Bradford Method: Colorimetric Protein Assay

Bradford method is a common colorimetric method to determine protein concentration in a sample solution. The Bradford method of protein determination is based on the binding of a dye, Coomasie Blue G, to the protein. This binding shifts the absorbtion maximum of the dye from red to blue. The absorbance of the solution is measured at 595 nm and is proportional to protein concentration when compared to a standard curve. Two types of assay are described here: the standard assay, which is suitable for measuring between 10 and 100 microgram of protein, and the microassay, which detects between 1 and 10 microgram of protein.

Materials

Coomasie Blue G

Preparation of the 5x Bradford Reagent

Dissolve 100 mg of Coomasie Blue g in 50 ml of ethanol and 100 ml of phosphoric acid (85%). The mixture is stirred for approximately 10 minutes. The solution is diluted to 200 ml with distilled water and filtered.

Ethanol

Phosphoric acid, 85% - Caution! Phosphoric acid can cause burns!

Test Tubes

Protein Standard

Protein standard. Bovine gamma-globulin at a concentration of 1 mg/mL (100 micrograms/mL for the microassay) in distilled water is used as a stock solution. This should be stored frozen at –20^oC. Since the moisture content of solid protein may vary during storage, the precise concentration of protein in the standard solution should be determined from its absorbance at 280 nm. The absorbance of a 1 mg/mL solution of gamma-globulin, in a 1-cm light path, is 1.35. The corresponding values for two alternative protein standards, bovine serum albumin and ovalbumin, are 0.66 and 0.75, respectively.

Spectrophotometer

Micropipettes and tips

Method-Protein Determination by the Bradford Method

Standard Assay Method

1. Pipet between 10 and 100 microgram of protein in 100 microLiter total volume into a test tube. If the approximate sample concentration is unknown, assay a range of dilutions (1, 1:10, 1:100, 1:1000). Prepare duplicates of each sample.
2. For the calibration curve, pipet duplicate volumes of 10, 20, 40, 60, 80, and 100 microliter of 1 mg/mL gamma-globulin standard solution into test tubes, and make each up to 100 microliter with distilled water. Pipet 100 microliter of distilled water into a further tube to provide the reagent blank.
3. Add 5 mL of protein reagent to each tube and mix well by inversion or gentle vortex mixing. Avoid foaming, which will lead to poor reproducibility.
4. Measure the A-595 nm of the samples and standards against the reagent blank between 2 min and 1 h after mixing. The 100 μg standard should give an A-595 value of about 0.4. The standard curve is not linear, and the precise absorbance varies depending on the age of the assay reagent. Consequently, it is essential to construct a calibration curve for each set of assays.

Microassay Method

This form of the assay is more sensitive to protein. Consequently, it is useful when the amount of the unknown protein is limited.

1. Pipet duplicate samples containing between 1 and 10 microgram in a total volume of 100 microliter into 1.5-mL polyethylene microfuge tubes. If the approximate sample concentration is unknown, assay a range of dilutions (1, 1:10, 1:100, 1:1000).
2. For the calibration curve, pipet duplicate volumes of 10, 20, 40, 60, 80, and 100 microliter of 100 microgram/mL gamma-globulin standard solution into microfuge tubes, and adjust the volume to 100 microliter with water. Pipet 100 microliter of distilled water into a tube for the reagent blank.
3. Add 1 mL of protein reagent to each tube and mix gently, but thoroughly.
4. Measure the absorbance of each sample between 2 and 60 min after addition of the protein reagent. The A-595 value of a sample containing 10 microgram gamma-globulin is 0.45. Following Figure shows the response of three common protein standards using the microassay method.

Variation in the response of proteins in the Bradford assay. The extent of protein–dye complex formation was determined for bovine serum albumin (Square), gamma-globulin (Circle), and ovalbumin (Triangle) using the microassay.

The Bradford method doesn’t measure the presence of peptide bonds but detects specific amino acids, which is believed to be responsible for the binding of the dye to the protein. The dye does not bind to free arginine or lysine, or to peptides smaller than about 3000 Da. The Bradford assay is not suitable for quantifying the amounts of such compounds.

Regards

Reference: Number 2 and 10 on References