Two enzymes, designated, E-2 and E-2', catalyze different oxidation reactions of an aci-reductone intermediate in the methionine salvage pathway. E-2 and E-2', overproduced in Escherichia coli from the same gene, have the same protein component. E-2 and E-2' are separable on an anion exchange column or a hydrophobic column. Their distinct catalytic and chromatographic properties result from binding different metals. The apo-enzyme, obtained after metal is removed from either enzyme, is catalytically inactive. Addition of Ni2+ or Co2+ to the apo-protein yields E-2 activity. E-2' activity is obtained when Fe2+ is added. Production in intact E. coli of E-2 and E-2' depends on the availability of the corresponding metals. These observations suggest that the metal component dictates reaction specificity.