NICOTINIC ACID 337 



gossypii and Eremolhecium ashbyii are less sensitive to iron (75, 301). 

 Moderate levels of glucose, a slightly acid pH, and aeration all improve 

 the yield of riboflavin (202, 243, 301). Riboflavin formation by Asper- 

 gillus niger is favored by any of a variety of nutritional factors, all of 

 which somewhat retard growth (230). 

 The structure of riboflavin is: 



CH 2 OH 

 (CHOH) 3 



Provision of certain purines, e.g., adenine or guanine, to Eremothe- 

 cium ashbyii increases the yield of riboflavin (26, 153, 155). The entry 

 of isotopic formate, carbon dioxide, and acetate (194) and of labeled 

 adenine (163) into the molecule also suggests that the synthesis of ring 

 C is similar to the synthesis of the pyrmidine ring of a purine or that 

 the pyrimidine ring of a purine is incorporated intact (156). 



Inhibition studies with l,2-dichloro-4,5-diaminobenzene led Woolley 

 (310, 311) to suggest that the natural precursor of ring A — and of 

 cyanocobalamin (p. 343) — is l,2-dimethyl-4,5-diaminobenzene. 



The cellular function of riboflavin is to provide the prosthetic group 

 of a number of oxidizing enzymes, the flavin enzymes (114). Flavin 

 enzymes known in fungi include glucose aerodehydrogenase and other 

 respiratory enzymes (Chapter 7) and several enzymes of nitrogen me- 

 tabolism — nitrate reductase, nitrite reductase, hydroxylamine reduc- 

 tase, and the amino acid oxidases (Chapter 8). Evidence from organ- 

 isms other than the fungi indicates that riboflavin plays a role in the 

 synthesis of nicotinic acid (41). 



Zalokar (335) has suggested that a riboflavin-protein complex may 

 be the photoreceptor in the light-activated synthesis of carotene by 

 Neurospora crassa (Chapter 6). 



9. NICOTINIC ACID 



Deficiencies for nicotinic acid — or its amide, which appears from 

 limited evidence to be physiologically equivalent to the free acid in 

 fungi — occur in relatively few fungi: Microsporum audouini (5), Bias- 



