850 PHYSIOLOGY OF DIGESTION AND SECRETION. 



group. Since arginin constitutes one of the split-products of the 

 protein during digestion and probably also one of the split-products 

 in the metabolism of the proteins of the tissues, there is reason to 

 believe that part of the urea actually formed in the body arises by 

 this method. It is possible that by some similar method the 

 nitrogen of the heterocyclic radical in other amino-acids (imidazol, 

 indol, pyrol) may give rise to urea, but nothing is known in regard 

 to this possibility. 



Origin and Significance of the Purin Bodies (Uric Acid, 

 Xanthin, Hypoxanthin, Adenin, Guanin). These bodies are 

 related chemically, and appear also to have a common physiological 

 significance. Their chemical relations have been described by 

 Emil Fischer, to whom we owe the term purin bodies. Fischer 

 pointed out that these and other substances belonging to this 

 group have a common nucleus: 

 N C 



C C N v which he named the purin nucleus. The 



hydrogen compound of this nucleus would be designated as purin, 



N = CH 



and would have the formula: HC C NH , C 5 H 4 N 4 . Addi- 



II II \ PTT 

 N C N^ CH 



tion of an atom of oxygen gives hypoxanthin, C 5 H 4 N 4 O: 

 HN CO 



HC C NH 



jjj. __ J __ N ^>CH. Addition of two atoms of oxygen gives xan- 

 HN CO 



thin, e 5 H 4 N 4 2 : CO C NH 



H |jq. _ |l _ N ^CH. And addition of three atoms 

 HN CO 



of oxygen gives uric acid, @ 5 H 4 N 4 O 3 : CO C NH , which 



HN C NH 



from this standpoint might be named trioxypurin. If one of the H 

 atoms in the purin is substituted by an amino-group, NH 2 , the com- 

 pound, adenin (C 5 H 5 N 5 ), is obtained, and the substitution of an 

 NH 2 group in hypoxanthin gives the compound guanin (C 5 H 5 N 5 O). 

 Moreover, caffein, the active principle of coffee and tea, and theo- 

 bromin, the active principle of cocoa, are respectively trimethyi 

 and dimethyl compounds of xanthin. We have to distinguish, 

 therefore, three classes of purin compounds, namely, the oxypurins, 

 comprising monoxypurin or hypoxanthin, dioxypurin or xanthin, 

 and trioxypurin or uric acid; the aminopurins, comprising adenin or 



