Information on Metal Binding Properties of Metallothioneins from Optical Spectroscopy

Abstract

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-S_x clusters form in both the α and β 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 β domain and a mixed-metal Cu₆, Cd₄ cluster in the α domain.