Evaluation of cadmium–metallothionein stability constants based on voltammetric measurements

A voltammetric study on the binding properties of the commercially available rabbit liver metallothionein (MT) Sigma M 7641 for cadmium ions was performed. The capacity of metallothionein to complex cadmium and the stability constants of Cd–T complex (T denotes the apoprotein or thionein molecule) have been determined from the direct titration of the defined MT concentration with the standard CdCl2 solution in 0.59 M sodium chloride medium at pH 7.9 and 2.0. At pH 7.9 the formation of the Cd–T complex has been followed measuring the specific anodic signal height of the complex. Stability constants of Cd–T have been evaluated from the experimental data using three different procedures. The calculated apparent stability constants K=(7.6±0.2)·108 dm3 mol−1 of Cd–T complex in 0.59 M NaCl at pH 7.9 and 25°C indicate that under the given experimental conditions all the three procedures are equally applicable.