AROMATIC COMPOUNDS 177 



that carbon may flow into new metabolic pools simply because of an 

 insufficiency of nitrogen. We may visualize this type of process 

 schematically: 



Glucose ^;A±^B^;C±^D±^E... (15) 



\\ \\ / 



A' B' — NH 2 



w w 



A" B" 



If compound D, for example, cannot be synthesized because of a rela- 

 tive nitrogen deficiency, enzymatic equilibria and factors of permeabil- 

 ity and solubility will determine which of the potential alternative 

 pathways is followed. In the specific instance of the benzenoid com- 

 pounds and their oxidation products, it is imaginable that the syn- 

 thesis of aromatic amino acids stops because nitrogen is no longer 

 available, and that a precursor of these amino acids, e.g., shikimic 

 acid, is then diverted to the synthesis of new compounds. 



This view of the origin of aromatic compounds puts them in the 

 same class as organic acids, fats, and polysaccharides, as products of 

 what have been termed metabolic shunts (200). Physiologically, the 

 model proposed above requires that the ratio of metabolic product to 

 mycelial nitrogen should increase with increasing glucose concentra- 

 tion, at least over a certain range. This has not been tested; the ratio 

 of product to dry weight of mycelium (243) is less informative, because 

 of the likelihood that carbon components of the mycelium may also be 

 products of metabolic shunts. 



It has often been suggested that fungal quinones play a specific role 

 in respiration, and the hypothesis is an attractive one. The frequent 

 simultaneous occurrence of the reduced and oxidized forms in one 

 organism suggests that the system could transport electrons and so 

 function in terminal respiration. However, satisfactory proof of 

 such a role in the intact cell has not been brought forward. Addition 

 of quinones to a culture often elicits a respiratory response (214, 389), 

 but this can only be taken as an indication that the compound could 

 function, not that it does in fact function. 



A respiratory role for the polyphenol oxidases of higher plants has 

 often been proposed, and the question is still open. In Glomerella 

 cingulata it appears that tyrosinase cannot be an essential respiratory 

 enzyme, inasmuch as it is not detectable during the growth phase (500). 



A pyridine nucleotide-menadione reductase occurs in Aspergillus 

 niger (564), and a pyridine nucleotide-quinone reductase in other 

 fungi (565). In principle, either of these could participate in a 



