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

The effect of acetylene ((C2H2)-C-14) 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 O-2-dependent. Cells pretreated with acetylene under oxic conditions were unable to oxidize ammonia with O-2 as oxidant. After these cell suspensions were supplemented with gaseous NO2, ammonia oxidation activity of about 140 mu mol 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.