I am not 100% sure but in my view, estimation of metalloprotien/ metallothionins content can be an important protocol for determining the interaction between metal and proteins.
Maybe this protocol helps.. I never tested it myself but ask the author. Greets https://www.researchgate.net/publication/45084876_Isolation_of_Acidic_Basic_and_Neutral_Metalloproteins_by_QPNC-PAGE
Article Isolation of Acidic, Basic and Neutral Metalloproteins by QPNC-PAGE