ENZYMES ATTACKING NUCLEIC ACIDS 603 



whether the resistance of adenine toward the cruder enzyme preparation 

 from rat Hver is a peculiarity of this species or whether it is caused by the 

 presence of an inhibitor in these enzyme preparations. Rowen and Korn- 

 berg-°' found that hver contains a nicotinamide nucleoside phosphorylase 

 and reported evidence in favor of its identity with the purine nucleoside 

 phosphorylase. Reduced nicotinamide riboside is not cleaved by the en- 

 zyme.^°* 



(2) Enzymes of Microorganisms. According to Heppel and Hilmoe,^^^ the 

 yeast enzyme splits only guanosine, hypoxanthosine, and nicotinamide 

 nucleoside, but is without action toward adenosine, xanthosine, and some 

 synthetic purine nucleosides. No information on the behavior of the deoxy- 

 ribonucleosides toward the yeast enzyme is available as yet. E. coli (strain 

 15, American Type Culture Collection Xo. 9723) contains potent purine 

 and pyrimidine deoxynucleoside phosphorylases according to Manson and 

 Lampen.-°^ 



Mechanism of the Reaction (see also Chapter 24). With hypoxanthosine 

 as substrate, the following reaction is catalyzed by the purine nucleoside 

 phosphorylase :^^^ 



hypoxanthosine + phosphate ;^ hypoxanthine + ribose-1-phosphate 



The phosphate in this reaction can be replaced by arsenate; Heppel and 

 Hilmoe state that the rate of arsenolysis of hypoxanthosine is approxi- 

 mately two-thirds of that of its phosphorolysis. 1-Riboarsenate undergoes 

 spontaneous hydrolysis according to Manson and Lampen f^^ this observa- 

 tion probably accounts for Klein's opinion (based on the results of a reducto- 

 metric assaj^ procedure) that arsenate would be a more efTective activator 

 of nucleosidase than phosphate. It follows from the eciuation that, for ex- 

 ample, a mixture of guanine riboside, hypoxanthine, and phosphate is 

 transformed by the enzyme into a mixture of guanine and hypoxanthine 

 ribosides and ribose-1 -phosphate. This has been verified by Friedkin and 

 Kalckar-"^ for the deoxyribosides. 



According to Friedkin and Kalckar, a mixture of ribo- and deoxyribo- 

 nucleosides is not phosphorylated at a higher rate than ecjuimolar amounts 

 of the pure nucleosides. This suggests that the phosphorolysis of purine 

 ribo- and deoxyribonucleosides is catalyzed by the same enzyme. 



Influence of pH. The optimum for the action of the yeast enzyme was 

 found near pH 7 by Heppel and Hilmoe ;^^^ the experiments with the liver 



"3 J. W. Rowen and A. Romberg, J. Biol. Chem. 193, 497 (1951). 

 2"^ H. M. Kalckar, Biochim. et Biophijs. Acta 12, 250 (1953). 

 ^oe L. A. Manson and J. O. Lampen, J. Biol. Chem. 193, 539 (1951). 

 "« L. A. Manson and J. O. Lampen, J. Biol. Chem. 191, 95 (1951). 

 2" M. Friedkin and H. M. Kalckar, /. Biol. Chem. 184, 437 (1950). 



