KIDNEY AND SKIN AS EXCRETORY ORGANS. 779 



The nucleins that are split off from the nucleoprotein are acted 

 upon first by an enzyme, nuclease, which has been demonstrated to 

 exist in various tissues, e. g., in the spleen, liver, lungs, and kidneys. 

 By the action of this enzyme the nuclein is split with the formation 

 of some of the purin bodies, adenin, guanin, xanthin, or hypoxan- 

 thin. The adenin and guanin are then deamidized and converted 

 respectively to hypoxanthin and xanthin. Jones * has given reasons 

 to believe that two specific deamidizing enzymes of this character 

 may exist in the body, namely, adenase and guanase. Their action 

 may be represented by the following equations : 

 C 5 H 5 N5 4- H 2 O = C 5 H 4 N 4 O + NH 3 



Adenin. Hypoxanthin. 



C 5 H 5 N 5 O + H 2 O = C 6 H 4 N 4 O 2 + NH 3 



Guanin. Xanthin. 



The hypoxanthin and xanthin thus formed are in turn oxidized 

 to uric acid by the action of an oxidase to which the specific naire 

 of xanthinoxidase has been given. Its action upon the hypoxanthin 

 or xanthin is represented by the series: 



C 5 H 4 N 4 O + O = C 5 H t N 4 O 2 



Hypoxanthin. Xanthin. 



C 5 H 4 N 4 2 f O = C 5 H 4 N 4 S 



Xanthin. Uric acid. 



Finally, as stated above, it can be shown that a portion of the uric 

 acid may be further metabolized by the action of a specific urico- 

 lytic enzyme and give rise to urea. The portion of the uric acid under- 

 going this last change varies in different animals, as may be demon- 

 strated by giving definite amounts of uric acid in the food. Ex- 

 periments of this kind have shown that in man about one-half 

 of the uric acid formed gives rise to urea, while in dogs and cats 

 only about -^V suffers this change. In rabbits the proportion is J. 

 According to a former view (Horbaczewsky) it was supposed that 

 the endogenous purin nitrogen represents an end-product of the 

 metabolism of the nuclein found in the nuclei of ceils, especially 

 in the nuclei of the leucocytes. But Burian has shown, on the 

 contrary, that most of this nitrogen in the excreta arises from a 

 metabolism in the muscular tissues. f Increased muscular activity 

 is followed within an hour or two by an increased output of uric 

 acid, and when an isolated muscle is perfused with a mixture of 

 defibrinated blood and Ringer's solution, uric acid is given off to 

 the circulating liquid. When the muscle under these last-mentioned 

 conditions is made to work a distinct increase in the hypoxanthin 

 and uric acid can be determined. It would seem, therefore, that 

 under normal conditions the uric acid and other purin bases are 



* Jones and Austrian, ' Zeitschrif t f. physioh Chem.," 1906, xlviii., 110. 

 t Burian, 'Zeitschrift f. physiol. Chemie., xliii., p. 532. 



