Reactivity of Cd-7-metallothionein with Cu(II) ions: evidence for a cooperative formation of Cd-3,Cu(I)(5)-metallothionein

Reaction of Cd-7-metallorhionein-2 (MT) with Cu(II) ions has been studied b y a variety of spectroscopic techniques including W-absorption, circular di chroism (CD) and luminescence spectroscopy. The addition of up to 5 Cu(II) equivalents to Cd-7-MT resulted in a cooperative formation of the monomeric Cd-3,Cu-5-MT form, as revealed by the analytical data and the presence of isosbestic or isodichroic points in the respective UV and CD spectra. The p resence of Cu(I) luminescence and the absence of Cu(II) EPR signal indicate d that copper is bound in the Cu(I) oxidation state, i.e., Cd-3,Cu-5(1)-MT. Consequently, the reduction of Cu(II) ions is accompanied by the oxidation of thiolate ligands of the protein. The absorption features and the lumine scence data at 77 K are consistent with the presence of an air-stable Cu(I) -clustwr in Cd-3,Cu-5(1)-MT The participation of other ligands, besides cys teine thiolates, in metal coordination cannot be ruled out, With men than 5 Cu(II) equivalents added a mixture of unstable MT metalloforms were formed . The concomitant reduction and binding of copper ions by metallated MT rep resent a new aspect of the MT structure. (C) 2001 Elsevier Science B.V. All rights reserved.