EXCRETION 



295 



Purine Bases 



adenase 



xanthine 

 oxidase 



Uric acid 



uncase 



\ 



Allantoin 



allantoinase 



Allantoic 

 acid 



allantoicase 



urease 



Urea 



Ammonia 



In the few flatworms and annelids which have been examined, enzymes 

 capable of deaminating purines are lacking, and purines are excreted. 

 Many other invertebrates possess uricolytic enzymes, namely actinians, 

 sipunculoids, molluscs, crustaceans and echinoderms. All four enzymes 

 capable of decomposing uric acid to ammonia — uricase, allantoinase, 

 allantoicase and urease — have been identified in certain species: such 

 include Sipunculus, Mytilus and Homarus. A small but significant propor- 

 tion of the excretory products shown in Table 7.2 is derived from purine 

 as distinct from protein metabolism. Data relating to processes of purine 

 deamination among invertebrates are very fragmentary, and more bio- 

 chemical information is desirable. 



Fish degrade purines part or all of the way to urea. The livers of selach- 

 ians {Raja) and of certain teleosts (scombrids et a I.) contain uricase, allan- 

 toinase and allantoicase; others (salmonids, anguillids, pleuronectids) lack 

 allantoicase. Urea is the final end-product of protein degradation in 

 selachians and it forms a significant proportion of the nitrogenous excreta 

 in some teleosts. Now urease converts urea into ammonia, and its absence 

 in marine fish in conjunction with the conservation of urea in selachians 

 and excretion of urea in teleosts, is significant for osmotic reasons which 

 have already been discussed. 



It has been pointed out by Florkin that the degradation of amino-acids 

 and purines in many animals tends to terminate in a common end-product. 

 In crustaceans, for example, the end-product of both protein and purine 

 catabolism is ammonia; and in elasmobranchs, urea. Uricolytic enzymes 

 show a rather patchy distribution among animals. Many invertebrates, 

 including some relatively simple groups, possess the full complement of 

 enzymes capable of degrading purines to ammonia. This appears to be a 

 primitive condition, and in the course of evolution certain groups of 

 animals have dropped various enzymes in the series. 



ELIMINATION OF NITROGENOUS EXCRETA: 

 RENAL AND EXTRARENAL ROUTES 



Invertebrates. Primitive invertebrate groups, including the protozoa, 

 sponges and coelenterates, possess no specialized excretory organs and 



