Toxic metal proteomics: reaction of the mammalian zinc proteome with Cd²⁺.

The hypothesis was tested that Cd(2+) undergoes measureable reaction with the Zn-proteome through metal ion exchange chemistry. The Zn-proteome of pig kidney LLC-PK1 cells is relatively inert to reaction with competing ligands, including Zinquin acid, EDTA, and apo-metallothionein. Upon reaction of Cd(2+) with the Zn-proteome, Cd(2+) associates with the proteome and near stoichiometric amounts of Zn(2+) become reactive with these chelating agents. The results strongly support the hypothesis that Cd(2+) displaces Zn(2+) from native proteomic binding sites resulting in the formation of a Cd-proteome. Mobilized Zn(2+) becomes adventitiously bound to proteome and available for reaction with added metal binding ligands. Cd-proteome and Zn-metallothionein readily exchange metal ions, raising the possibility that this reaction restores functionality to Cd-proteins. In a parallel experiment, cells were exposed to Cd(2+) and pyrithione briefly to generate substantial proteome-bound Cd(2+). Upon transition to a Cd(2+) free medium, the cells generated new metallothionein protein over time that bound most of the proteomic Cd(2+) as well as additional Zn(2+).