j METABOLISM, NUTRITION AND DIETETICS 507 



difficult} 7 than the first, undergoes proteolysis in the usual way. 

 For the decomposition of the nucleic acid (or rather acids, since 

 different nucleo-proteins contain different nucleic acids), still 

 more drastic treatment is required namely, heating with hydro- 

 chloric acid in a sealed tube. Thus treated, nucleic acid yields 

 a number of products, among which phosphoric acid and purin 

 bases (adenin, hypoxanthin, guanin, xanthin) are always present, 

 and probably a carbo-hydrate group also. Pyrimidin bases 

 (uracil, cytosine, thymine) are also present, although, perhaps, 

 not in all nucleic acids. The empirical formulae for the purin 

 bodies of greatest physiological interest are as follows : 



Purin - C 5 H 4 N 4 . 



^ . f Hypoxanthin (a monoxypurin) - C-H 4 N 4 O. 



a o J Xanthin (a dioxypurin) - C 5 H 4 N 4 O 2 . 



3 rt I Adenin (an amino-purin) - C 5 H 3 N 4 .NH 2 . 



^ [Guanin (an amino-oxypurin) - C 5 H 3 N 4 O.NH 2 . 



Uric acid (a trioxy purin) - - C 3 H 4 N 4 O 3 . 



Purin has not been found in the body. The purin bases and 

 uric acid are widely spread in the tissues, although in very small 

 amounts. 



As to the manner in which uric acid arises from the nuclein 

 substances in the tissues, 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 the nucleic 

 acid produced in the alimentary canal in the digestion of nuclein- 

 containing substances, are then decomposed by another ferment, 

 nuclease, which, along with phosphoric acid and the carbo- 

 hydrate group, liberates purin (and pyrimidin) bases, especially 

 adenin and guanin. Then follows the action of ferments 

 (adenase and guanase), which remove the ammo-group from 

 these purin bases, transforming adenin into hypoxanthin and 

 guanin into xanthin. By means of an oxidizing ferment or 

 oxydase we may next imagine that hypoxanthin is oxidized into 

 xanthin and xanthin into uric acid. Evidence of the existence 

 of all these ferments, and of their wide distribution, 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 endogenous portion. The latter moiety, which 

 generally amounts to about o'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, 



