Formation of Nitrate from 3-Nitropropionate by Aspergillus flavus

Extracts of the hyphae of a nitrifying strain of Aspergillus flavus formed nitrite and nitrate from 3-nitropropionate. Nicotinamide adenine dinucleotide phosphate and nicotinamide adenine dinucleotide enhanced the production of nitrate but not nitrite, whereas cysteine and diethyldithiocarbamate increased nitrite but diminished nitrate synthesis. Quinacrine reduced the extent of conversion of the nitro compound to nitrite and nitrate, but only the inhibition of nitrite formation was completely reversed by flavine coenzymes. Molecular oxygen was essential for this part of the nitrification sequence. 3-Chloropropionate stimulated the oxidation of nitrite by hyphae or enzyme preparations. Although the fungus contained a noncytochrome-linked nitrite-oxidizing enzyme, partially purified preparations free of this enzyme formed both nitrite and nitrate from 3-nitropropionate. Possible mechanisms of this latter stage of heterotrophic nitrification are discussed.