536 



A. E. BRAUNSHTEIN 



tigators, working cither with animal tissues or with bacteria and fungi (in higher 

 plants the paths of amino acid synthesis and catabolism have not been thoroughly 

 studied as yet). For a survey of these investigations see the review by Braunshtein 



[15]. 



Some authors (F. Ccdrangolo [20]; S. Kaplanskii and coworkers [21]) suppose 

 that in animal tissues L-alanine and L-aspartic acid are synthesized and deamin- 

 ated directly, rather than via glutamic acid. V. Kretovich [22] holds the same 

 opinion with regard to plants. 



As a matter of fact, the importance of the mechanisms of transreamination 

 and transdeamination can be demonstrated in the most convincing manner 

 precisely for alanine and aspartic acid. It should be noted that these two amino 

 acids, along with glutamic acid, are immediately related to the citric acid cycle; 

 in surviving tissues they are transaminated, synthesized, and deaminated more 

 actively than all other L-amino acids. 



Glutamic 

 dehydrogenose 



NH3, 2H 



Fig. 3. 



In perfect agreement with the mechanism of transreamination, the rate of 

 formation of amino acids from ammonia and pyruvate or oxaloacetate is greatly 

 lowered (whereas the amination of oxoglutarate is not impaired) in the tissues 

 of rats (Braunshtein & Azarkh [18]) or of birds (Efimochkina) when the amino- 

 pherase activity is suppressed, e.g. in vitamin Be-deficient animals subjected to 

 acute intoxication with zsonicotinylhydrazide (a carbonyl fixative blocking the 

 CO-group of pyridoxal). Under these conditions, the depressed amination of 

 pyruvate to alanine in liver homogenate can be restored by the addition, in vitroy 

 of purified glutamic-alanine aminopherase (Table i). 



As expected in the case of transreamination, the amination of pyruvate or 

 oxaloacetate (but not of oxoglutarate) is also depressed if formation of oxo- 

 glutarate in the surviving tissue is inhibited with the aid of fluoroacetate, fluoro- 

 citrate [19], or other specific inhibitors of the citric acid cycle, e.g. cocaine or 

 w^sotartratc [15]. 



In such experiments the synthesis of amino acids, e.g. from pyruvate, can be 

 completely restored if the transreamination mechanism is restituted by supple- 

 mentation of the system with glutamic acid or other oxoglutarate precursors 

 situated below the inhibited link of the respiratory cycle (Table 2). It should be 

 noted that even normally the rate of oxidation of pyruvate is the limiting factor 

 of the overall rate of the transreamination process ; the addition of small amounts 

 of glutamate or oxoglutarate does, therefore, greatly stimulate the formation 

 of NHo-N and of alanine from pyruvate even in the absence of inhibitors. 



