190 



Comparative Animal Physiology 



converted to creatinine and excreted, although creatine may also be excreted 

 unchanged. 



HN^CO 



I I H 



OC C-N^ 



HN-C-N 



/ 



CO 



Uric Acid 



H 



Uricase 



I O 

 H.N C-NH 



OC 



CO 



Allantoin 



H 



Allantoicase 



COOH 

 H 



+ 2 0=^ 



/ 



NH2 



Ui 



Gl^oxylic 

 Acid 



NH2 

 Urease 

 2 NHa + CO. 



Fig. 3.^. Sequence of breakdown of uric acid to ammonia. From Florkin.'" 



Nitrogenous Pigments. Some nitrogenous pigments resulting from the 

 metabolism of hemochromogens (e.g., hemoglobin) are excreted, particularly 

 in mammals. 



Pterines. These nitrogenous pigments deposited in butterfly wings are 

 sometimes regarded as excretory products. 



Pyrimidines. Pyrimidines may be excreted as such, but in mammals, at least, 

 are partly converted to urea. 



DISTRIBUTION OF EXCRETORY PRODUCTS 

 OF PROTEIN METABOLISM 



In no group of animals is nitrogenous excretion limited to one product. 



