METABOLISM OF THE NUCLEIC ACIDS 425 



Man and other primates, birds, terrestrial reptiles 

 Uric acid Cyclostomes 



I Insects (except Diptera) 



I 

 (Uricase) Mammals (except man and other primates) 



I Diptera 



AUantoin Gastropods 



I 



I 

 (Allantoinase) One group of Teleosts 



l (Salmonidae, Pleuronectidae, Anguillidae) 



Allantoic acid 



I 



I 

 {Allantoic ase) Selachii, Dipnoi, Crossopterygii 



One group of Teleosts (Cyprinidae, Esocidae, and Scombri- 

 dae) 

 Urea Amphibia 



Fresh-water lamellibranchs 



1 



Fig. 5. Comparative biochemistry of the breakdown of uric acid (Florkin"^). 



studied, and it would appear from the earlier studies recently summarized 

 by Christman,*' Franke,^^ and Klemperer^^- and from the more recent work 

 of Brown and his co-workers^^"''^^^ that a symmetrical product such as 

 hydroxyacetylene-diureine-carboxylic acid may be produced as an inter- 

 mediate in this reaction (Fig. 4). Brown's studies have shown that after 

 the administration of adenine or uric acid labeled with N^^ in the 1- and 

 3-positions, the isotope content of the excreted allantoin is evenly dis- 

 tributed between the imidazole ring and the urea side chain. This would be 

 possible only if a rearrangement of the molecule between uric acid and 

 allantoin had taken place. 



Allantoin is the main end-product of purine metabolism in most mam- 

 mals. In some fish, the allantoin is further broken down by allantoinase to 

 allantoic acid (Figs. 4 and 5), while other fish and amphibia split the al- 

 lantoic acid to urea and glyoxylic acid by the action of allantoicase.^'^^'^^'^ 



1" F. W. Klemperer, /. Biol. Chein. 160, 111 (1945). 



1" G. B. Brown, P. M. Roll, and L. F. Cavalieri, /. Biol. Chem. 171, 835 (1947). 



1'^ G. B. Brown, Cold Spring Harbor Symposia Quant. Biol. 13, 43 (1948). 



"5 M. Florkin, "Biochemical Evolution" (edited, trans., and augmented by S. 



Morgulis). Academic Press, New York, 1949. 

 ''* E. Baldwin, "An Introduction to Comparative Biochemistry." Cambridge Univ. 



Press, England, 1949. 



