Nitrogen Excretion 201 



PHYLOGENY OF ENZYMES OF PROTEIN BREAKDOWN PRODUCTS 



Deaminases catalyze the liberation ot ammonia from amino acids in all 

 phyla. Urea can be formed by the ornithine cycle (see p. 188), by the action 

 of arginase on dietary arginine independently of the ornithine cycle, and by 

 unknown reactions. Arginase is absent, or present in mere traces, in ammono- 

 telic invertebrates. No arginase was found in the hepatopancreas of a series 

 of marine molluscs; a trace was found in Anodonta and Carchuis and small 

 amounts in several fresh-water snails.^ In the snail Helix the hepatopancreas 

 contains much arginase, but no urea is formed by the ornithine cycle: possibly 

 arginase has some role in uric acid synthesis in Helix.^ The chloragogue 

 tissue of the earthworm intestine can synthesize urea, although the ornithine 

 cycle may not operate as in mammals. Urea production by chloragogue tissue 

 was increased in one set of observations by peptone but not by arginine or 

 ornithine.^^ However, worms fed arginine increased their urea output, and 

 there was a small increase after the feeding of citrulline but not ornithine.'^ 

 Teleost fish fall into three groups, having low (cod, salmon, bullhead, 

 trout), intermediate (herring), and high (dogfish) arginase content in their 

 livers,'"'^ but the ornithine cycle is said to be absent from teleosts."' •'■^' *"■' 

 Elasmobranchs contain large amounts of arginase in all tissues except blood 

 and brain; the heart is remarkably rich in arginase. ''■^- ''^ Whether the orni- 

 thine cycle functions in elasmobranchs is not known. 



Urea is formed by the ornithine cycle in the livers of ureotelic tetrapods, 

 frogs, turtles, and mammals. In mammals the liver is rich in arginase and 

 the kidney contains a small amount, whereas other tissues are negative for 

 arginase.'^'^- •''■''• *" The ornithine cycle is lacking in uricotelic snakes and 

 birds, although bird but not snake kidneys, liver and testes have some argin- 

 ase. '•''• -^- '''■^' •''•''• "" In the animal kingdom urea is formed by several methods, 

 arginase content of liver is correlated with ureotelism, and the ornithine cycle 

 is a specialized development not found among invertebrate animals. The orni- 

 thine cycle and most of the usefulness of arginase are lost by birds and higher 

 reptiles. 



Conclusions Regarding Excretion of Protein Nitrogen. The form of nitro- 

 genous waste produced by protein catabolism is related to stress of water supplv 

 in both embryonic and adult life of an animal. Ammonia diffuses freely out 

 of and away from aquatic invertebrates and fish. Where there is some osmotic 

 stress but ample dilution, urea appears, as in amphibians and mammals. 

 Marine tcleosts are subject to osmotic stress, and some of them excrete tri- 

 methylamine oxide. Extreme need for water retention, in insects, land gas- 

 tropods, snakes and lizards, and birds, is satisfied by uric acid excretion; this 

 is associated with cleidoic eggs. Trimethylamine oxide excretion may have 

 arisen only once, but uric acid excretion became dominant in several unrelated 

 groups and represents convergent evolution. 



Urea production has arisen by several means: urea may be built up from 

 peptones and arginine in Luinhricns, by arginase from dietary arginine in 

 snails and bony fish, by the ornithine cycle from ammonia in amphibians and 

 mammals, and still other routes of urea synthesis probably exist. The ornithine 

 cycle has been lost in reptilian and avian evolution. Embryonic retention of a 

 biochemical trait is shown in nitrogen excretion patterns. Blowfly larvae 

 excrete ammonia before they excrete uric acid, tadpoles produce ammonia 



