Direct metal analysis of the bacteriolytic exoenzyme zoocin A failed to unequivocally identify a putative metal cofactor. Hence, indirect experiments utilizing NMR were undertaken to settle this question. Cd +2 as a surrogate metal ion was reconstituted into EDTA-treated, metal-free recombinant zoocin, and 113 Cd-NMR was employed to explore binding in the protein for this ion. The Cdsubstituted enzyme was found to have 75% of native streptococcolytic activity. A major 113 Cd resonance at 113.6 ppm was observed which with time split into resonances at 113.6 and 107.2 ppm. A minor 113 Cd resonance at 87.3 ppm was observed which increased in intensity with time. These Cd chemical shifts are indicative of two nitrogen atoms and two oxygen atoms ligating directly to the metal site. Based upon conserved amino acid residues in a homologous protein of known structure, LytM, the ligands in zoocin are tentatively assigned to H45, D49, H133, and some combination of water or buffer ions as the fourth oxygen donor in zoocin A. Comparison of the combined intensities for 113 Cd-substituted zoocin with a known quantity of another Cd-substituted protein gave Cdbinding as approximately stoichiometric (1.2 +/-0.2) with protein. Additional metal-removal and reconstitution experiments on the recombinant catalytic domain of zoocin implicate Zn +2 as the metal cofactor. Therefore, the evidence supports zoocin as a single Zn ion binding metalloenzyme.