Lab Protocol: BCA Protein Quantification in a 96-Well Plate

Why is this done?

Knowing how much protein is present helps calculate specific activity and compare purification fractions. It does not say how much of that protein is an active enzyme.

How does it work?

React matched samples and BSA standards with BCA reagent in a 96-well plate, read the colour at 562 nm and interpolate only within the standard curve. Pair the concentration with a separate enzyme activity assay.

Materials and equipment

• Pierce BCA reagents A and B or an equivalent kit with its current manual.
• BSA standard prepared in the sample buffer or matched diluent, clear flat-bottom 96-well plate, plate shaker and 562-nm reader.
• Diluted lysates and purification fractions with their dilution factors recorded.

Procedure

1. Prepare fresh working reagent by mixing 50 parts reagent A with one part reagent B for the cited Pierce BCA kit. Use enough for 200 µL per well plus dispensing allowance.
2. Prepare a blank and standards spanning the kit's working range; for example 0, 125, 250, 500, 1000, 1500 and 2000 µg/mL. Make standards in a matrix matching the unknowns where practical. Use duplicate or triplicate wells.
3. Pipette 25 µL standard or diluted sample into each assigned well. Add 200 µL working reagent, mix on a plate shaker for 30 seconds, cover and incubate 30 minutes at 37°C. Cool to room temperature and read absorbance near 562 nm.
4. Subtract the mean blank reading and fit the standards according to the kit guidance. Dilute samples outside the valid range and repeat. For very low or very high samples, change dilution rather than extrapolate.

Controls and blanks

Include reagent blank, an independent protein control, and a sample-buffer blank for each materially different matrix, especially imidazole- or reducing-agent-containing fractions. Check the current kit compatibility table before analysing unusual buffers.

Calculations and reporting

Unknown concentration (mg/mL) = interpolated standard-curve concentration (µg/mL) × dilution factor / 1000. If a 1:10 sample reads as 500 µg/mL, the original is 5.0 mg/mL. Specific activity = independently measured enzyme U/mL divided by protein mg/mL, giving U/mg.

Quality checks and troubleshooting

A low blank and a monotonic standard curve are necessary. BCA responses differ by protein, so BSA-equivalent concentration may differ from the target's true mass; if accurate absolute quantification is critical, use a purified target standard. Reducing agents and metal chelators can interfere.

Method scope

This is a research and process-development protocol. Verify the selected enzyme grade, reagent supplier instructions, equipment capacity and local laboratory SOP before use. Report the exact assay definition and conditions; results obtained with different substrates or methods are not automatically interchangeable.

Illustrative BCA standard curve dataset

Illustrative, simulated values for graphing and calculation practice. These are not measured STL product data or acceptance limits.

x-axis: BSA concentration in assay sample (µg/mL); y-axis: blank-corrected A₅₆₂ (dimensionless absorbance).

Data pairs (x → y): 0→0.000; 125→0.045; 250→0.090; 500→0.180; 1000→0.360; 1500→0.540; 2000→0.720.

Simulated idealized data: y = 0.00036x, where x is µg/mL; a diluted unknown at A₅₆₂ = 0.180 corresponds to 500 µg/mL (0.5 mg/mL) in the well. A 1:10 dilution implies 5.0 mg/mL in the original. Real BCA responses can curve; fit only the validated range and use the measured blank.

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