higher 

 than that of the phosphorylase, namely, at about pH 8. 



b. Uridine Hydrolase of Yeast 



Carter^^^ obtained from autolysates of bakers' yeast in 1950 a highly spe- 

 cific uridine nucleosidase which did not require phosphate or arsenate for 

 its activity. No hydrolytic uridine nucleosidase has been found so far in 

 mammalian tissues. The enzyme was partially purified 10 to 15 times by 

 ammonium sulfate fractionation. Adenine, inosine, guanosine, cytidine, and 

 thymidine were not degraded by the enzyme. The reaction followed a uni- 

 molecular course up to 85% of hydrolysis. The pH optimum was found to 

 be at pH 7. 



