542 A. E. BRAUNSHTEIN 



importance, has been transformed, by the addition of the arginase reaction, into 

 a cycle leading to the production of the new nitrogenous end-product, urea. It 

 is appropriate to point out that the second phase of this cycle is not merely the 

 final step of nitrogen catabohsm in the liver. In mammals this phase has acquired 

 the significance of an obligatory condition, or prerequisite, for the dissimilation 

 of nitrogen; in its absence, the transformations of amino acids in the liver are 

 mainly confined to reversible shutthng of the amino nitrogen in transamination 

 reactions. 



This fact serves to explain a biochemical paradox of long standing, namely, 

 the well-known incongruity between the ease and rapidity with which the 

 natural amino acids are catabolized to urea in the living mammalian organism, 

 and the considerable inertia usually displayed by the same amino acids in 

 attempts to bring about their dissimilation with the aid of surviving tissues. 



In this connection it is possibly significant that in some lower organisms 

 excreting ammonia, citruUine may be transformed into arginine by reversible 

 condensation with ammonia, rather than by the interaction with aspartic acid. 

 This has been observed by Szorenyi and his co-workers [28] in the crayfish; it 

 is possible that the same mechanism of arginine synthesis occurs in those types 

 of bacteria in which this amino acid is spht into citrulline and ammonia by arginine 

 dcsiminase [3, 25]. 



Green plants and yeasts, which normally do not form ammonia and actively 

 assimilate nitrogen, apparently synthesize arginine via argininosuccinic acid, 

 like themammals. 



As shown by Efimochkina {vide supra), L-amino acids are deaminated at fairly 

 high rates, directly in part and partly by way of transdeamination, in avian liver 

 and kidneys. Active L-amino acid oxidases are also present in the tissues (and 

 venom) of snakes. It would seem that there is some relation, as yet obscure, 

 between the active deamination of amino acids in birds and snakes, and their 

 uricoteUc type of nitrogen catabolism. 



Yet it is remarkable that the mechanism of synthesis of uric acid implies, at 

 least in principle, that in uricotelic organisms one half of the protein nitrogen 

 may undergo dissimilation by way of transfer reactions without intermediary 

 deamination, similarly to what has been shown experimentally for the mammals 

 {vide supra). 



It is known, indeed (cf. [3]), that, of the four nitrogen atoms in the purine 

 ring of uric acid, one atom (N-i) is derived from the amino group of aspartic 

 acid and another (N-7) belongs to a molecule of glycine, into which the nitrogen 

 of other amino acids is incorporated by transamination. And only the remaining 

 two nitrogen atoms (N-3 and N-9), i.e. one half of the nitrogen of uric acid, 

 originate from ammonia via the amide group of glutamine [29]. 



We do not yet know what the relations are between direct deamination and 

 the indirect paths of oxidative dissimilation of the amino acids in cold-blooded 

 vertebrates with ureotelic (amphibia, ganoid and selachian fishes) and ammonio- 

 telic (freshwater teleosts, amphibian larvae) nitrogen metabohsm, and also in 

 insects (uricotelic metabolism). No data are available on the reactions leading 

 from citrulline to arginine in most invertebrates, in fungi and in other lower 



