Redox Reactions of and Transformation between Cysteine-Mercury Thiolate and Cystine in Metallothioneins Adsorbed at a Thin Mercury Film Electrode

Voltammetric studies of rabbit liver metallothioneins (MTs) adsorbed onto thin mercury films preformed onto glassy carbon electrodes were carried out in MT-free phosphate buffers in an attempt to shed light on the possible redox-modulated MT metal-transfer processes. The redox behavior of the surface-confined MTs was studied by cyclic voltammetry and differential pulse voltammetry, and the amount of MT adsorption was quantified by a flow-injection quartz crystal microbalance. Two reversible redox waves, with Ep values at -0.63 and -0.91 V, respectively, were observed for the first time. These values and the overall voltammetric characteristics were found to be remarkably analogous to those of cysteine adsorbates at thin mercury films. The peak at Ep ) -0.63 V is attributable to the reduction of the Cys-mercury thiolates formed between the adsorbed MTs and the mercury electrode, whereas that at Epc ) -0.91 V is assigned to the reduction of Cys-Cys (cystine analogue) present in the portion of the MT adsorbate that is not in direct contact with the mercury film. The two redox waves were observed to be interchangeable through preelectrolysis at a negative potential (e.g., -1.1 V) to reduce the MT adsorbate, or at a more positive potential (e.g., -0.1 V) to oxidize the adsorbed MTs. On the basis of these voltammetric results, we proposed a simple schematic to elucidate the redox reactions of and the transformation between the cysteinemercury thiolates and cystines that are present in the MT adsorbates under different electrochemical redox conditions. Since the electrochemical reduction of the cystine analogues in the MT adsorbates to the corresponding cysteines (a process facilitating metal complexation) and the reoxidation of the cysteine residues back to cystine analogues (a process causing metal release) are both reversible, it appears that the metal transfer between MT and a substrate might accompany the variation of the redox states of the MT-metal complexes. Our voltammetric studies of MTs thus provide supportive evidence for the mechanism of the MT metal transfer in cytoplasmic milieu proposed by Vallee and co-workers (Maret, W.; Vallee, B. L. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 3478-3482).