INORGANIC SOURCES OF NITROGEN 245 



The passage of electrons is visualized: 



Reduced triphosphopyridine nucleotide -> flavin 



adenine dinucleotide -> molybdenum -+ nitrate (1) 



That is, nitrate is reduced with concurrent oxidation of some other, 

 presumably carbon, compound by a triphosphopyridine-linked dehy- 

 drogenase. The enzyme of Hansenida anomala differs in that either 

 di- or triphosphopyridine nucleotide is functional (483). 



Two other flavin enzymes of Nearospora crassa, nitrite reductase and 

 hydroxylamine reductase, also inducible metal-dependent enzymes, 

 have not as yet been separated. Together, and possibly with the aid 

 of a third enzyme, they catalyze the reduction of nitrite to ammonia 

 (384,385,451). 



From these enzymatic studies, and from the somewhat uncertain 

 occurrence of hydroxylamine in nitrate cultures (322, 396, 494), an 

 inorganic pathway of nitrogen reduction has been proposed (384): 



N0 3 _ -» N0 2 ~ -> ? -> NH 2 OH -> NH 3 (2) 



The unknown intermediate, possibly hyponitrite, is included on the 

 assumption that the four-electron change from nitrite to hydroxyla- 

 mine is unlikely to be a single reaction. 



Whether this inorganic pathway is the sole and obligatory sequence 

 of nitrate reduction in fungi is not yet certain. Since molybdenum 

 deficiency reduces both nitrate reductase content and the ability to 

 grow on nitrate in N. crassa and Aspergillus niger, it seems probable 

 that the first proposed step is essential. Genetic studies bear out this 

 conclusion (484). However, we cannot be sure that succeeding steps 

 are necessarily inorganic in nature, as implied in Equation 2; incor- 

 poration into organic combination may occur at an oxidation level 

 above that of ammonia (220, 338, 484). 



A nitroaryl reductase system in Neurospora crassa catalyzes the re- 

 duction of m-dinitrobenzene by reduced pyridine nucleotides to ra-ni- 

 troaniline (384). This system, although possibly not specific, is of 

 interest as a model of the reduction of organic nitro compounds as an 

 alternative to the inorganic pathway of Equation 2. 



The increased toxicity of thiourea to Aspergillus niger in nitrate 

 media suggests that the poison interferes with assimilation (158); 

 chlorate toxicity is also more evident in nitrate media (203). Fluoride 

 causes the accumulation of nitrite in fungi, presumably acting spe- 

 cifically on the nitrite reductase system (373). 



