314 



F. SCHLENK 



220 240 260 280 300 



WAVE LENGTH mp 



Fig. 2. Absorption spectra of some oxypurines:^''-0 — 0-, hypoxanthine;-X — X-, 

 xanthine; -• — •-, uric acid. 



2. Purine Riboside Hydrolases 



The discovery of the phosphorolysis of purine nucleosides by Kalckar^' 

 led to the concept that all nucleosides are metabolized according to this 

 mechanism. The first observation to the contrary concerned the hydrolytic 

 cleavage of uridine. More recently, purine riboside hydrolases have been 

 found in bacteria and in yeast, and these do not show the restricted range 

 of action of the phosphorylase. 



A comprehensive study of the nucleosidases of yeast was made by Heppel 

 and Hilmoe.^* The phosphorolytic and hydrolytic enzymes could be sepa- 

 rated ; the former behaved much like the corresponding enzyme from animal 

 tissues while the latter was active with a variety of purine ribosides such as 

 inosine, guanosine, adenosine, xanthosine, and a number of synthetic nu- 

 cleosides. Wang and Lampen^" found in Lactobacillus pentosus a hydrolytic 

 enzyme splitting adenosine to adenine and ribose; in addition, inosine, 

 guanosine, xanthosine, and uric acid riboside were hydrolyzed.^' The crude 

 enzyme was found sensitive to dialysis but, in contrast to the tissue enzyme, 

 phosphate or arsenate did not restore the activity. 



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



20 T. P. Wang and J. O. Lampen, J. Biol. Chem. 192, 339 (1951). 

 =1 J. O. Lampen and T. P. Wang, /. Biol. Chem. 198, 385 (1952). 



