540 A. E. BRAUNSHTEIN 



In these tissues the natural amino acids are dcaminated either very slowly or 

 not at all, with the exception of a few L-amino acids, like histidine, the hydroxy 

 and mercapto amino acids, which are partly degraded by non-oxidative reactions 

 with hbcration of ammonia. 



We have studied the paths of dissimilation of amino acids in animal tissues 

 with the aid of procedures, mentioned above, for the suppression of trans- 

 amination reactions (pyridoxine deficiency; isoniazid) or of oxoglutarate form- 

 ation (fluoroacetate and other inhibitors of the citric acid cycle). In this way, it 

 could be shown that the slight formation of ammonia from natural amino acids 

 (beside those subject to non-oxidative deamination, as stated above) in the liver 

 and kidney of rats proceeds only by way of transdeamination [15]. 



But the extent of this process is quite restricted, no more than 2-3 % of the 

 theoretical amount of ammonia being formed; the thermodynamic conditions 

 of equilibrium in the reversible deamination of glutamic acid are such that 

 accumulation of the products of transdeamination even in low concentrations 

 will result in dominance of the opposite process, or transreamination. 



E. F. Efimochkina has established that the rates of deamination of natural 

 amino acids in liver and kidney of birds (chicks, turkey poults, pigeons) are 

 considerably higher than in mammahan tissues, alanine, aspartic acid and, partly, 

 phenylalanine and leucine being dissimilated via transdeamination. In addition 

 to this indirect mechanism, however, the avian tissues contain special amino 

 acid oxidases, absent in the mammals, which effect the direct deamination of 

 basic L-amino acids (Boulanger & Osteux), of L-tryptophan and glycine 

 (Efimochkina), and of some other L-amino acids. 



PreHminary experiments (Efimochkina) indicate that deamination of alanine 

 and aspartate proceeds by the indirect path in frog Uver, which forms urea, 

 and probably by straight oxidation in Uver of freshwater fish belonging to the 

 ammoniotelic type. 



In manmials deamination (anaerobic cleavage of some L-amino acids, direct 

 oxidation of L-glutamate and L-proline, and slow transdeamination of other 

 L-isomers and glycine) is not the only path of dissimilation of amino acid 

 nitrogen. 



A large part of the catabolized protein nitrogen (about one half of this nitro- 

 gen) is converted to urea in the mammahan Uver via reactions of intermolecular 

 transfer of amino groups, without the intermediary hberation of ammonia. 



It is known, at present, that in the 'ornithine cycle' of ureogenesis one nitro- 

 gen atom of the urea molecule is derived from ammonia (at the ornithine -^ citrul- 

 line stage), whereas L-aspartic acid acts as the immediate source of the second 

 nitrogen atom (in the citrulline -> arginine step; Ratner & Pappas, 1947 [25]), 

 as shown opposite. 



In perfect agreement with this scheme, it has been demonstrated in our 

 laboratory (I. V. Klyugc [26]) that most natural amino acids fail to be deamin- 

 ated at physiological concentrations, in surviving rat liver tissue. Yet under 

 appropriate conditions ensuring the rapid formation of arginine in the second 

 phase of the ornithine cycle, the nitrogen of these amino acids (as well as 

 ammonia N) is readily transferred to citruUine and converted to urea in liver 



