856 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 the arginin, formed in the cleavage of the 

 protein molecule 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, thy- 

 mus, muscle, etc., arginase, which is capable of splitting arginin 

 into urea and ornithin. The reaction may be represented by the 

 following equation: 



Arginin (guanidin-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. 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, 

 indpl, 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 Nv 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- 



tion of an atom of oxygen gives hypoxanthin, C 5 H 4 N 4 0: 

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



