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Bacillus halodenitrificansproduced a dimeric, manganese-containing superoxide dismutase constitutively when grown either aerobically or as a denitrifier. The molecular mass of the enzyme was determined by sedimentation equilibrium to be 41.4±3 kDa with each subunit estimated at 26 kDa. Plasma emission spectroscopy indicated the presence of 1.22 mol manganese atoms/mol holoenzyme. The electronic absorption spectrum displayed a broad band centered at approximately 474 nm (e=560 mM-1· cm-1) and a shoulder at 595 nm. In the ultraviolet range, the spectrum exhibited split maxima at 278 nm and 283 nm and a shoulder at 291 nm, thus resembling the spectra of superoxide dismutase fromBacillus subtilisandEscherichia coli. The amino acid composition of theB. halodenitrificansenzyme differed slightly quantitatively but little qualitatively from counterpart enzymes from other sources. Like the superoxide dismutases ofMycobacterium lepraemuriumand human mitochondria, theB. halodenitrificansenzyme exhibited several cysteine residues. As expected from the capacity superoxide dismutase exhibits for protecting NO as neutrophil cytotoxicity factor, theB. halodenitrificanssuperoxide dismutase did not interfere with accumulation of NO produced by the organism's nitrite reductase.