164 PHYSIOLOGICAL CHEMISTRY 



deposited in the joints in gout and arthritis, and shows other 

 interesting variations in disease and on varying diets. 



In birds and reptiles uric acid is the chief nitrogenous excre- 

 tion. The urine of these animals is semisolid, since it contains 

 large amounts of uric acid crystals.. 



Uric acid is 2, 6, 8 trioxypurine and has the following formula : 



HN = 

 = NH 



= 



,. / 



HN NH 



Uric Acid. 



It is extremely insoluble and frequently precipitates from the 

 urine as a crystalline sediment which usually is highly colored 

 by other substances. The crystal form varies, but is frequently 

 whetstone or gunboat shape. It dissolves fairly readily in alka- 

 lies, in boiling glycerol, and in concentrated sulphuric acid on 

 slight warming without decomposition. It forms salts, those of 

 sodium and amonium often being present in urine sediment. Uric 

 acid is easily decomposed in alkaline solution, and yields various 

 products such as urea, dialuric acid, etc. It reduces Fehling's 

 solution, and is readily oxidized by alkaline permanganate and 

 other oxidizing agents. Alloxan, allantoin, oxalic and carbonic 

 acids are among the oxidation products of uric acid. 



If a crystal of uric acid is moistened with concentrated HN0 3 

 and evaporated to dryness on the water bath a reddish spot re- 

 mains, which becomes a deeper red on the addition of ammonia. 

 This is known as the murexide test. Other purines (xanthine 

 and guanine) give this test, but may be distinguished from uric 

 acid by the fact that the red color persists on warming, whereas 

 that from uric acid disappears on warming. The red substance 

 from uric acid is ammonium purpurate. 



The source of the uric acid was long a matter of uncertainty. 

 We now know, however, that it is derived from the purine por- 



