Western Blot Sample Preparation

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Protein Extraction Methods

Choosing the correct protein extraction method is crucial, ultimately making the difference between a blank blot and a beautiful blot. To extract protein for the western blot analysis, the first step is to prepare for the lysate from samples. Cell culture and tissue are the most common samples to start with.


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Protein Extraction

Protein Extraction from Cell Culture

Culture cells in the cell culture dish or tube until 80% confluency.

Digest cells with 0.05% trypsin (Catalog# AR1007), or scrape adherent cells off the dish or tube using a cold plastic cell scraper. Be as gentle as possible to avoid inducing cell stress pathways.

Place the dish on ice and rinse cells in ice-cold PBS buffer for 2 times (Catalog# AR0030; 10 mM Na2HPO4 and 1.8 mM Na2H2PO4 with 0.2% Tween 20; pH 7.4).

Aspirate the PBS and add ice-cold lysis buffer (150 mM NaCl, 1% NP-40, 50 mM Tris-HCl and protease inhibitors).

Centrifuge the cells at 3,000 rpm at 4°C for 2-3 minutes.

Remove the supernatant and wash it 2 times with ice-cold PBS buffer.

Gently transfer the cell precipitate into an ice-cold tube.

Add Cell Lysis Buffer (Catalog # AR0103) into the tube (v/v: 6/1: extraction reagent/cell precipitate), which is 5-7 times the volume of the precipitation. Re-suspend vigorously.

If the above solution is murky, sonicate the solution for 10-15 seconds to break up the proteins. Ultrasonic processes for two seconds and pauses for two seconds. The power should be 100-120 watt. Turn off the power when the murky solution becomes clear.

Lyse the cells in RIPA lysis buffer (Catalog# AR0105) on ice for 4-5 hours.

If the solution remains murky, sonicate and lyse again.

Centrifuge at ~10,000 rpm at 4°C for 10 minutes. The centrifugal force and time can be varied depending on the cell type.

Discard lipid (at top) and cell debris (at bottom). Take out the protein solution in the middle and put it in a fresh tube.

Mix the protein solution with Boster SDS-PAGE loading buffer 2X (Catalog# AR0131) in the same volume or with SDS-PAGE loading buffer 5X (Catalog# AR1112) in volume ratio of 4:1 evenly.

Denature the solution in 100°C water bath for 5 minutes. The solution can be used immediately or be aliquotted for storage. Store at -20°C for several months or at 4°C for 1-2 weeks.

Protein Extraction from Tissue

Place surgically resected tissue in pre-cooled (4°C) normal saline. Make sure to wash off any blood from the tissue.

Chop the tissue into small pieces (0.1g to 1g each) after weighing.

Add Broad Spectrum Protease Inhibitor Cocktail (Catalog # AR1182) to homogenize.

Add 10 mL Mammalian Tissue Protein Extraction Reagent (Catalog # AR0101) per 1g of tissue.

Mince the tissue and place the minced tissue in the tissue homogenizer. Grind the tissue until fully homogenized.

Add ice-cold lysis buffer (E.g. For a 5 mg piece of tissue, add 300 µL of buffer. Buffer volume should be determined in relation to the amount of tissue present).

After treated by an ultrasonic processor, lyse the tissue homogenate on ice for 4-5 hours or at 4°C (high speed) for 5 minutes.

If necessary, sonicate until no tissue chunks remain.

Centrifuge at ~10,000 rpm at 4°C for 10 min. The centrifugal force and time can be varied depending on the sample type.

Discard lipid (at top) and cell debris (at bottom). Take out the protein solution in the middle and put it into a fresh tube.

Mix the protein solution with SDS-PAGE loading buffer 2X (Catalog# AR0131) in the same volume or with SDS-PAGE loading buffer 5X (Catalog# AR1112) in volume ratio of 4:1 evenly.

Denature the solution in 100°C water bath for 5 minutes. The solution can be used immediately or be aliquotted for storage. Store at -20°C for several months or at 4°C for 1-2 weeks.

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Protein Quantitation Assay

After protein sample collection, the concentration of proteins should be quantified to ensure equivalent loading amount of each protein.

The most commonly used assay kits are Boster's BCA protein assay kit (AR0146), micro BCA protein assay kit (AR1110), and Commassie plus protein assay kit (AR0145) in order of popularity.

Coomassie plus protein assay is easy to carry out and consumes less sample. However, different proteins perform with great variety and a poor standard curve linearity may be generated. The pH value could also change due to various factors, such as high concentration of Tris, EDTA, urea, glycerol, sucrose, acetone, ammonium sulfate, and detergent. This will subsequently affect color development and interfere with protein quantification.

BCA protein assay is valued for its stronger stability, higher sensitivity, and higher flexibility. The colored complex formed is more stable with little interfering substance. Furthermore, BCA protein assay performance is not influenced by detergent, unlike the Coomassie plus protein assay.

The test tube protocol for our BCA (Bicinchoninic Acid) Protein Assay Kit (Boster Catalog# AR0146) is described here. Refer to our datasheet for the microplate protocol as appropriate.

Reagent Preparation

Reconstitute the albumin standard ampules (BSA) with 0.9% NaCl or PBS to create a working solution of 2000 µg/mL (Tube A).

Mix thoroughly 50 mL BCA Reagent A with 1 mL BCA Reagent B (i.e., Diluent).

Prepare the diluted BSA standards by mixing the diluent and BSA as follows:

Tube Diluent Volume (µL) BSA Volume (µL) BSA Concentration (µg/mL)
A 0 600 (From Tube A) 2000
B 100 300 (From Tube A) 1500
C 300 300 (From Tube A) 1000
D 200 200 (From Tube B) 750
E 300 300 (From Tube C) 500
F 300 300 (From Tube E) 250
G 300 300 (From Tube F) 125
H 400 100 (From Tube G) 25
I (Blank) 300 0 0

Quantitation Assay (For the test-tube protocol, ratio of sample to working range is 1:20).

Pipette 0.1 mL of each standard and unknown sample replicate into an appropriately labeled test tube.

Add 2.0 mL of the working range (WR) to each tube and mix well.

Cover and incubate tubes with one of the protocols:

  • Standard Protocol: 37°C for 30 min (WR: 25 to 2,000 μg/mL).
  • RT Protocol: Room temperature for 2 hrs (WR: 25 to 2,000 μg/mL).
  • Enhanced Protocol: 60°C for 30 min (WR: 5 to 250 μg/mL).

Note: Increasing the incubation time and temperature can increase the net 562 nm absorbance for each test and decrease both the minimum detection level of the reagent and the working range of the protocol. Use a water bath to heat the tubes for either Standard (37°C incubation) or Enhanced (60°C incubation) Protocol. Using a forced-air incubator can introduce significant error in color development because of uneven heat transfer.

    Cool all tubes to room temperature

    With the spectrophotometer set to 562 nm, “zero” the instrument on a cuvette filled only with water. Subsequently, measure the absorbance of all the samples within 10 min.

Note: Because the BCA Assay does not reach a true end point, color development will continue even after cooling to room temperature. However, because the rate of color development is low at room temperature, no significant error will be introduced if the 562 nm absorbance measurements of all tubes are made within 10 min of each other.

Subtract the average 562 nm absorbance measurement of the blank standard replicates from the 562 nm absorbance measurement of all other individual standard and unknown sample replicates.

Prepare a standard curve by plotting the average blank-corrected 562 nm measurement for each BSA standard vs. its concentration in μg/mL. Use the standard curve to determine the protein concentration of each unknown sample.