Characterisation of zinc metallothioneins by electroanalytical techniques

The electrochemical behaviour of zinc metallothionein (MT) from commercially available rabbit liver, with respect to solution pH as well as the influence of the addition of cadmium and/or zinc was studied using differential pulse polarography and square wave voltammetry. This Zn-MT has an extremely low content of cadmium in the molecule. The complexation equilibrium of dissociation and the formation from their depleted form was investigated, going from basic to acid solution and vice versa. Results obtained allow one to conclude that the equilibrium of complexation is reversible in the pH range studied. Three different responses due to the Zn(II) reduction were distinguished, with a better resolution over the basic pH value range indicating different forms of complexation of zinc with the thionein. Additions of zinc to the MT do not provoke changes in the features of voltammograms indicating that the zinc, if it is incorporated in the molecule, does not modify its structure. On the contrary the additions of cadmium, which is quickly incorporated into the molecule, provoke a clear reorganisation of its structure affecting not only peaks of the Cd(II) reduction attributed to two forms of complexation of cadmium-thionein, but also changes in the co-ordination of zinc complexes. The pH plays an important role in the stability of the different sorts of cadmium and zinc complexes, Voltammetric peaks attributed to different species are interrelated. (C) 1999 Elsevier Science B.V. All rights reserved.