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

• Novel experiments reveal the reaction of the Zn-proteome with Cd2 +.
• Cd2 + displaces Zn2 + from Zn-proteins in approximately stoichiometric fashion.
• Much of the Zn-proteome can be converted to Cd-proteome.
• Both apo-metallothionein (MT) and Zn7-MT bind Cd2 + from Cd-proteome.
• Cellular Cd-proteome reacts with MT following the induction of MT.

The hypothesis was tested that Cd2 + 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 Cd2 + with the Zn-proteome, Cd2 + associates with the proteome and near stoichiometric amounts of Zn2 + become reactive with these chelating agents. The results strongly support the hypothesis that Cd2 + displaces Zn2 + from native proteomic binding sites resulting in the formation of a Cd-proteome. Mobilized Zn2 + 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 Cd2 + and pyrithione briefly to generate substantial proteome-bound Cd2 +. Upon transition to a Cd2 + free medium, the cells generated new metallothionein protein over time that bound most of the proteomic Cd2 + as well as additional Zn2 +.

Cd2+ displaces Zn2+ from native proteomic binding sites in vitro to generate a Cd-proteome. Mobilized Zn2+ becomes adventitiously bound to proteome. Apo-metallothionein sequesters Cd2+ from Cd-proteome. Zn-metallothionein exchanges metal ions with Cd-proteome, possibly restoring functionality to Cd-proteins. Similar behavior was observed in cells exposed to Cd2+ and pyrithione.Figure optionsDownload as PowerPoint slide