Information on metal binding properties of metallothioneins from optical spectroscopy

Absorption, circular dichroism and emission measurements made during titrations of rabbit liver Zn-MT and calf liver Cu, Zn-MT with Cd2+ and Cu+ are reported. There are systematic changes in the CD and emission spectra that can be associated with the formation of several species during these titrations. Addition of Cu+ to Zn-MT results in the formation of distinct species that form at specific stoichiometries, these are: Cu6-MT, Cu12-MT and Cu20-MT. The emission intensity due to Cu+ provides a sensitive indication of the presence of Cu-S clusters for the Cu6-MT and Cu12-MT species, suggesting that Cu6-Sx clusters form in both the alpha and beta domains of the protein. The data also demonstrate that Cd7-MT will bind 12 Cu+ to form a species with the stoichiometry of 12 Cu: 4 Cd, and that, surprisingly, Cu12-MT will also bind Cd2+ to form this same new species. It is suggested that the new species incorporates a Cu6 cluster in the beta domain and a mixed-metal Cu6, Cd4 cluster in the alpha domain.