METABOLISM OF PROTEINS 



583 



is 2-amino-6-oxypurin, NH a being united with C(2) and oxygen with C( 6 j 

 in purin. Uric acid is 2, 6, 8-trioxypurin i.e., purin in which oxygen 

 is united to the carbon atoms 2, 6, and 8. Hypoxanthin is 6-oxypurin, 

 oxygen being introduced at the position of C (6 ) in purin. By removal 

 of the amino-group from adenin hypoxanthin is formed. Xanthin is 

 2, 6, dioxypurin, oxygen being introduced at C (2 ) and C< 6 ) in the purin 

 molecule. Xanthin can be derived from guanin in the same way as 

 hypox?,nthin from adenin. 



i 



C (5) N 



(7) 



N {3) CMJ N (9 ) 



Purin nucleus. 



N =CH 

 HC C NH 



N =C.NH a 

 HC C NH 



./ 



Purin. 



N C N 



Adenin. 



NH 



NH CO NH CO N=C.OH NH CO 



.C C NH CO C NH HC C NH CO C 



\CH I \CO II | \CH 



NH 



N C N 



Guanin. 



NH C NH 



Uric acid. 



N C N 



Hypoxanthin. 



NH C N 



Xanthin. 



H 



Besides the purin bases combined in the nuclein substances, purin bases 

 and uric acid are widely spread in the tissues in the free state, although 

 in very small amounts. 



A portion of the intake of purin bodies is therefore ready formed, 

 especially in the animal constituents of the food, and does not require 

 the decomposition of nucleic acid for its liberation. The nuclei of 

 vegetable cells contain nucleo-proteins, and accordingly can contribute 

 to the purin intake. The most interesting contribution of vegetable 

 origin has been previously alluded to (p. 475) namely, the methyl 

 purins forming the active principles of tea, coffee, and cocoa, caffein, 

 or i, 3, y-trimethylxanthin (C 8 H 10 N 4 O 2 ), theobromin, or 3, y-dimethyl- 

 xanthin (C 7 H 8 N 4 O 2 ), and theophyllin, or i, 3 - dimethylxanthin 

 (C 6 H 8 N 4 2 ). 



CH 3 .N CO 



CO C N.CH 



CH 3 .N C N 



Caffein. 



> C 



NH CO 



CO C N.CH 3 



CH 3 .N CO 

 CO C NH 



CH.N 



Theobromin. 



CH 3 .N 



Theophyllin. 



> H 



N 



Nucleic acid, as stated, can be partially decomposed by the 

 succus entericus, by means of a ferment called nuclease or, more 

 accurately, nucleic-acidase. The groups into which it is split 

 are nucleotids (see above). By another ferment, nucleotidase, a 

 portion, at any rate, of the nucleotids is further decomposed to 

 yield nucleosides, bodies of the glucoside class containing a com- 

 pound of a purin base with the carbo-hydrate group of the nucleic 

 acid, to which phosphoric acid is also coupled. Beyond this stage 

 the hydrolysis of nucleic acid does not proceed in the intestine. 



