Metal ion release from metallothioneins: proteolysis as an alternative to oxidation

Metallothioneins (MTs) are among others involved in the cellular regulation of essential Zn-II and Cu-I ions. However, the high binding affinity of these proteins requires additional factors to promote metal ion release under physiological conditions. The mechanisms and efficiencies of these processes leave many open questions. We report here a comprehensive analysis of the Zn-II-release properties of various MTs with special focus on members of the four main subfamilies of plant MTs. Zn-II competition experiments with the metal ion chelator 4-(2-pyridylazo)resorcinol (PAR) in the presence of the cellular redox pair glutathione (GSH)/glutathione disulfide (GSSG) show that plant MTs from the subfamilies MT1, MT2, and MT3 are remarkably more affected by oxidative stress than those from the E-c subfamily and the well-characterized human MT2 form. In addition, we evaluated proteolytic digestion with trypsin and proteinase K as an alternative mechanism for selective promotion of metal ion release from MTs. Also here the observed percentage of liberated metal ions depends strongly on the MT form evaluated. Closer evaluation of the data additionally allowed deducing the thermodynamic and kinetic properties of the Zn-II release processes. The Cu-I-form of chickpea MT2 was used to exemplify that both oxidation and proteolysis are also effective ways to increase the transfer of copper ions to other molecules. Zn-II release experiments with the individual metal-binding domains of E-c-1 from wheat grain reveal distinct differences from the full-length protein. This triggers the question about the roles of the long cysteine-free peptide stretches typical for plant MTs.