NITROGENOUS WASTE-PRODUCTS 549 



the Diurates, containing two molecules of base. The so-called quadri- 

 urates are non-existent. 



Uric acid yields a variety of characteristic color reactions, among 

 which the Murexide Test, already described in connection with the 

 purine bases, must be included. Uric acid is a reducing agent and 

 reduces an alkaline cupric hydroxide solution; the quantity of uric 

 acid which is present in urine is, however, insufficient to produce an 

 appreciable precipitation of cuprous oxide. If a drop of uric acid dis- 

 solved in sodium carbonate be placed upon a filter-paper moistened 

 with silver nitrate solution, reduction occurs with the production of a 

 yellow or brown spot (Schiff's Reaction). If a weak alkaline solution of 

 uric acid in water is treated with a soluble zinc salt a white precipitate 

 is produced which gradually turns blue if exposed to light and air, or 

 immediately, if treated with sodium persulphate (Ganassini's Reaction). 

 With a certain mixture of phosphoric and phosphotungstic acids uric 

 acid yields a blue coloration (Folin and Macallum's Reaction), the 

 origin of which is unknown. 



The elimination of uric acid is definitely increased by a diet which 

 contains excess of purines or of Nucleic Acids. This is due to the fact 

 that the adenine and guanine, split off from the nucleic acids, are 

 transformed in the tissues into Hypcxanthine and Xanthine, by the 

 deaminizing enzymes adenase and guanase. The hypoxanthine is sub- 

 sequently converted into xanthine and the xanthine into uric acid 

 by a specific oxidizing enxyme which is found in a variety of animal 

 tissues, and is designated Xanthine-oxidase : 



HN CO HN CO HN CO 



! I I I II 



HC C NH\ OC C NH\ OC C NH, 



J| || \ CH + O -> | || VJH + O -* | || >CO 



N _C_ N + HN C N * HN C NH/ 



Hypoxanthine. Xanthine. Uric acid. 



Nevertheless, the elimination of uric acid continues on a purine or 

 nuclein-free diet. In a series of experiments on himself and others, 

 Folin was able to reduce the daily elimination to 0.3 grams on a diet 

 of cream and starch, but this minimum could not be reduced. Evi- 

 dently, therefore, there is, as in the case of amino-acids and other 

 foodstuffs, an Endogenous Metabolism of purines as contrasted with an 

 Exogenous Metabolism. That the endogenous metabolism represents 

 the actual breaking down of tissues is shown by the fact that if destruc- 

 tion of tissue is remarkably augmented, as in pneumonia, leukemia, 

 or in severe burns, the uric acid excretion rises decisively. 



There is no evidence that mammalian tissues can synthesize uric 

 acid from any other source than purines. It is true that the elimination 

 of uric acid, and of purine bases also, is increased by an increase in the 

 dietary intake, but this is true whether the increase be nitrogenous or 

 non-nitrogenous, and it follows very rapidly upon the intake of food. 



