Nitrogen Excretion 



203 



dominant in a given species depend more on its immediate osmotic needs than 

 on its ancestry. 



DISTRIBUTION OF ENZYMES AND PRODUCTS 

 OF PURINE METABOLISM 



A small amount of the nitrogen excreted hy animals comes from nucleic 

 acid metabolism, particularly from purine bases. The amount of nucleic acid 

 eaten varies with the diet, being greater in meat than in plant food. Enzymes 

 of purine metabolism differ from those of protein metabolism, even though 

 some of the end-products are the same. An animal may excrete some uric acid 

 from purines, regardless of whether its protein is degraded to ammonia, urea, 

 or uric acid. The various end-products of purine metabolism were listed above 

 (Fig. 33). Purines may be excreted as such, e.g., adenine or guanine, or they 

 may be deaminated to xanthine, which is converted to uric acid by xanthine 

 oxidase. If uricase is present, allantoin is then formed; allantoinase converts 

 this to allantoic acid; allantoicase converts this to urea; urea may be broken 

 down to ammonia by urease. Numerous invertebrates— annnelids, molluscs, 

 and arthropods— have deaminases of the free bases (adenase and guanase); 

 many vertebrate livers lack adenase but can deaminate the nucleosides by 

 adenosinase and guanosinase.-^'^- ^^ The type of purine breakdown product 

 in different animals has been identified partly by analysis of excretory products 

 and excretory organs, and partly by identification of the enzymes in tissues, 

 particularly liver (or hepatopancreas). A detailed summary of purine products 



TABLE 31. DISTRIBUTION OF URICOLYTIC ENZYMES 

 -f indicates presence; — indicates absence. 



