THE BIO-CHEMISTRY OF ANIMALS AND PLANTS 107 



in the animal than in the plant. In accordance with, this view, 

 we find in animals a much more pronounced nitrogenous 

 katabolism than in plants ; the healthy animal organism is in 

 a state of nitrogenous equilibrium — i.e. approximately the same 

 amount of nitrogen is excreted in the urine and faeces as is 

 assimilated from the food. In plants this nitrogenous equilibrium 

 does not exist, but the nitrogen contents of the organism grow 

 steadily without nitrogen being lost in any other way than 

 by the decay of older parts. 



Final Products of N -Katabolism. — The question arises whether 

 the plant ever completely breaks down proteins in the manner 

 they are broken down in the animal body. The two common 

 end-products of nitrogenous katabolism in animals, urea and 

 uric acid, have certainly never been found in the vegetable 

 world. E. Fischer has, however, isolated the mother substance 

 of uric acid and other materials closely related to it ; this he 

 termed purine, and uric acid is an oxygenated derivative of this 

 body. 1 It is therefore interesting to note that other purine 

 derivatives occur in certain plants, and occasionally in quite 

 large quantities, and are possibly end-products of metabolism. 

 Of these caffeine (trimethyl-dioxy-purine) and theobromine 

 (dimethyl-dioxy-purine) are the most important. The purine 

 derivatives are especially abundant in those parts of the plant 

 which are rich in protein, and in which metabolism is most 

 active {e.g. in parenchymatous tissue, as in buds, young leaves, 

 etc.). There is, therefore, some justification for assuming that 

 they are formed from the proteins. It must be remembered, 

 however, that we have so far no definite proof of this, and 

 that they may just as likely be derived from the nucleins. 

 E. Fischer's discovery of the pyrrolidine compounds (proline 

 and oxyproline) among the cleavage products of proteins is 

 suggestive as indicating a relationship of the latter to the 

 alkaloids, in many of which (nicotine, cocaine, hygrine, atropine, 

 hyoscyamine) the pyrrolidine ring occurs in connection with 

 the pyridine ring. Still another cleavage product of proteins, 

 namely tryptophane or indole-amino-propionic acid (isolated 

 by Hopkins and Cole and recently synthesised by Ellinger) r 

 helps us to explain the relationship to proteins of another 

 class of substances found as end-products in both the animal 

 and plant organism — the indole derivatives. Trytophane is 



1 See E. Fischer, Untersuchungen in der Puringriippe (1882 — 1906), 



