Biochemistry and Evolution 65 



parallelism between urea synthesis in these fishes, in amphibia, in 

 chelonian reptiles, and in mammals. We have since done more detailed 

 work on the stages between citrulline + aspartate and urea, and there 

 is no doubt that the reactions take place in the usual way. But where 

 we have completely failed in more than two years of concentrated 

 work is in the biosynthetic production of carbamyl phosphate by 

 elasmobranchs. We have used more delicate and refined methods for 

 its detection, we have tried all sorts of conditions of temperature and 

 pH, we have tried putting in everything from sodium chloride to the 

 proverbial kitchen sink, but still we have not been able to set a 

 synthesis of carbamyl phosphate. Parallel experiments with rat liver, 



Table 11. Formation of urea from citrulline and aspar- 

 tate * 



(M. calif ornicus) 

 j 



D . j /Cimole urea/g ,umole urea formed/e 



Keactant omitted 



powder powder 



All 



None 



Citrulline 



Aspartate 



ATP 



Phosphoglycerate 



Arginase 



* 1 ml enzyme and 1 ml reaction mixture as for Table 10. Incu- 

 bation 30 min at 30°C. 



source: Baldwin (1958, 1960). 



frog liver, and slaughterhouse material produce carbamyl phosphate 

 readily enough, but the elasmobranchs systematically refuse to do so. 

 Brown and Cohen ( 1960 ) have since reported similar failures. 



We shall not try to put any interpretation on these results except 

 to say that there seem to be two possibilities. One is that we may he 

 dealing with a case of evolutionary convergence. The other is that 

 the scheme for urea synthesis on which we are working, and which is 

 widely accepted, may be wrong. Bach, for example, has long main- 

 tained that there are more ways than one in which urea may be 

 synthesized and it is even possible that, working on elasmobranch 

 fishes, we may yet find out how urea is really synthesized in mammals. 



The author was brought up to believe that the job of biochemistry 

 is to solve biological problems by chemical methods, and he felt in 



