Metallothionein Electron Transfer In The Nafion-mercury Film Modified Electrode And The Metal Release Process Research

Metallothionein (MT) is a type of metalloproteins characterized by a large number of cysteine residues, low molecular weight and the absence of aromatic or histidine residues. It exerts its functions in regulating essential metals, detoxifying heavy metals, scavenging free radicals and controlling the intracellular redox potential. Several groups have described the use of dropping mercury electrodes (DMEs), hanging mercury electrodes, and thin mercury film electrodes to examine the redox properties of MTs. However, there are certain inherent limitations which lead to an unambiguous determination of the redox potentials of MTs. To alleviate these problems, Voltammetric studies of rabbit liver metallothioneins (MTs, containing both Zn and Cd ions) and Zn₇-MT were carried out at Nafion-coated mercury film electrodes (NCMFEs). The accumulation of MT molecules into the NCMFEs enhances the voltammetric signals and the electrostatic interaction between the Nafion membrane and MT.Due to the comparable sizes between the Nafion pores and MT molecules and the permselective exchange between MT molecules and cations in Nafion above the mercury film surface, two pairs of well-defined redox peaks, with Epc values at -0.740 V and -1.173 V, respectively, were observed for the first time. The peak at Epc = -0.740 V is attributable to the reduction of the Cd-MT complex, whereas that at Epc = -1.173 V was assigned to the reduction of Zn-MT complex. The NCMFE was found to be more advantageous than thin mercury film electrode (MFE), because the pristine metal ions in MTs (e.g., Cd²⁺ and/or Zn²⁺) are not significantly replaced by Hg²⁺ The NCMFE provides an effective avenue for fast and very sensitive exploration on the redox characteristics of surface-confined MT.To quantify the redox active Zn-cysteine residues per MT molecule, the electrochemical behavior of Zn₇-MT entrapped in the NCMFE was examined. One pair of redox peaks corresponding to the reduction of Zn-cysteine was observed. The percentage of the redox active Zn-cysteine residues per MT molecule was determined to be 22%. Evidently, the NCMFE is highly desirable for quantification studies of Zn containing MT isoforms owing to its high hydrogen over potential. The above results are highly complementary to AAS determinations. As a consequence, NCMFEs provide a feasible method for the quantification of the metal release associated with the MT redox reactions.