Why is this done?
Lipases act at an oil–water interface, so a convenient chromogenic ester is useful for screening but cannot alone establish performance on an industrial oil.
How does it work?
Disperse p-nitrophenyl palmitate consistently, start each reaction with enzyme and follow the early A₄₀₅ increase. Subtract substrate and solvent blanks, then use a matched pNP calibration to calculate initial-rate activity.
Materials and equipment
- Fresh 3 mM pNPP stock in an appropriate solvent, 0.25% polyvinyl alcohol dispersing solution in buffered assay medium, and pNP calibration standards.
- Clear flat-bottom 96-well plate, reader at 405 nm with temperature control and suitable mixing.
- Enzyme dilution series; solvent, substrate and sample-matrix blanks.
Procedure
- As an illustrative 200 µL scale-down of a published pNPP method, combine 160 µL buffered 0.25% PVA solution, 20 µL fresh 3 mM pNPP stock and 20 µL enzyme dilution. All wells, including standards and controls, must receive matching solvent and dispersant.
- Set an assay pH at which pNP can be quantified reproducibly and confirm enzyme compatibility. Start each column with enzyme and mix consistently without creating stable bubbles. Read A405 every 30–60 seconds over the initial linear interval at a controlled temperature.
- Prepare pNP standards in the same plate geometry and final buffer/solvent matrix. If a different stop or alkaline development step is used, treat standards and blanks identically and state the changed assay definition.
- Fit the initial net slope after subtracting the no-enzyme substrate slope. Repeat at multiple enzyme dilutions to show proportional response.
Controls and blanks
Include substrate without enzyme to track spontaneous hydrolysis, enzyme without substrate for sample colour, solvent-only blank and a positive lipase control. Check that pNPP stays dispersed throughout the read; turbidity can produce false A405 changes.
Calculations and reporting
Convert the corrected A405/min slope to µmol pNP/min using the same-plate calibration slope expressed as absorbance per µmol. Divide by enzyme volume in mL and multiply by sample dilution to report U/mL of original preparation. If standards are fitted in nmol/well, convert nmol to µmol before defining U.
Quality checks and troubleshooting
A drifting blank or precipitating substrate invalidates the comparison. Lipase at a real triglyceride–water interface may rank differently from pNPP; use a separate oil or FAME assay for the intended application. Avoid comparing units from different pNPP emulsifiers or pH values.
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 lipase kinetic dataset
Illustrative, simulated values for graphing and calculation practice. These are not measured STL product data or acceptance limits.
x-axis: reaction time after mixing (min); y-axis: blank-corrected A₄₀₅ (dimensionless absorbance).
Data pairs (x → y): 0→0.020; 1→0.090; 2→0.160; 3→0.230; 4→0.300; 5→0.325.
Simulated initial slope from 0–4 min = 0.070 A₄₀₅/min; the fifth minute deviates from the initial rate. Convert with a matrix- and plate-matched pNP calibration to µmol/min and correct for sample dilution. Report pNPP formulation and optical read conditions because emulsion and solvent affect the result.
Lab Protocol: BCA Protein Quantification in a 96-Well Plate
