PHYSIOLOGICAL CHEMISTRY 423 



NH CO HN C=NH 



HN=C C NH CH C NH 



\ || >CH; X II >CH. 



NH C N" ^N C N^ 



Guanine. Adenine. 



The atoms in purine are numbered so as to facilitate the description 

 of the location of the side groups. 



The lowest oxidation product of purine is hypoxanthine (6 oxypurine). 

 It occurs abundant^ in muscle extract (p. 356) and in the extracts of 

 other tissues, and also in the urine. It always exists along with 

 xanthine, which is 2, 6 di-oxypurine. 



If the oxygen in hypoxanthine be replaced by an imino group 

 ( = NH), the result is adenine, which occurs in nucleic acids. 



A similar derivative of xanthine is called guanine. It is the only 

 purine found in the variety of nucleic acid called guanylic acid, and 

 exists in certain pigments of insects and fishes. It occurs abundantly 

 in guano. 



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

 oxypurine), and this is the form in which nearly all the " tissue 

 purines " are excreted in the urine. 



The empirical formulae for these bodies are therefore : 



Purine, C 5 H 4 N 4 . 



f Hypoxanthine, C 5 H 4 N 4 0. 



Purine I Xanthine, C 5 H 4 N 4 2 . 



Bases j Adenine, C 5 H 5 N 5 . 



I Guanine, C 5 H 5 N 5 O. 



Uric Acid, C 5 H 4 N 4 3 . 



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

 the purine bases are most abundant in the tissues. In metabolism the 

 latter form the precursors of the former. 



The alkaloids of tea and coffee are methyl derivatives of xanthine. 

 Thus, caffeine and theine are 1, 3, 7 trimethyl, 2, 6 dioxypurine, and 

 theobromine (the alkaloid in cocoa) is 3, 7 dimethyl, 2, 6 dioxypurine. 



The constitutional formula of uric acid given above indicates that it 

 is a diureide, containing two urea groups in the molecule. This fact is 

 demonstrated by the syntheses of uric acid and by the nature of its 

 oxidation products. 



The simplest synthesis of uric acid, brought about by heating urea 

 with a derivative of lactic acid, trichlor lactamide, is of some physio- 

 logical importance, as there is experimental evidence to show that in 

 birds uric acid, which forms their principal nitrogenous excretive, is 



