266 NITROGEN NUTRITION AND METABOLISM 



amino ac id pool, (2) transfer of the nitrogen from one amino acid to 

 another, and (3) biosynthesis of the carbon chains of known amino 

 acids either before or after incorporation of nitrogen. 



Most attention has been directed toward the synthesis of the pro- 

 tein amino acids. Even a superficial survey reveals that many other 

 amino acids are formed by fungi either free or in combined form. 

 Gamma-aminobutyric acid, probably arising from glutamate by decar- 

 boxylation, is rather common (374, 380, 486, 612). Djenkolic acid is 

 found free in extracts and culture fluids of Tilletia caries (380). Beta- 

 lysine occurs in several actinomycete antibiotics (56, 76, 382). Antibi- 

 otics of fungi and actinomycetes yield on hydrolysis numerous N-methyl 

 amino acids, for example, sarcosine, N-methylvaline, and N-methyliso- 

 leucine (78, 417). Alpha-hydroxy amino acids occur combined in ly- 

 comarasmin (609) and in ergot compounds (76), while D-a-methylserine 

 is a constituent of the antibiotic amicetin (160). 



Entrance of Nitrogen into the Amino Acid Pool. We have observed 

 that most fungi utilize ammonium nitrogen as the major or sole nitro- 

 gen source; other inorganic forms are, we may assume with certain 

 reservations, converted to ammonia before assimilation. The best- 

 established reaction is the amination of «-ketoglutaric acid derived 

 from carbohydrate via the tricarboxylic acid cycle, to form glutamic 

 acid, i.e., by the reversal of Equation 5. As mentioned, the only fungus 

 in which this enzyme has been unequivocally demonstrated is Nenro- 

 spora crassa; distribution of carbon-14 in glutamate is consistent with 

 the operation of this reaction in Streptomyces griseus (89, 199). 



Reversal of aspartase action occurs in bacteria (145, 183) and the 

 enzyme is known in fungi (p. 262). It is not yet, however, known 

 whether the reverse reaction: 



Fumaric acid + NH 3 —> n-aspartic acid (8) 



makes a significant contribution to amino acid synthesis in fungi. This 

 leaves us for the moment with only one mechanism for the entrance 

 of nitrogen into amino acids. On grounds of comparative biochem- 

 istry, however, it seems likely that future work will uncover other 

 mechanisms, especially in the synthesis of amides and of tryptophan, 

 histidine, serine, and other amino acids. BlastocladieUa emersonii, 

 which will not grow with ammonia as sole nitrogen source, forms glu- 

 tamate from labeled glucose, but isotopic equilibrium with a -ketoglu- 

 tarate is not observed; this suggests that glutamic acid arises in some 

 other way (91). 



