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A TEXTBOOK OF PHYSIOLOGY 



action of the enzymes adenase and guanase present in. these tissues, 

 converted by deaminization into hypoxanthin and xanthin respec- 

 tively (cf. p. 50) : 



C 5 H 3 N 4 .NH 2 + KjO - C 5 H 4 N 4 + NH 3 



Adenin Hypoxanthin 



C 5 H 3 N 4 O.NH 2 + H 2 O - C 5 H 4 N 4 O 2 + NH 3 



Guanin Xanthin 



By the action of oxidases also present in the tissues the hypo- 

 xanthin and xanthin are converted into uric acid (tri-oxy-purin). 

 The whole process may be graphically represented as follows : 



ADENIN -> 



(by action of adenase) 

 C 5 H 3 N 4 .NH 2 

 (amino purin) 



GUANIN > 



(by action of guanase) 



C 5 H 3 N 4 O.NH 2 

 (amino oxy-purin) 



HYPOXANTHIN 



(+ O by oxidase) 



C 5 H 4 N 4 



(mon-oxy-purin) 



j 



i 



XANTHIN 

 (+0 by oxidase) 



C 5 H 4 N 4 2 

 (di-oxy-purin) 



URIC ACID 



C 5 H 4 N 4 3 



(tri-oxy-purin) 



Some of the uric acid thus formed may further be converted into 

 urea by the presence of a uricolytic enzyme. This enzyme occurs in 

 the liver, muscles, and kidneys, and probably destroys a considerable 

 amount of the uric acid formed in the body. Indeed, uric acid, even 

 when given in the food, owing to the presence of this enzyme, causes 

 no increase in the uric acid output of the body. 



Uric acid is, however, regarded as the chief end product of nuclein 

 and purin metabolism. The uric acid thus formed is taken to the 

 kidneys for excretion, but in what form is not exactly known. Uric 

 acid is about forty times more soluble in blood than in distilled water. 

 This is not attributable to the alkalinity of the blood (which is in 

 reality neutral), since uric acid is also much more soluble in acidified 

 blood-serum than in distilled water. Probably uric acid is carried 

 in the blood in combination with some other organic body, and not, 

 as was once generally supposed, with sodium salts (sodium urate and 

 sodium acid urate). The nature of the organic complex is not known. 

 It is by some supposed to be thyminic acid, but no such compound 

 has yet been isolated from the blood. 



Experiment shows that the uric acid which occurs in the urine 

 has two sources an exogenous, from the purins of the food; an endo- 

 genous, from the purins liberated by the breaking down of the cell 

 nuclei, and possibly also from other bodies. The presence in the urine 

 of uric acid of endogenous origin is shown by the fact that upon a 

 purin -free diet uric acid is excreted in the urine. The amount then 

 excreted is fairly constant for each individual. This excretion reaches 



