The Zn- and Cd-Clusters of Recombinant Mammalian MT1 and MT4 Metallothionein Domains Include Sulfide Ligands

Recombinant (E. coll) synthesis of mammalian MT1 and MT4 domains as separate peptides in Zn(II) and Cd(II) enriched growth media has rendered metal complexes containing sulfide anions as additional ligands. The Cd preparations show higher sulfide content than the Zn preparations. Also, the βMT1 and βMT4 fragments exhibit higher sulfide/peptide ratios than the respective α fragments. Titration of Zn₃-βMT1 with Cd(II) followed by addition of several sodium sulfide equivalents shows that the Cd(II)-βMT1 species can incorporate sulfide ligands in vitro, with a concomitant evolution of their UV-vis and CD fingerprints to those characteristic of the Cd-S^2- chromophores. Current results have also provided full understanding of previous data collected by this group in the characterization of the Cd-βMT1 preparations obtained from large-scale fermentor synthesis by allowing identification of at least 2S^2- ligands per Cd-βMT1 species. Furthermore, the results here presented have revealed that synthesis of βMT4 in Cd-supplemented cultures yielded Cd,S^2--containing clusters instead of the proposed heterometallic Zn, Cd-βMT4 complexes. Finally, a global evaluation of our results suggests that the higher the Cu-thionein character of a MT peptide, the higher is its tendency to harbor nonproteic ligands (i.e., sulfide anions) when building divalent metal clusters, especially Cd-MT complexes.