394 PHYSIOLOGICAL CHEMISTRY 



introduce a crystal of urea or a small amount of concentrated solution of urea. 

 Through the influence of the sodium hypobromite, NaOBr, the urea is decom- 

 posed and carbon dioxide and nitrogen are liberated. The carbon dioxide is 

 absorbed by the excess of sodium hydroxide, while the nitrogen is evolved and 

 causes the marked effervescence observed. This property forms the basis for 

 one of the methods in common use for the quantitative determination of urea. 

 Write the equation showing the decomposition of urea by sodium hypobromite. 

 It is claimed that all ammonium compounds and all compounds 

 containing the amino ( NH 2 ) group yield nitrogen when treated with 

 hypobromite as in this test. 



HN CO 



I I 



URIC ACID, OC C-NH 



HN C NH 



Uric acid is one of the most important of the constituents of the 

 urine. It is generally stated that normally about 0.7 gram is excreted 

 in 24 hours, but that this amount is subject to wide variations, particu- 

 larly under certain dietary and pathological conditions. It has been 

 shown, however, that the average daily excretion of uric acid for ten 

 men ranging in age from 19 to 29 years and fed a normal mixed diet 

 was 0.597 g ram > a value somewhat lower than the generally accepted 

 average of 0.7 gram for such a period. On a purine-free diet the uric 

 acid output maybe 0.1-0.5 gram per day, whereas a high purine diet may 

 yield a daily output of 2 grams. Uric acid is a diureide and consequently 

 upon oxidation may yield two molecules of urea. It acts as a weak di- 

 basic acid and forms two classes of salts, neutral and acid. The neutral 

 potassium and lithium urates are the most easily soluble of the alkali 

 salts; the ammonium urate is difficultly soluble. The acid-alkali urates 

 are more insoluble and form the major portion of the sediment which 

 separates upon cooling the concentrated urine; the alkaline earth urates 

 are very insoluble. Ordinarily uric acid occurs in the urine in the form 

 of urates and upon acidifying the liquid the uric acid is liberated and 

 deposits in crystalline form. This property forms the basis of one of 

 the older methods for the quantitative determination of uric acid 

 (Heintz Method.) 



Uric acid is very closely related to the purine bases as may be seen 

 from a comparison of its structural formula with those of the purine 

 bases given on page 126. According to the purine nomenclature it is 

 designated 2-6-8- trioxypurine. Uric acid forms the principal end- 

 product of the nitrogenous metabolism of birds and scaly reptiles; 

 in the human organism it occupies the fourth position inasmuch as here 



