364 PHYSIOLOGICAL CHEMISTRY. 



and which have held our attention for a longer or shorter 

 period have been founded on very weak chemical evidence, 

 and this, it should be mentioned, is the real factor in the case. 

 Under the older view, as explained already, uric acid was 

 supposed to be but a step in the formation of urea, the normal 

 end product in protein metabolism, and numerous disorders 

 were attributed to the accumulation of uric acid in the blood 

 through some failure in the final oxidation processes. But it 

 appears now from the evidence available that uric acid is not 

 a natural step in the oxidation of the ordinary proteins; it 

 does result, however, from the breaking down of the complex 

 nucleo proteids which are represented to a limited extent only 

 in the body, as compared with the muscle proteins, for exam- 

 ple. The glandular organs rich in cells furnish the chief 

 amount of the nuclein complexes. In the katabolism of these, 

 true proteins and the residues rich in phosphorus known as 

 nucleinic acids result; the proteins undergo the usual further 

 oxidation probably, while the nucleinic acids break down into 

 a variety of products of which the xanthine bases, ammonia, 

 thymine and carbohydrate groups are the most important. 

 The xanthine bodies in turn doubtless give rise to the uric 

 acid. As pointed out in Chapter V several nucleinic acids 

 exist ; their structural formulas are not known, but empirically 

 these formulas have been given to acids from different sources. 



wOzsP* ................................ Salmon milt 



C4oH e6 Ni4O 26 P4 ................................ Salmon milt 



C 3 6H4sNi4O3oP 4 ................................ Yeast cells 



ieOsiP* ................................ Wheat embryo 



The cleavage products of these acids are not constant, since 

 from different acids different xanthine bases have been made. 

 Those found in the animal body are the following: xanthine, 

 hypo xanthine, guanine, adenine, hetero xanthine, paraxanthine 

 and epiguanine. In order to show the relations of these com- 

 pounds to uric acid, E. Fischer proposed to consider them all 

 as derivatives of a nucleus group which he called purine. 

 As the chemistry of these bodies is complex it may be well 



