Bio-Rad Bradford Protein Assay User Manual

Quick Start™Bradford Protein Assay

Instruction Manual

For technical service call your local Bio-Rad office, or in the US, 1-800-4BIORAD (1-800-424-6723)

Table of Contents

Section 1 Introduction 1

1.1 Principle 1

1.2 Selecting a Protein Standard 5

1.3 Product Description 9

Section 2 Instructions 11

2.1 Standard Assay Protocol 11

2.2 Microassay Protocol 14

Section 3 Data Analysis 18

Section 4 FAQs and Troubleshooting 22

Section 5 Ordering Information 26

Section 6 References 28

Section 7 Appendix 30

Section 1 Introduction

The Quick Start Bradford protein assay is a simple and accurate procedure for determining the concentration of protein in solution. It provides ready-to-use convenience by supplying the dye reagent at 1x concentration and two protein assay standards at seven prediluted concentrations. The prediluted standards are conveniently packaged in 2 ml screwcap vials, eliminating wasteful and sharp ampoules, and ensuring protein stability over the shelf life of the product.

1.1 Principle

The Bradford assay is a protein determination method that involves the binding of Coomassie

Brilliant Blue G-250 dye to proteins (Bradford

1976). The dye exists in three forms: cationic (red), neutral (green), and anionic (blue) (Compton and Jones 1985). Under acidic conditions, the dye is predominantly in the doubly protonated red cationic form (A

max

= 470 nm). However, when the dye binds to protein, it is converted to a stable unprotonated blue form (A

max

= 595 nm) (Reisner et al. 1975, Fazekes de St. Groth et al. 1963, Sedmack and Grossberg 1977). It is this blue protein-dye form that is detected at 595 nm in the assay using a spectrophotometer or microplate reader.

Cation « Neutral form « Anion 470 nm (red) 650 nm (green) 595 nm (blue)

Work with synthetic polyamino acids indicates that Coomassie Brilliant Blue G-250 dye binds primarily to basic (especially arginine) and aromatic amino acid residues (Compton and Jones 1985). Spector (1978) found that the extinction coefficient of a dye-albumin complex solution was constant over a 10-fold concentration range. Thus, Beer's law may be applied for accurate quantitation of protein by selecting an appropriate ratio of dye volume to sample concentration.

Certain chemical-protein and chemical-dye interactions interfere with the assay. Interference from non-protein compounds is due to their ability to shift the equilibrium levels of the dye among the three colored species. Known sources of interference,

such as some detergents, flavonoids, and basic protein buffers, stabilize the green neutral dye species by direct binding or by shifting the pH (Compton and Jones 1985, Fanger 1987). Nevertheless, many chemical reagents do not directly affect the development of dye color when used in the standard protocol and the more common reagents are listed in Table 1. The microassay is compatible with lower concentrations of reagents

than listed in Table 1 due to the larger sample volume-to-dye ratio. Since every protein-chemical reagent combination has not been assayed, it is possible that some of the listed reagents interfere in combination with certain proteins. However, with respect to proteins such as bovine serum albumin (BSA) and bovine

gamma--globulin, the listed reagents show little or no interference.

1.2 Selecting a Protein Standard

In any protein assay, the ideal protein to use as a standard is a purified preparation of the protein being assayed. In the absence of such an absolute reference protein, another protein must be selected as a relative standard. The best relative standard to use is one that gives a color yield similar to that of the protein being assayed. Selecting such a protein standard is generally done empirically. Alternatively, if only relative protein values are desired, any purified protein may be selected as a standard. The two most common protein standards used for protein assays are BSA and gamma-globulin.

With the Quick Start Bradford protein assay, dye color development is significantly greater with BSA than with most other proteins, including gamma--globulin. Therefore, the BSA standard would be an appropriate standard if the sample contains primarily albumin, or if the protein being assayed gives similar response to the dye. For a color response that is typical of many proteins, the gamma--globulin standard is appropriate.

Table 1. Reagents compatible with the Quick Start Bradford protein assay when using the standard procedure.*

Acetone, 10% Acetonitrile, 10% Ammonium sulfate, 1 M Ampholytes, 3–10, 0.5% ASB-14, 0.025% Ascorbic acid, 50 mM Bis-Tris, pH 6.5, 0.2 M b-mercaptoethanol, 1 M Calcium chloride, 40 mM CHAPS, 10% CHAPSO, 10% Deoxycholic acid, 0.2% DMSO, 5% Dithioerythritol (DTE), 10 mM Dithiothreitol (DTT), 10 mM Eagle's MEM Earle's salt solution EDTA, 0.2 M EGTA, 0.2 M

Ethanol, 10% Glucose, 20% Glycerol, 5% Glycine, 0.1 M Guanidine-HCl, 2 M Hank's salt solution HCl, 0.1 M HEPES, 0.1 M Imidazole, 0.2 M Magnesium chloride, 1 M MES, 0.1 M Methanol, 10% Modified Dulbecco's PBS MOPS, 0.1 M NAD, 2 mM Nonidet P-40, 0.25% Octyl b-glucoside, 0.5% Octyl b-thioglucopyranoside, 1% PBS

Coomassie is a trademark of Imperial Chemical Industries. Triton is a trademark of Union Carbide Corp. Tween is a trademark of ICI Americas, Inc.

*The concentration limits for compatibility with the microassay are 1/25of the values in Table 1.

TBP, 5 mM TBS (25 mM Tris, 0.15 M NaCl, pH 7.6), 0.5x TCEP, 20 mM Thio-urea, 1 M Tricine, pH 8, 50 mM Triethanolamine, pH 7.8, 50 mM Tris, 1 M Tris-glycine (25 mM Tris, 192 mM glycine) Tris-glycine-SDS, (25 mM Tris, 192 mM glycine, 0.1% SDS),

0.5x Triton X-100, 0.05% Tween 20, 0.01% Urea, 4 M

Phenol Red, 0.5 mg/ml PIPES, 0.2 M PMSF, 2 mM Potassium chloride, 2 M Potassium phosphate, 0.5 M SB 3–10, 0.1% SDS, 0.025% Sodium acetate, pH 4.8, 0.2 M Sodium azide, 0.5% Sodium bicarbonate, 0.2 M Sodium carbonate, 0.1 M Sodium chloride, 2.5 M Sodium citrate, pH 4.8 or

6.4, 0.2 M Sodium hydroxide, 0.1 M Sodium phosphate, 0.5 M Sucrose, 10%

1.3 Product Description

All kit components have a 1 year shelf life at 4°C. Standards are provided in a 0.9% NaCl, 0.05% NaN

solution.

1x Dye Reagent: 1 L of dye solution containing methanol and phosphoric acid. One bottle of dye reagent is sufficient for 200 assays using the standard 5 ml procedure or 4,000 assays using the microplate procedure.

BSA Standard, 2 mg/ml: Provided in 2 ml tubes.

Bovine Gamma-Globulin Standard, 2 mg/ml:

Provided in 2 ml tubes.

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