Which protein assay method can cause a color change due to interaction with copper?

The protein assay method that can cause a color change due to interaction with copper is the Lowry assay. This assay is based on a chemical reaction in which proteins in a sample react with the reagent containing copper ions, leading to the formation of a colored complex. Specifically, the copper ions in the reagent interact with the peptide bonds of the protein, and this results in a color change that can be quantitatively measured.

The Lowry assay utilizes the Folin-Ciocalteu reagent, which enhances the color change produced by the interaction of copper with proteins. This property makes the Lowry method sensitive and suitable for detecting and quantifying proteins across various concentrations.

In contrast, the Bradford assay relies on the binding of dye (Coomassie Brilliant Blue) to proteins, which causes a color shift but does not involve copper ions. ELISA (Enzyme-Linked Immunosorbent Assay) and Western blotting are techniques primarily used for detecting specific proteins rather than quantifying total protein concentrations through interactions with copper. Hence, the unique interaction with copper in the Lowry assay is what distinguishes it as the correct answer.