ELEMENTAEY PHYSIOLOGICAL CHEMISTRY 259 



EXPERIMENT VI. By applying Trommer's test to an alkaline 

 solution of uric acid, it will be noticed that reduction ensues. The 

 reduction precipitate is, however, of a dull brown colour instead of 

 being yellowish red as it usually is. This is because a certain amount 

 of the cuprous oxide unites with some of the uric acid to form a brown 

 compound. 



Quantitative Estimation of Uric Acid in Urine. In this country 

 the usual method adopted is that of Hopkins , the principle of which 

 is as follows. The uric acid is precipitated as ammonium urate by 

 saturating the urine with ammonium chloride crystals. The urate 

 is then collected on a filter paper, washed with ammonium chloride or 

 sulphate solution, and dissolved in warm water. The dissolved urate 

 is then decomposed by adding a strong mineral acid, and the acid 

 thereby liberated either collected and weighed, or titrated with a 

 standard solution of permanganate of potassium, which it decolourises 

 (see p. 462). 



Metabolism of Purin Bodies. The urinary purin bodies are derived 

 from two sources, the food and the tissues. That portion which comes 

 from food is called the exogenous moiety; the portion which comes from 

 the tissues the endogenous moiety. 



The Exogenous Moiety arises from food stuffs containing purin 

 bodies. These may exist in the food in three states: (1) in combination 

 with albumin and phosphoric acid, as nuclein (e.g. in sweetbreads, or 

 other cellular tissues, p. 179), (2) in a free state, as in muscle extract, 

 (p. 442) ; (3) as a methyl derivative of purin, such as thein, theobromin 

 and caffein. 



The purins of the first two groups (i.e. guanin, adenin, hypoxanthin 

 and xanthin) are oxidised in their passage through the organism into 

 uric acid, and appear in the urine as such, whereas those of the third 

 group lose their methyl radicle, and are excreted as xanthin and 

 hypoxanthin in the urine. 



The whole of the administered purin does not, however, reappear 

 in the urine, a certain fraction being further broken down by a 

 splitting of the purin ring, the decomposition products being probably 

 excreted as urea. In the case of hypoxanthin and xanthin (which are 

 the purins existing in muscle, liver and spleen) about one half reappears 

 in the urine, whereas in the case of adenin, and probably quanin (ad- 

 ministered as nuclein), only one fourth can be recovered. In the case of 

 the methyl derivatives, about one third can be recovered as purin bases. 



The Endogenous Moiety. This can be estimated ,by placing the 

 person on a purin-free diet such as bread, eggs, milk, cheese, and 

 butter. It usually amounts to 0'6 grammes l in the twenty-four hours, 



1 Calculated as uric acid. 



