Equilibrium and Kinetic Analysis of the Interaction of Mercury(II) with Cadmium(II) Metallothionein

The reaction of Cd—metallothionein (MT) with mercury(II) has been studied. The Cd₇-MT was titrated with from 1 to 14 equiv of HgCl₂, following which the cadmium(II) and mercury(II) contents, the partition coefficient on a size-exclusion HPLC column, and the UV spectra were measured. The results showed that mercury(II) quantitatively displaces cadmium(II) from the protein. An analysis of the UV spectral data by the method of singular value decomposition (SVD) was used to determine the number of spectrally distinct components and their fractional abundance throughout the titration. A minimum of three spectrally distinct components were found by this method; they corresponded to tetrahedral Cd-S, tetrahedral Hg-S, and linear Hg-S coordination geometries. Initially the mercury(II) occupies tetrahedral sites; however, above a Hg/MT stoichiometry of 4, there is a shift to linear coordination. The size-exclusion data indicate a shift to a more compact structure upon addition of mercury(II) to the metallothionein. Finally, the analysis of a UV spectrum of a mixture of the all-cadmium(II) and the all-mercury(II) metallo-thioneins indicates that the two forms undergo facile interprotein metal ion exchanges. The analysis of the kinetics of this reaction is best fit by second-order kinetics, which would be consistent with a mechanism whereby there was a direct interaction between the two MTs.