KIDNEY AND SKIN AS SECRETORY ORGANS. 859 



The nucleosides (adenosin and guanosin) may first be converted 

 to the corresponding oxy-compounds (xanthosin and inosin), and 

 then by hydrolysis the carbohydrate is split off with the liberation 

 of xanthin and hypoxanthin. 



4. The xanthin and hypoxanthin, under the influence of an oxi- 

 dase (xanthinoxidase), are in part oxidized to uric acid, CsEUN^a. 

 In man this oxidase seems to occur only in the liver, so that the 

 immediate production of uric acid must be referred to this organ. 



5. In man the uric acid, 'xanthin, and hypoxanthin represent 

 the final end-products of the metabolism of the nucleic acid or, 

 rather, of the purin nucleotide portion of its molecule, and to 

 the extent that they occur in the urine they indicate so much 

 nucleic acid broken down. In the other mammals the oxidation 

 process goes a step further the uric acid is converted to allantoin 

 by an oxidase known as uricolytic enzyme or uricase, whose action 

 may be represented by the equation : 



403 + H 2 + O = C4H 6 N 4 3 + CO 2 



Uric acid. Allantoin. 



Origin and Significance of the Creatinin and Creatin. 



Creatinin (C 4 H7N 3 O) occurs in the urine, and it was formerly as- 

 sumed that it is derived from the creatin (C 4 H 9 N 3 O2) found in 



/NH - CO 

 muscle. Its structural formula is given as NHC^ I and its 



X N(CH 3 )CH 2 



chemical relations are indicated by the fact that it may be prepared 

 synthetically from methyl-glycocoll and cyanamid that is, the 

 union of these two substances gives creatin, from which in turn 

 creatinin may be obtained by loss of a molecule of water. 



N=C-NH 2 + NH(CH 3 )CH 2 COOH 



Cyanamid. Methyl-glycocoll. Creatin. 



Creatinin occurs in the urine constantly and in amounts equal to 

 1 to 2 gms. per day, or, according to Sh'affer,* there is an excretion 

 of from 7 to 11 mg. of creatinin nitrogen per kilogram of body- 

 weight. Next to the urea and the ammonia compounds it forms 

 the most important of the known nitrogenous constituent of the 

 urine. Its physiological history is imperfectly known. Under 

 constant conditions of life the amount of creatinin formed in the 

 body is independent of the quantity of protein eaten, and this 

 fact indicates (Folin) that it represents an end-product of the 

 metabolism of living or organized protein tissue rather than one 

 of the results of the metabolism of the food protein. This con- 

 clusion is strengthened by the fact that in fevers and other patho- 



* Consult Shaffer, "American Journal of Physiology/' 33, 1, 1908. 



