846 PHYSIOLOGY OF DIGESTION AND SECRETION. 



living on its own tissues, urea continues to be formed, so that in 

 part the urea of the urine represents nitrogen which has prob- 

 ably arisen from the destruction of protein tissue. The inter- 

 mediate steps in this latter process are not definitely known, but 

 probably they are analogous to those described, that is to say, the 

 protein passes through the stage of amino-acids and subsequently 

 undergoes deaminization. 



3. Urea arises from arginin by conversion of the contained 

 guanidin radicle. Kossel and Dakin* have demonstrated the 

 existence of a ferment, present in the liver especially, but found 

 also in the kidney, thymus, muscle, etc., arginase, which is cap- 

 able of sphtting arginin into urea and ornithin. The reaction may 

 be represented by the following equation: 



NHC<^§^(CHol3CHNHoCOOH+H20=CO<^2;;+NH2(CH2)3CHNH2COOH 



Arginin (guaniriin-diamino-valerianic acid). Urea. Diamino-valerianic acid. 



Unlike cases 1 and 2, the urea in this instance is formed from the 

 guanidin residue contained in the arginin and not from the amino- 

 group. The diamino-valerianic acid formed in the reaction may 

 also doubtless, like the other amino-acid, undergo deaminization, 

 and thus have its nitrogen appear in the form of urea. Since 

 arginin constitutes one of the split-products of the protein during 

 digestion and probably also one of the split-products in the metab- 

 olism 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 radicle 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-. which he named the purin nucleus. The 



>I - C - N^ 



hydrogen compound of this nucleus would be designated as purin, 



N = CH 



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



N — C — N;:^CH 

 tion of an atom of oxygen gives hypoxanthin, CjH^N^O:: 

 *"Zeitschrift f. physiol. Chemie," 1904, xlii., 181. 



