Interprotein metal ion exchange between cadmium-carbonic anhydrase and apo- or zinc-metallothionein

The ligand substitution reactions of cadmium-carbonic anhydrase with EDTA and pyridine-2,6-dicarboxylic acid were compared with one in which rabbit apometallothionein was the competing metal-binding agent. This last reaction occurred more rapidly than the other two at a much smaller ratio of competing ligand to Cd-carbonic anhydrase. It was characterized as a second-order reaction, first-order in Cd-carbonic anhydrase and in apometallothionein, having a rate constant of 5.8+/-0.1 M-1 s(-1) at 25 degrees C and pH 7.4 in Tris.HCl buffer and 0.1 M KCl. At 25 degrees C, Zn-7-metallothionein also exchanged metal ions with Cd-carbonic anhydrase with a rate constant of 0.33 +/- 0.02 M-1 s(-1) to reconstitute enzymatically active protein. Cd-carbonic anhydrase reacted within the time of mixing with the peptide sequence 49-61 of rabbit metallothionein 2 which contains four cysteinyl residues, leading to the exchange of most of the Cd2+ into the peptide. At pH 7.4 and 25 degrees C, Cd2+ has higher affinity for apometallothionein than for the apo-peptide.