424 PRACTICAL PHYSIOLOGY 



synthesised in the liver from oxidation products of lactic acid. 

 Whether such a synthesis takes place also in mammals is at present 

 unknown. 



On oxidation with potassium permanganate uric acid yields allantoin, 

 which is present in the urine of the dog and cat, and occasionally in 

 that of man. The formula for allantoin is : 



/NH CH NH V 



C0< | >CO. 



X NH CO NH/ 



On oxidation with nitric acid, uric acid yields alloxan, carbon dioxide, 

 and nitrogen. An intermediate oxidation product, alloxantin, is formed 

 at the same time, which with ammonia forms a red dye, murexide. This 

 reaction is used as a test for uric acid. Alloxan is the ureide of 

 mesoxalic acid : 



Further oxidation yields the ureide of oxalic acid. 



Ordinarily uric acid behaves as a monobasic acid, being soluble in 

 alkalies (caustic soda, ammonia, and boiling solution of sodium carbonate) 

 with the formation of the corresponding salts, which are more soluble 

 in water than the free acid. The dibasic salts can, however, be obtained 

 by the use of excess of concentrated alkali, so that it is the custom 

 to call uric acid a dibasic acid and its ordinary salts acid salts, although 

 their solutions are alkaline, not acid. Strictly speaking, uric acid is a 

 tetrabasic acid, as all four hydrogen atoms in the molecule have latent 

 acid properties. 



Uric acid is the principal nitrogenous excretive of birds and reptiles. 

 Together with other purine bodies it is always present in the urine of 

 man, having a twofold origin, exogenous and endogenous. The exo- 

 genous purines come from purine bodies in the food (nucleo-proteins in 

 cellular structures, xanthine and hypoxan thine in meat, caffeine, etc.). 

 The endogenous output of purines is fairly constant for a given indi- 

 vidual under ordinary conditions, and is to be traced, partly at any rate, 

 to the breakdown of nucleo-proteins in the body. Burian and Schur 

 found the daily purine excretion of a normal individual on an ordinary 

 diet to be about 1 grm. On a purine free diet this was reduced to 6'0 

 grin., and was practically independent of the amount of nitrogen in the 

 food. Violent muscular exercise and pyrexia both increase the output 

 of purine bases and uric acid on a purine free diet. This effect is pre- 

 sumably connected with the xanthine and hypoxanthine of muscle 

 (see p. 354). 



