Biological Assimilation and Dissimilation of Nitrogen 539 



tion of glutamine or asparagine may represent a widespread mechanism for the 

 indirect synthesis of amino acids (especially those with complicated carbon 

 skeletons) ; this possibility is in need of direct experimental verification. 



The paths of nitrogenous catabolism in different organisms are much more 

 variable than the relatively imiform mechanisms of nitrogen assimilation. 



In green plants and in markedly aminoautotrophic micro-organisms (yeasts 

 and some types of bacteria) the assimilation of nitrogen greatly outweighs its 

 dissimilation. Under normal conditions these organisms do not form ammonia 

 or other excretory nitrogenous metabolites. This is due, in part, to the active 

 reutihzation of nitrogen in biosynthetic processes, but chiefly to low activity of 

 the enzymes of amino acid catabolism. 



Other organisms, particularly the animals and saprophytic micro-organisms 

 feeding on products of protein cleavage, handle the nitrogen of amino acids less 

 economically, and ultimately convert it into ammonia or other excretory end- 

 products. 



The simplest and most common excretory form of nitrogen in micro-organisms 

 and lower types of animals is anmionia, liberated from amino acids, amines, and 

 other nitrogen compounds by the action of various deaminating enzymes. 



There exist several mechanisms for the deamination of amino acids; among 

 these the most important, especially in animals, are processes of oxidative de- 

 amination, either direct, by the action of amino acid oxidases, or indirect, by 

 way of transdeamination. 



Our discussion will be chiefly confined to the dissimilation of nitrogen in 

 animals. A number of different end-products of nitrogen metabolism are known 

 to occur in the various classes and types of animals. In the course of evolution, 

 the predominance of one form of nitrogen excretion or another in the several 

 groups of animals has arisen mainly as a result of adaptation to changing environ- 

 mental conditions of Hfe and development. According to the well-known con- 

 ceptions of J. Needham, E. Baldwin and others, the major factors affecting the 

 evolution of the type of nitrogen catabolism in animals are : the conditions of 

 water metabolism at different stages of their ontogeny, and in some cases the 

 participation of nitrogenous metabolites in osmoregulation [23, 24]. Little is 

 known, for the present, about the changes in enzyme systems which form the 

 basis of the shifts in type of nitrogen catabolism. 



It will be recalled that aquatic invertebrates and freshwater fishes excrete 

 mainly ammonia (ammoniotelic type of nitrogen catabohsm); in insects, many 

 reptiles (snakes, Uzards), and in birds the excretion of uric acid is predominant 

 (uricoteUc nitrogen catabohsm); in amphibia, turtles, the mammals (including 

 man) and in ganoid and selachian fishes the chief excretory product is urea 

 (ureotehc type of nitrogen catabohsm). 



We can leave aside some special nitrogenous end-products occurring in 

 particular groups of animals, e.g. trimethylamine oxide in the marine teleostians, 

 guanine in spiders, etc. 



It is a familiar fact that the tissues of mammals are almost devoid of L-amino 

 acid oxidases, apart from L-glutamic dehydrogenase. 



