Catalyzes the reduction of hydroxylamine to form NH(3) and H(2)O. Is also able to reduce hydroxylamine analogs such as methylhydroxylamine and hydroxyquinone. Might have a role as a scavenger of potentially toxic by-products of nitrate metabolism. A + H2O + NH4(+) = AH2 + H(+) + hydroxylamine Binds 1 [2Fe-2S] cluster. Binds 1 hybrid [4Fe-2O-2S] cluster. Inhibited by oxygen. Activated by cyanide except in the prolonged presence of excess cyanide, where the enzyme is inactivated. Optimum pH is 9.0. By Fnr, NarL and NarP under anaerobic conditions in the presence of either nitrate or nitrite. Belongs to the HCP family.