318 



F. SCHLENK 



Fig. 5. Absorption spectrum of uracil, -O — 0-, and uridine, -• — •-, at pH 11.5. 

 The enzymic splitting of uridine can be measured by adjustment of aliquots of the 

 incubation mi.xture with alkali and observation of the increase in density at 290 m/<. 

 An alternative is the continuous observation of the density at 280 m/i in neutral 

 medium.""^^ 



Lampen and Wang-^ found a nucleoside hydrolase in extracts from L. 

 pentosus which hydrolyzed cytidine and uridine. Divalent ions such as 

 HPOr~, HASO4 , SO4 , or succinate stimulated the reaction. Lampen^^ 

 believes that these ions increase the stability of the pyrimidine nucleosidase 

 which otherwise is completely inactive after one minute at 37° C. Free sugar 

 and base are the products of the reaction. 



6. Pyrimidine Deoxyriboside Phosphorylase 



The occurrence of pyrimidine deoxyriboside nucleosidase in kidney was 

 observed by W. Klein. ^- The preparations of Wajzer-^ and of Friedkin and 

 Kalckar-- act on various deoxyribose nucleosides, and the preparation from 

 horse liver^ catalyzes the following reactions: 



Thymine + deoxyribose-l -phosphate 



(8) 



thymine deoxyriboside + phosphate 



Uracil + deoxyribose-l -phosphate ^ uracil deoxyriboside + phosphate (9) 



According to Friedkin^ the protozoon Tetrahymena geleii is another suitable 

 source of this enzyme. Pyrimidine deoxyriboside phosphorylase from E. coli 

 has been studied by Manson and Lampen. ^^ The specificity of this enzyme 

 was the same as that of the tissue enzyme. Cytosine deoxyriboside was not 

 split. 



There are as yet no studies on record concerning 5-methylcytosine deoxy- 



