Three metallo forms of peptide deformylase (PDF, EC 3.5.1.31) of Escherichia coli were prepared and crystallized (space group C2, diffraction limit 1.9 A) for initiating the X-ray structure determination of the metal center in correlation with the catalytic functionality of this enzyme. The native Fe2+ containing enzyme species was directly isolated from overproducing bacteria by using catalase as a buffer additive, which stabilizes the catalytic activity against oxidative destruction. The Ni2+ containing form, which is oxygen-insensitive, was obtained by metal exchange with free Ni2+ and found to be catalytically equally effective (kcat/KM = 10(5) M-1 s-1 for N-formyl-Met-Ala). The Zn2+ form, prepared from the apoenzyme or by displacement of bound Ni2+ by free Zn2+, proved virtually inactive.