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Ammonia oxidation by Nitrosomonas eutropha with NO₂ as oxidant is not inhibited by acetylene

Department of Microbiology, University of Nijmegen, Toernooidveld 1, 6525 ED Nijmegen, The Netherlands¹
Institute for General Botany, Department of Microbiology, University of Hamburg, Ohnhorststraße 18, 22609 Hamburg, Germany²

Author for correspondence: Ingo Schmidt. Tel: +31 24 3652568. Fax: +31 24 3652830. e-mail: i.schmidt{at}sci.kun.nl

The effect of acetylene (¹⁴C₂H₂) on aerobic and anaerobic ammonia oxidation by Nitrosomonas eutropha was investigated. Ammonia monooxygenase (AMO) was inhibited and a 27 kDa polypeptide (AmoA) was labelled during aerobic ammonia oxidation. In contrast, anaerobic, NO₂-dependent ammonia oxidation (NO₂/N₂O₄ as oxidant) was not affected by acetylene. Further studies gave evidence that the inhibition as well as the labelling reaction were O₂-dependent. Cells pretreated with acetylene under oxic conditions were unable to oxidize ammonia with O₂ as oxidant. After these cell suspensions were supplemented with gaseous NO₂, ammonia oxidation activity of about 140 µmol NH₄⁺ (g protein)^-1 h^-1 was detectable under both oxic and anoxic conditions. A significantly reduced acetylene inhibition of the ammonia oxidation activity was observed for cells incubated in the presence of NO. This suggests that NO and acetylene compete for the same binding site on AMO. On the basis of these results a new hypothetical model of ammonia oxidation by N. eutropha was developed.

Keywords: acetylene, nitrogen dioxide, ¹⁴C₂H₂-labelling, NO₂, O₂-dependent ammonia oxidation

Abbreviations: AMO, ammonia monooxygenase

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