ObjectiveTo purify a low-temperature hydroxylamine oxidase (HAO) from aheterotrophicnitrifying bacteriumAcinetobactersp.Y16 and investigate the enzyme property. MethodsA HAO was purifiedby an anion-exchange and gel-filtration chromatography from strain Y16. The purity and molecular mass were determined by RP-HPLC and SDS-PAGE. The HAO activity was detected by monitoring the reduction of potassium ferricyanide using hydroxylamine as substrate and ferricyanide as electron acceptor. The partial amino acid sequence was determined bymass spectrometry. ResultsThe low-temperature HAO with a molecular mass of 61 kDa was purified from strain Y16 by an anion-exchange and gel-filtration chromatography. The enzyme exhibited an ability to oxidize hydroxylamine in wide temperature range (4-40 °C)in vitro using hydroxylamine as substrate and ferricyanide as electron acceptor. It was stable in the temperature range of 4 to 15 °C and pH range of 6.0 to 8.5 with less than 30% change in its activity. The optimal temperature and pH were 15°C and 7.5, respectively. Three peptides were determined by mass spectrometry which were shown to be not identical to other reported HAOs. ConclusionThis is the first study to purify a low-temperature HAO from aheterotrophicnitrifier Acinetobactersp. It differs from other reported HAOs in molecular mass andenzyme properties.The findings of the present study have suggested that the strain Y16 passes through a hydroxylamine-oxidizing process catalyzed by a low-temperature HAO for ammonium removal.