The electrochemical behaviour of Cd,Zn thioneins depending on the solution pH using differential pulse polarography

The electrochemical behaviour of Cd,Zn thioneins (MT) from rabbit liver and horse kidney with respect to solution pH at the dropping mercury electrode was studied using differential pulse polarography. The equilibrium M + T ⇆ MT concerning the formation of metallothionein from their metal depleted form for assessing the possibility of reproducing the same original molecule was investigated as a function of pH, going from basic to acid solutions and vice versa. The equilibrium is reversible and dissociation and association are quickly reached. Consequently the apothionein, metallothionein free of metal ions and exhibiting a mainly random-coil structure, in acid solution is still able to maintain its metal binding capability and in neutral or basic solution to complex metal ions through its thiol groups. However, the polarographic response from acid to basic solution is not the same as for the variation of pH in the opposite direction. It is possible that the reconstituted Cd, Zn thionein possesses properties and characteristics which are different to their native form. The apparent stability constants of the complexes CdT, ZnT and HgT were estimated at different pH values. The order of affinity of the metal ions to the thionein is well determined: ḰHgT ⪢ ḰCdT > ḰZnT.