PYRIDOX1NE 335 



biotin, <i/-Oxybiotin (O-heterobiotin), has slight activity for fungi 

 (190), as it does for other organisms (92). Specific anti-biotin activity 

 against fungi is reported for avidin (317), biotin sulfone (321), and 

 y-(3,4-ureylenecyclohexyl)butyric acid (191). 



7. PYRIDOXINE 



Pyridoxine (vitamin B G , adermin) appears to be required by fewer 

 fungi than is biotin or thiamine. Several species of Ceratostomella 

 (Ophiostoma) require it (67, 215), as do Ascoidea rubescens and Tri- 

 chophyton discoides (67, 215, 222). The requirement is highly species- 

 specific within the genus Ceratostomella. 



Pyridoxineless mutants of Neurospora crassa and A r . sitophila grow 

 better at limiting pyridoxine concentration if thiamine is present (263, 

 279). This relation between two vitamins has been explained by the 

 postulate that thiamine competitively inhibits the endogenous destruc- 

 tion of pyridoxine (96). Conversely, pyridoxine inhibits the biosyn- 

 thesis of thiamine, probably by preventing the incorporation of the 

 pyrimidine moiety (98). 



As mentioned above, certain mutants of Neurospora crassa require 

 pyridoxine only at low pH. The mechanism of this conditioned de- 

 ficiency is a requirement for free ammonia in the absence of pyridoxine; 

 at low pH, insufficient free ammonia is present and the vitamin is 

 therefore needed for growth (268). 



The coenzyme form of pyridoxine is pyridoxal-5-phosphate, known 

 to be the coenzyme for enzymes of amino acid metabolism, specifically 

 decarboxylases, transaminases, and racemases (286). Insofar as these 

 enzymes are present in fungi (Chapter 8), we may assume the involve- 

 ment of the coenzyme also. In addition, pyridoxal phosphate has 

 been specifically shown to be active in several isolated enzyme systems 

 of Neurospora crassa (201, 268, 287, 327). Pyridoxine deficiency in A r . 

 crassa mutants is accompanied by accumulation of nitrite from nitrate, 

 indicating a role of pyridoxal phosphate in nitrite reduction (255). 



Although pyridoxine, pyridoxamine, and pyridoxal are equally usa- 

 ble by fungi (261), other modifications of the basic molecule are usu- 

 ally inactive or toxic (218); the few that are active can easily be guessed 

 to be converted in the cell to pyridoxine (198, 218). 



The growth of a pyridoxineless mutant of Neurospora sitophila is 

 inhibited by 4-deoxypyridine; the inhibition is competitively reversed 

 by pyridoxine and its derivatives (197, 198). 



