Biochemistry and Evolution 63 



preformed urea, but it has been possible to overcome this, first by the 

 use of well-washed tissue preparations and later (Joseph, unpub- 

 lished) by the use of ion-exchange resins. A few experiments have 

 been done on urea synthesis in organs other than the liver but have 

 led nowhere so far. 



Hitherto it had been usual to think of urea production as a means 

 of detoxicating ammonia, and as something that originated in animals 

 at somewhere about the amphibian level of evolution. This attitude 

 could accommodate not only the modern amphibia but also the lung- 

 fishes which, apart from the elasmobranchs, are the only group of 

 fishes in which urea formation is a serious industry. Why, then, and 

 at what stage in their history, did these wholly acpiatic elasmobranchs 

 develop the machinery for making urea? It could, of course, be a 

 matter of evolutionary convergence. Alternatively it could be, perhaps, 

 as Romer and others have suggested, that the earliest ancestral fishes 

 lived under conditions similar to those the lungfishes experience 

 today, and that they all used urea as a means of survival. Later on, 

 it could be argued, urea formation became redundant and disappeared 

 from the majority of fishes. The elasmobranchs, it might be thought, 

 had discovered its osmotic advantages in the meantime and retained 

 the synthetic apparatus. If this were true, it might perhaps be an- 

 ticipated that some vestiges of the one-time synthetic apparatus might 

 be discovered in other kinds of fishes todav, and new work is going 

 on to look into this. In the meantime it is known that arginase 

 is present in the livers of a considerable number of teleostean fishes, 

 and that appreciable quantities of urea are often to be found in their 

 excreta and even in their tissues. So this looks like a helpful line to 

 follow up. Finally, there are still a few fishes that, morphologically 

 speaking, have remained practically unchanged since the Jurassic, for 

 example, Amict and Lepidosteus. Polypterus is still with us but has 

 changed little since the Carboniferous or thereabouts. There are, too, 

 the coelocanths. All these and many more would be well worth study- 

 ing as potential sites of vestigial urea-producing enzymes. So far the 

 only information we have comes from Brown and Cohen (1960), who 

 could find neither carbamylphosphate synthetase nor ornithine trans- 

 carbamylase in Amia or Lepidosteus. 



In the earlier part of our own work, we were able without difficulty 

 to show that laboratory-synthesized carbamyl phosphate reacts with 

 ornithine and gives citrulline, and that, in the presence of citrulline, 

 aspartate, and ATP, urea is formed. If any of the reactants was 

 omitted, urea synthesis stopped (Tables 9 to 11 L All these results have 

 • since been abundantly confirmed. So far there is obviously a close 



