Study of the additions of cadmium and zinc to the cadmium, zinc metallothioneins using differential pulse polarography Influence of the pH

The binding properties and metal ion exchanges of Cd, Zn Metallothioneins (MT) from rabbit liver and horse kidney were studied using differential pulse polarography. The influence of the additions of cadmium or/and zinc to the non-saturated MT was investigated. The addition of cadmium does not remove the zinc initially bound to the protein, as one would expect from the values of stability constants of CdT and ZnT complexes, but produces the splitting of the peak attributed to the Cd-thionein complex into two: one corresponding to the initial one, CdT, and another peak, CdT', at a less cathodic potential. The addition of zinc provokes the transformation of the CdT peak, found for the native MT, into CdT'. Assuming that cadmium forms two complexes with the thionein the equilibrium between these two species CdT reversible arrow, CdT' was studied as a function of added metal ion concentration and of the solution pH. The pH plays a very important role in the stabilization of each of the two cadmium complexes. All ion exchange equilibria are quickly reached.