The two acireductone dioxygenase (ARD) isozymes from the methionine salvage pathway of Klebsiella oxytoca are the only known pair of naturally occurring metalloenzymes with distinct chemical and physical properties determined solely by the identity of the divalent transition metal ion (Fe2+ or Ni2+) in the active site. We now show that this dual chemistry can also occur in mammals. ARD from Mus musculus (MmARD) was studied to relate the metal ion identity and three-dimensional structure to enzyme function. The iron-containing isozyme catalyzes the cleavage of 1,2-dihydroxy-3-keto-5-(thiomethyl)pent-1-ene (acireductone) by O2 to formate and the ketoacid precursor of methionine, which is the penultimate step in methionine salvage. The nickel-bound form of ARD catalyzes an off-pathway reaction resulting in formate, carbon monoxide (CO), and 3-(thiomethyl) propionate. Recombinant MmARD was expressed and purified to obtain a homogeneous enzyme with a single transition metal ion bound. The Fe2+-bound protein, which shows about 10-fold higher activity than that of others, catalyzes on-pathway chemistry, whereas the Ni2+, Co2+, or Mn2+ forms exhibit off-pathway chemistry, as has been seen with ARD from Klebsiella. Thermal stability of the isozymes is strongly affected by the metal ion identity, with Ni2+-bound MmARD being the most stable, followed by Co2+ and Fe2+, and Mn2+-bound ARD being the least stable. Ni2+- and Co2+-bound MmARD were crystallized, and the structures of the two proteins found to be similar. Enzyme–ligand complexes provide insight into substrate binding, metal coordination, and the catalytic mechanism.