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Copper Iodide Staining of <strong>Protein</strong>s 381<br />

51<br />

Copper Iodide Staining of <strong>Protein</strong>s<br />

and Its Silver Enhancement<br />

Douglas D. Root and Kuan Wang<br />

1. Introduction<br />

Copper iodide staining and silver-enhancement is designed to quantify proteins<br />

adsorbed to solid surfaces such as nitrocellulose, nylon, polyvinylidene difluoride<br />

(PVDF), silica, cellulose, and polystyrene (1–5) and has important applications in<br />

Western blotting and thin layer chromatography (3,6). The binding of cupric ions to the<br />

backbone of proteins under alkaline conditions and their reduction to the cuprous state<br />

is the basis of several protein assays in solution including the biuret, Lowry, and<br />

bicinchoninic acid methods (1–3,7 and see Chapters 2–4). In the case of copper iodide<br />

staining, the protein binds copper iodide under highly alkaline conditions. This protein<br />

assay demonstrates sensitivity, speed, reversibility, low cost, and the lack of known<br />

interfering substances (including nucleic acid; refs. 4,5). Copper iodide staining is sufficiently<br />

sensitive to permit the quantification of proteins adsorbed to microtiter plates<br />

(5). The information is particularly useful for the quantitative interpretation of enzymelinked<br />

immunosorbent assay (ELISA) and protein binding experiments. The precision<br />

of the determination of protein adsorbed to the microtiter plate by copper iodide staining<br />

is typically about 10–15%. The high sensitivity of copper iodide staining (about 40 pg/µL)<br />

may be increased several fold by a silver-enhancement procedure that allows the detection<br />

of protein down to about 10 pg/µL, which is more sensitive than common solutionbased<br />

assays (7). The sensitivity of the assay can be increased by repeated applications<br />

of the protein on a membrane to concentrate it. <strong>Protein</strong> concentrations may be estimated<br />

from copper iodide staining from very dilute protein solutions or when only<br />

small amounts of a precious protein are available such as for the analysis of chromatography<br />

fractions.<br />

2. Materials<br />

2.1. Copper Iodide Staining<br />

1. Prepare the copper iodide staining reagent by mixing 12 g of CuSO4·5H2O, 20 g of KI,<br />

and 36 g of potassium sodium tartrate with 80 mL of distilled water in a glass beaker (see<br />

Note 1). As the slurry is vigorously stirred, 10 g of solid NaOH is slowly added. The<br />

From: The <strong>Protein</strong> <strong>Protocols</strong> Handbook, 2nd Edition<br />

Edited by: J. M. Walker © Humana Press Inc., Totowa, NJ<br />

381

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