The metabolism of hydroxylamine to nitrite by Nitrosomonas

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Abstract
Washed-cell suspensions of the autotrophe Nitrosomonas, which obtains its free energy for growth from the oxidation of NH3 to nitrite, rapidly oxidized low concentrations of NH3 and hydroxylamine, but not hyponitrite to nitrite. Extracts produce small amounts of nitrite from hydroxylamine but did not produce nitrite from NH3. Anaerobically in the presence of the extract and methylene-blue, hydroxylamine disappeared and negligible amounts of nitrite were formed. Nitric-oxide and nitrous-oxide were produced in amounts equivalent to the hydroxylamine added. Aerobically in the presence of extracts, methylene-blue or mammalian cytochrome-c as carrier, the rate of disappearance of hydroxylamine and the nitrite production increased. The disappearance of hydroxylamine in the presence of cytochrome-c was inhibited by cyanide. In the presence of methylene-blue, cyanide-poisoned extracts metabolized hydroxylamine and nitrite appeared. The metabolism of hydroxylamine may be a 2-stage process. In stage-1, hydroxylamine is dehydrogenated to nitroxyl which decomposed to nitrous-oxide and which could be further dehydrogenated to nitric-oxide. The oxidation of a reduced natural Hydrogen carrier, with which methylene-blue was in equilibrium, involved a cytochrome and cytochrome oxidase. Stage-2 was the conversion of nitric-oxide into nitrite by an enzyme system requiring O2. The metabolism of nitric to nitrite aerobically may involve a cytochrome.