METABOLISM OF PROTEINS 593 



uric acid arises from the nuclein substances, we may picture the 

 process as taking place by the following steps : Certain organs have 

 been shown to contain ferments which split up nucleo-proteins into 

 protein and nucleic acid. This nucleic acid, or nucleic acid arising 

 in other ways in the metabolism of nuclein, and also any nucleic 

 acid absorbed as such from the alimentary canal in the digestion of 

 nuclein-containing substances, are then decomposed by another 

 ferment, similar to or identical with the nuclease or nucleic-acidase 

 previously encountered in the intestine. The resulting nueleotids are 

 split up by a special ferment (nucleotidase) so as to yield nucleosides. 

 These are in turn decomposed by appropriate enzymes (nucleo- 

 sidases), so that we finally arrive at the individual ' building-stones,' 

 the nucleic acid molecule, phosphoric acid, the carbo-hydrate group, 

 pyrimidin and purin bases, especially adenin and guanin. Then 

 follows the action of ferments (adenase and guanase), which remove 

 the amino-group from these purin bases, transforming adenin into 

 hypoxanthin, and guanin into xanthin (Jones). The deaminiza- 

 tion is associated with hydrolysis. Thus : 



C 6 H 5 N 6 + H 8 =C 6 H 4 N 4 p + NH 3 ; C 6 H 5 N 5 O + H 2 O =C 5 H 4 N 4 O a + NH 3 . 



Adenin. Hypoxnnthin. Guanin, Xanthin. 



By oxidation hypoxanthin is changed into xanthin and xanthin 

 into uric acid, and the oxidation seems to be accomplished by a 

 separate oxidizing ferment, xanthin oxidase, whose action may be 

 thus represented : j 



C B H 4 N 4 + O = C 6 H 6 N 4 2 ; C 5 H 5 N 4 O a + O = C B H 4 N 4 O g . 



Hypoxanthin. Xanthin. Xanthin. Uric Acid. 



Evidence of the existence of these ferments, and of their wide dis- 

 tribution, has been obtained by making experiments on the various 

 substances mentioned with extracts of different tissues. 



The portion of the uric acid which comes from the food (mainly 

 from the purin bodies in it) is sometimes denominated the exogenous 

 portion, while that which arises from the tissues is called the endog- 

 enous portion. The latter moiety, which generally amounts to 

 about 0-6 gramme in the twenty-four hours, can be estimated by 

 restricting the diet to articles of food free from purin bodies, such as 

 bread, milk, cheese, eggs, and butter. It is stated that the endog- 

 enous uric acid remains practically constant in the same individual 

 under constant conditions, and is unaffected by changes in the diet. 



The total excretion of uric acid (and the other purin bodies) is. 

 by no means identical with the sum of the uric acid taken in as 

 purin bases in the food and that produced in the body. A con- 

 siderable destruction of uric acid (and other purin bodies) goes on 

 in the body, and mainly in the liver. The quantity of endogenous 

 uric acid excreted by the kidneys bears a certain ratio to the total 

 amount which has entered the circulation. This ratio varies much 

 in different mammalian species. In man a full half is said to be 



