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 oxid ation 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-dep endent. Cells pretreated with acetylene under oxic conditions were unable t o oxidize ammonia with O-2 as oxidant. After these cell suspensions were su pplemented with gaseous NO2, ammonia oxidation activity of about 140 mu mol NH4+ (g protein)(-1) h(-1) was detectable under both oxic and anoxic condi tions. A significantly reduced acetylene inhibition of the ammonia oxidatio n activity was observed for cells incubated in the presence of NO. This sug gests that NO and acetylene compete for the same binding site on AMO. On th e basis of these results a new hypothetical model of ammonia oxidation by N . eutropha was developed.