294 THE BIOLOGY OF MARINE ANIMALS 



formation of ammonia by enzymes known as deaminases. However, as we 

 have seen, not all groups excrete the nitrogenous end-products of protein 

 metabolism as ammonia ; some species convert the latter into urea or uric 

 acid. In mammals, ammonia is synthesized into urea by the Krebs 

 (ornithine) cycle, according to the following schema. 



Arginine- 

 Ammonia/ \*Urea 



Citrulline Ornithine 



\ 



Ammonia. 

 + Carbon dioxide 



In this process arginine is converted into ornithine and urea through the 

 catalytic action of arginase present in the liver. The resulting ornithine 

 interacts with ammonia and carbon dioxide, and is reconverted into argin- 

 ine in a repetition of the cycle. Urea may also be formed directly from 

 dietary arginine by the action of arginase. 



The elasmobranch fishes, in which a high degree of uraemia is a natural 

 condition, contain arginase and synthesize urea in all tissues of the body 

 except the brain and blood; the liver in particular is rich in arginase. It 

 is not known, however, whether the ornithine cycle operates in this group. 

 Teleost fishes show much specific variation in liver-arginase content, but 

 this never attains the high level of selachians. The ornithine cycle is reported 

 to be absent from bony fishes, and the urea excreted is probably formed 

 from dietary arginine. 



The ornithine cycle does not appear to be present in any invertebrate 

 group. Most marine invertebrates are ammonotelic, and in these animals 

 arginase is absent or occurs only in very small quantities. Thus in a series 

 of marine molluscs investigated by Baldwin, the hepatopancreas (liver) 

 was found to be free of arginase (2). 



Purine Metabolism 



From studies of excretory products and enzyme complements, certain 

 tentative conclusions have been drawn about the course of purine meta- 

 bolism in invertebrates. Purine bases which are released by the hydrolysis 

 of nucleic acids are sometimes excreted unaltered. But some animals 

 possess specific enzymes, adenase and guanase, which deaminate the 

 purine bases adenine and guanine, converting them to hypoxanthine and 

 xanthine. These, in turn, may be oxidized to uric acid by xanthine oxidase. 

 Some or all of these compounds are excreted in various proportions by 

 different groups of animals. Various invertebrates possess a further com- 

 plement of enzymes capable of breaking down uric acid through a series 

 of steps to ammonia. An outline for the course of purine degradation is as 

 follows — 



