232 THE BIOCHEMISTRY OF B VITAMINS 



found to participate as a substrate in the reversible nicotinic acid systems. 

 For this reason, ketoglutaric acid may be an extremely important inter- 

 mediate for the fixation of ammonia into organic molecules. Equally 

 significant is the fact that of the known transamination reactions, by which 

 the a-amino group of one acid can be passed on to other keto acids, all 

 have as one amino acid component glutamic acid. 35 Although only a few 

 transaminases are known, it seems not unlikely that others exist. If so, a 

 general scheme for the formation of these amino acids would be the 

 reductive amination of ketoglutaric acid, followed by a transfer of the 

 amino group to other keto acids. In this way the ketoglutaric acid- 

 glutamic acid system could function as a type of ammonia carrier and the 

 system would be one requiring nicotinic acid and pyricloxal. Since the 

 amination of ct-ketoglutaric acid is reductive, it entails having the reac- 

 tion coupled with the dehydrogenation of some organic substrate in order 

 to supply the hydrogen atoms which will keep regenerating the reduced 

 form of the nicotinic acid coenzyme. Thus the energy for the conversion 

 of ammonia to an organic amine is derived indirectly from the "oxidation" 

 of another substrate. In addition to pyridoxal and nicotinic acid, biotin 

 may also be another B vitamin directly involved in the biosynthesis of 

 at least one amino acid, aspartic acid (p. 172). 



For many microorganisms the amides of aspartic and glutamic acids, 

 asparagine and glutamine, are better sources of nitrogen than are inorganic 

 ammonium salts. It has been shown that glutamine is formed by a reac- 

 tion in which inorganic ammonia is fixed at the expense of a high energy 

 phosphate bond in adenosine triphosphate, 31 and this conversion of inor- 

 ganic nitrogen into an intermediate amide nitrogen may often constitute 

 an essential step in the formation of amino compounds from ammonia. 



A number of different observations offer evidence indicating that 

 the requirements for specific amino acids by organisms reflects a defi- 

 ciency in their ability to form the carbon skeleton of the essential mole- 

 cules rather than an incapacity to form the specific amino acid itself. 

 These observations include (1) the ability of most organisms to utilize 

 the corresponding keto acids in place of essential amino acids; (2) the 

 demonstrated equilibrium existing between inorganic ammonia and the 

 a-amino nitrogen of all amino acids except lysine; 36 and (3) the utiliza- 

 tion of ammonia by mammals in place of the nitrogen usually supplied 

 by the nonessential amino acid. 37 



Identified reactions in which one amino acid is formed from another 

 by changes leaving the original a-amino and carboxyl groups intact 

 include several transformations which are known to depend upon the 

 presence of a B vitamin coenzyme: 



