Quantitative determination of cadmium, zinc thioneins using differential-pulse polarography

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Abstract
The quantitative determination of five commercial Cd–Zn thioneins (MT) and of two organic molecules intrinsic to the structure of metallothioneins, the peptidic fragment Lys-Cys-Thr-Cys-Cys-Ala thionein fragment [56-61] MT I and cysteine, was carried out using differential-pulse polarography (DPP). Metallothioneins of different origins, rabbit liver and horse kidney, presenting different isoforms and with a wide range of total contents of cadmium and zinc were studied. At pH 7.5, polarograms exhibit three peaks, labelled pMT, pCdT and pZnT and previously attributed to the oxidation of the mercury electrode in the presence of thiol groups and to the reduction of cadmium and zinc complexed by the thionein, respectively. Linear relationships between peak intensities and concentrations were observed. The intensity of the peak corresponding to the organic part of the molecule as a function of three times the molar concentration of metal ions initially contained in the MT was a single calibration curve for the five metallothioneins. DPP is proposed as a quantitative determination method.