390 THE METABOLISM OF THE PURIN BODIES 



it yields urea and oxalic acid. Allantoin is, as we will see later, 

 of peculiar interest from a physiological standpoint because it 

 appears in the urine of certain animals (dogs and cats) when a 

 large excess of uric acid is present in the tissues. 



So far, then, we have seen that uric acid is a diureid ; it 

 contains in its molecule two urea radicles linked together by a 

 carbon residue. Now any diureid can be built up by the con- 

 densation of oxy-acids with urea. To form uric acid, lactic acid 

 is the most suitable oxy-acid, and, for the purpose of this 

 synthesis, a substitution product of it, viz. trichlorlactamide, is 

 used, because it is much more capable of uniting with such bodies 

 as urea than the acid itself would be. (The chlorine atoms and 

 the amido groups which this derivative contains are more easily 

 replaced than hydrogen atoms.) If then trichlorlactamide and 

 urea be heated together the following reaction ensues : 



Cl 



HNH C1-C-C1 /H 



I I HN< 



CO C - HOH >CO 



| II HN< 



HNH CO - NH 2 X H 



( Urea) ( Trichlorlactamide) ( Urea) 



The groups printed dark unite to form uric acid. Ammonia, 

 hydrochloric acid, and water are evolved as bye-products. 



Before leaving this subject of its chemical structure, another 

 reaction of uric acid must be mentioned. If uric acid be hydro- 

 lysed (caused to take up water) by heating it in a closed tube 

 to 170 C., it yields glycin (amido acetic acid), carbonic acid, 

 and ammonia : 



C 5 H 4 N 4 O 3 + 5H 2 O - CH 2 (NH) 2 COOH + 3CO 2 + 3NH 3 



(Uric acid) (Glycin) 



and if glycin and urea be melted together and heated to 220-230 C. 

 uric acid is formed. Now glycin is abundantly present in the 

 organism, and the above reactions might lead one to expect a 

 relationship between it and uric acid. We will discuss this 

 question later. 



So far we have spoken only of uric acid. It must be 



