Ammonia monooxygenase (AMO) of Nitrosomonas europaea is a metalloenzyme that 
catalyzes the oxidation of ammonia to hydroxylamine. This study shows that AMO 
resides in the cytoplasm of the bacteria in addition to its location in the 
membrane and is distributed approximately equally in both subcellular fractions. 
AMO in both fractions catalyzes the oxidation of ammonia and binds 
[(14)C]acetylene, a mechanism-based inhibitor which specifically interacts with 
catalytically active AMO. Soluble AMO was purified 12-fold to electrophoretic 
homogeneity with a yield of 8%. AMO has a molecular mass of approximately 283 
kDa with subunits of ca. 27 kDa (alpha-subunit, AmoA), ca. 42 kDa (beta-subunit, 
AmoB), and ca. 24 kDa (gamma-subunit, cytochrome c(1)) in an 
alpha(3)beta(3)gamma(3) sub-unit structure. Different from the beta-subunit of 
membrane-bound AMO, AmoB of soluble AMO possesses an N-terminal signal sequence. 
AMO contains Cu (9.4+/-0.6 mol per mol AMO), Fe (3.9+/-0.3 mol per mol AMO), and 
Zn (0.5 to 2.6 mol per mol AMO). Upon reduction the visible absorption spectrum 
of AMO reveals absorption bands characteristic of cytochrome c. Electron 
para-magnetic resonance spectroscopy of air-oxidized AMO at 50 K shows a 
paramagnetic signal originating from Cu(2+) and at 10 K a paramagnetic signal 
characteristic of heme-Fe.