256 PRACTICAL PHYSIOLOGY 



From a study of these formulae, it will be seen that purin really con- 

 sists of two urea radicles joined together by a central chain of three 

 carbon atoms. It would, however, be more accurate to describe it as 

 consisting of a radicle of urea united with one of alloxan, which is pro- 

 duced by condensation of mes-oxalic acid with urea. 



COOH NH 



CO/ + CO = CO CO + 2H 2 0. 



COOH NH 2 ' 



Mes-oxalic acid. Urea. Alloxan. 



These substances are, therefore, sometimes named the alloxuric 

 bodies. (For further details of chemistry, see p. 206.) For convenience 

 of description the various atoms in the purin ring are numbered. 



The lowest oxidation product of purin is Hypoxanthin (6 oxy- 

 purin). It occurs abundantly in muscle extract (p. 207) and in the 

 extracts of other tissues, and also in the urine. It always exists 

 along with Xanthin which is 2, 6 di-oxypurin. 



If the oxygen in hypoxanthin be replaced by an amido group 6 

 amino- purin results. This is adenin, and is the chief purine body 

 found in nuclein prepared from the thymus gland. It only exists in 

 traces in the urine. 



A similar derivative of xanthin (2 amino 6 oxypurin) is called 

 guanin. It is the chief purin found in nuclein prepared from the 

 pancreas, and exists in certain pigments of insects and fishes. It occurs 

 abundantly in guano, but only exists in traces in urine. 



If three oxygen atoms be present we have uric acid (2, 6, 8 trioxy- 

 purin), and this is the form in which nearly all the " tissue purins " are 

 excreted in the urine. 



The empirical formulae for these bodies are therefore : 



Purin, C 5 H 4 N 4 . 



! Hypoxanthin, C 5 H 4 N 4 O. 

 Xanthin, C 5 H 4 N 4 2 . 



Adenin, C 6 H 4 N 4 NH. 



Guanin, C 5 H 4 N 4 ONH. 



Uric acid, C 5 H 4 N 4 3 . 



Of these the uric acid is by far the most abundant in urine, whereas 

 the purin bases are most abundant in the tissues. In metabolism, 

 therefore, the latter form the precursors of the former. 



Preparation and Properties of Uric Acid. EXPERIMENT I. To 

 100 c.c. urine are added 5 c.c. HC1 (con.), and allow the mixture 



