538 A. E. BRAUNSHTEIN 



For contemporary organisms the rôle of oxoglutaric acid as the principal or 

 unique acceptor of ammonia in aminoautotrophic nitrogen assimilation is firmly 

 established. 



From the evolutionary viewpoint, however, the question arises whether this 

 keto acid could have performed the same function in very ancient organisms, at 

 the dawn of the autotrophic mode of nitrogenous nutrition. 



Contemporary organisms synthesize oxoglutaric acid by way of the tri- 

 carboxylic acid cycle. This is a very elaborate process which necessitates a 

 highly organized system of many enzymes and coenzymes, embodying most 

 of the B- vitamins. 



Therefore, the tricarboxylic acid cycle must have arisen relatively late in the 

 course of phylogeny. Most probably it has been preceded by simpler cycles of 

 oxidative transformations of organic acids, e.g. by C4-dicarboxylic acid cycles, 

 such as those still important in the respiratory metabolism of some present-day 

 micro-organisms . 



The formation of fumarate in such cycles and its conversion to aspartic acid 

 by aspartase is a process requiring a considerably simpler set of biocatalysts than 

 the synthesis of ketoglutarate and glutamate via the citric acid cycle. 



At present, this process plays only a subordinate part, in certain bacterial 

 species and, possibly, in some plants. But it seems possible that the synthesis 

 of aspartic acid by the aspartase reaction was the principal mechanism of primary 

 nitrogen assimilation at the dawn of aminoautotrophism. One is tempted to 

 suppose that, on these primitive levels of biological evolution before the citric 

 acid cycle had developed, glutamic acid and the other amino acids of the C5- 

 family could have been synthesized from aspartic acid by way of secondary 

 condensation reactions. 



A suggestive analogy in support of the possibility of this different, more 

 ancient mode of glutamate formation is the existence, in contemporary organ- 

 isms, of two parallel pathways for the biogenesis of lysine : — from aspartic acid, 

 via threonine and diaminopimelic acid in bacteria, and in the higher organized 

 fungi — by was of a-oxo- and a-amino-adipic acids, arising probably through a 

 special homocitric acid cycle superposed to the ordinary tricarboxylic cycle 

 (cf. Fig. I). 



The existence in contemporary organisms of aminopherases effecting direct 

 transfer of NH2-groups from aspartate to a-keto acids is subject to doubt. 

 However, amino acids could have been synthesized by transreamination via 

 aspartic acid (instead of glutamic) in ancestral organisms, if they possessed 

 aspartic aminopherases, which disappeared at later stages of evolution, or if the 

 transfer of amino groups was preceded by the synthesis of asparagine. Highly 

 active asparagine and glutamine aminopherases transferring the a-amino groups 

 of these amides to diverse a-keto acids have been discovered in mammahan liver 

 by A. Meister [13]. Similar enzymes were later found in various plants 

 (Olenicheva, 1955) and in some baaeria (Domaradskiï, 1955) [15]. 



It is well known that asparagine and glutamine act as the most efficient single 

 nitrogen sources for protein synthesis and growth in many micro-organisms and 

 plant tissues. It is possible that transreamination with the intermediary forma- 



