172 IMMUNO-CATALYSIS 



The oxidation of hypoxanthine 

 H— N— C = H— N— C = H— Ni— eC^O 



H 



H— C C— N 



H 



->0 = C C— N 



H 



-_).0 = C2 ^C— 'N 



CH 



^ 



N— C— N 

 Hypoxanthine 



H— N— C— N 



Xanthine 



^ 



CH 



H- 



«c=o 



-N3— "G— ^N— H 

 Uric Acid 



shows that the point of attack is at carbon 8 in the purine ring when 

 xanthine is oxidized to uric acid and at carbon 2 when hypoxanthine is 

 oxidized to xanthine. The fact that uric acid as a reaction product 

 inhibits the oxidation of hypoxanthine and xanthine shows that it 

 reacts with the same active grouping of the enzyme molecule which 

 combines also with xanthine or hypoxanthine. As a result of this re- 

 action the two xanthines become activated but uric acid not being 

 activated by the enzyme can act only as a competitive inhibitor. 



12. Inhibition of the Hydrolysis of Guaninedesoxyribose 

 by the Reaction Product Guanine 



Klein (1935) prepared an enzyme from spleen which was highly 

 specific in hydrolyzing purine nucleosides. This enzyme preparation 

 did not act on pyrimidine nucleosides, purine or pyrimidine nucleo- 

 tides, or on d-ribose- or desoxyribose-nucleic acids. 



To determine the specific affinity of nucleosidase for various sub- 

 stances he resorted to inhibition experiments, using guanine desoxy- 

 riboside as substrate. 



enzyme 

 Guaninedesoxyriboside >guanine-|-desoxyribose 



The experimental procedure yielded 51 per cent hydrolysis of the 

 nucleoside. Studying the effect of 18 different substances he found 

 that the addition of 1 mg. each of guanine, hypoxanthine and ade- 

 nine to the reaction system produced, respectively, 47, 53 and 21 per 

 cent inhibition. Under identical conditions xanthine produced 10 

 per cent inhibition and uric acid had no effect whatsoever. It is evi- 



