The Pathways of Biological Assimilation 



and Dissimilation of Nitrogen and Some 



Aspects of Their Evolution 



A. E. BRAUNSHTEÏN 



Institute of Biological and Medical Chemistry, Academy of Medical Sciences 

 of the U.S.S.R., Moscow 



Biochemical evolution is not my special field of research, and I do not intend 

 to consider the whole problem of the evolution of nitrogen metabolism. My aim 

 is to discuss a few selected questions concerning the formation of amino acids 

 from ammonia and the paths of nitrogen dissimilation. 



This seems justified insofar as amino acids are the first organic products of 

 nitrogen assimilation and the main precursors of secondary nitrogenous meta- 

 bolites, including vitamins, porphyrins, purines etc. On the other hand, ammonia 

 is the immediate mother substance of organic nitrogen. Although many organisms 

 can utiHze nitrates, nitrites or molecular nitrogen, it would appear that these 

 nitrogen sources of higher oxidation level must always undergo preliminary 

 reduction to ammonia. 



One of the most impressive general results of modem biochemical research 

 is the recognition of far-reaching unity of chemical structures and processes in 

 all contemporary forms of life. In all organisms, the proteins, nucleic acids, 

 coenzymes and other important nitrogenous body constituents are made up 

 from nearly identical building units, arranged into similar structural patterns. 

 There is also relatively Httle difference in the paths of biosynthesis of the simpler 

 subunits. In aU organisms of our epoch, the synthesis of amino acids, vitamins, 

 heterocyclic bases etc. proceeds through similar steps and is effected with the 

 aid of closely related catalytic agents. 



Such unity implies that the biosynthetic mechanisms operating in present- 

 day organisms are of exceedingly ancient origin. Only the latest evolutionary 

 changes of these intricate and highly perfected mechanisms can be evaluated 

 on the basis of the data of comparative biochemistry. In attempting to arrive 

 at some understanding of how these mechanisms could have arisen from simpler 

 or, at any rate, quahtatively very different primordial modes of abiogenic and 

 biological synthesis, we must heavily lean upon conjecture and indirect reasoning. 



Two different ways of nitrogenous nutrition provide for the synthesis of pro- 

 teins and other nitrogenous body constituents in contemporary organisms. On 

 the one hand, the organisms can utiHze amino acids, heterocyclic bases, etc., 

 synthesized in other organisms and absorbed as food; this is the heterotrophic 



527 



