528 A. E. BRAUNSHTEIN 



type of nitrogenous nutrition. On the other hand, the same sub-units can be 

 synthesized by the organisms themselves from nitrogen-free organic compounds 

 and inorganic nitrogen, chiefly ammonia; that is what we call the aminoauto- 

 trophic type of nutrition. All grades of transition are known from total amino- 

 autotrophy (in green plants and many micro-organisms) to high degrees of 

 nitrogen hetcrotrophy, e.g. in the animals, which fail to synthesize B-vitamins 

 and a number of amino acids. Hetcrotrophy reaches extreme levels in lactic 

 acid bacteria and in certain parasitic organisms exhibiting still more restricted 

 capacities for the synthesis of amino acids, nitrogenous heteroc}'cles etc. [i]. 



The widely differing abilities of various organisms to synthesize the amino 

 acids chiefly reflect variations of their capacity to build the corresponding 

 carbon skeletons. 



Here again we meet with a wide range of intermediate levels between the 

 absolute carboautotrophy of chemo- and photosynthesizing organisms, and the 

 greatly narrowed synthetic potentialities of animals and those microbes which 

 thrive on proteins or protein fragments. But even the extreme aminohetero- 

 trophs are able, to a lesser or greater extent, to utilize ammonia and incorporate 

 it into amino acids, including the essential ones, when supplied with adequate 

 nitrogen-free precursors, e.g. in the form of ac-keto acids. 



The more closely the type of nutrition of an organism approximates to total 

 carbon and nitrogen autotrophy, the more complete and elaborate must neces- 

 sarily be the array of enzyme systems at its disposal for the synthesis of proteins 

 and all other body constituents. This implies that chemo- or photosynthesizing 

 autotrophs can only have arisen, through gradual evolution, from primordial 

 heterotrophic forms of life which thrived on organic compoimds of abiogenic 

 origin [2]. The heterotrophy of the present-day organisms, which feed on the 

 products of biosynthesis of carbo- and aminoautotrophs, is of secondary origin; 

 its development involves adaptive regression and simplification of superfluous 

 enzyme systems. 



During the last decades great progress has been achieved in elucidating the 

 principal stages of the biogenesis of all amino acids (and of important secondary 

 nitrogenous metabolites), and the major steps of their catabolic degradation ial 

 various organisms [3-5]. This progress has been due, to a considerable extent,] 

 to ingenious studies involving the use of isotopically labelled compounds [6],| 

 and especially to the utilization of artificially induced mutations in micrc 

 organisms, resulting in the blocking of single cnzymic reactions [3, 5]. 



These investigations have revealed the striking uniformity of the pathways] 

 for biogenesis of amino acids and other nitrogen compounds in all organisms] 

 (to the extent they are able to synthesize these metabolites), and the close simi- 

 larity of the enzymes and coenzymes involved in these syntheses in organisms! 

 belonging to widely differing types. Lysine is the only amino acid synthesized} 

 by two independent routes (in bacteria and in moulds, respectively) [5]. 



Contemporary organisms build the structural skeletons of the amino acidsj 

 from intermediates of the biosynthesis of sugars and of their anaerobic or oxi- 

 dative breakdown, i.e. from triose and pentose phosphates, phosphoglycerate, 

 pyruvate and the acids involved in the respiratory tricarboxyUc acid cycle. 



