302 Biological Chemistry. 



that the organs of different animals vary very consider- 

 ably as regards their content in the oxidases bringing 

 about the oxidation of the various purine substances. 

 Thus the following organs contain an oxidase capable 

 of converting (in the presence of air) xanthine into uric 

 acid : human liver, ox liver, ox spleen, adult pig's liver. 

 In the following organs the xanthine oxidase is absent : 

 human organs other than the liver, ox-thymus, pig's spleen, 

 embryo pig's liver, all the organs of the rat. It is not 

 easy to generalize the results obtained on the oxidation of 

 the purine substances ; they are quoted as examples of 

 detailed work, in which attempts have been made to 

 follow out the intermediary metabolism of nucleic acid 

 in vitro. In all cases the experiments have been carried 

 out by the incubation of the pure intermediary products 

 with the minced organ in the presence of air. 



Numerous experiments have been also performed with 

 the object of ascertaining the mechanism by means of 

 which deaminization (scission of the amino group) takes 

 place in such substances as the amino-acids and substances 

 containing amino groups derived from products of nucleic 

 acid degradation, such as guanine or adenine. No satis- 

 factory explanation of enzymatic deaminization has as yet 

 been obtained. 



One other series of investigations is also worthy of 

 note before concluding this section, namely, those dealing 

 with the conversion of ammonium salts into urea. It is 

 known that this process takes place in the liver ; it is of 

 interest to recall the fact that the reverse process is readily 

 brought about by an enzyme in the soya-bean (p. 279). 



