Enzyme Activity Calculator
Translate spectrophotometric assay data into enzyme units per milliliter and total units using the Beer-Lambert relation.
Accounts for dilution of sample prior to the assay.
Activity (U/mL)
0
Total units in sample
0
How to Use This Calculator
Measure delta absorbance
Record the change in absorbance during the linear region of the assay and convert to per minute if needed.
Enter assay parameters
Supply extinction coefficient, path length, reaction volume, and sample volume consistent with the assay setup.
Account for dilution and time
Include dilution factors and elapsed time to scale the activity to the original sample.
Review calculated activity
Results show enzyme units per milliliter and total units in the aliquot used.
Formula
Activity (U/mL) = (dA/dt) * dilution * Vtotal / (epsilon * l * Vsample)
dA/dt in absorbance per minute, epsilon in M^-1 cm^-1, l path length in cm, volumes in mL. Total units equal activity multiplied by sample volume in mL.
Example
For dA/dt = 0.06 min^-1, epsilon = 6220, l = 1 cm, Vtotal = 1 mL, Vsample = 0.05 mL, dilution = 1: activity = 0.06 * 1 * 1 / (6220 * 1 * 0.05) about 0.00019 U/mL.
Full Description
Enzyme activity assays often rely on changes in absorbance generated by substrate consumption or product formation. Converting those changes into enzyme units standardizes results across experiments.
The formula assumes a linear response region and uses the Beer-Lambert relation to relate absorbance change to molar conversion, scaled to volume, dilution, and time.
Frequently Asked Questions
What units are enzyme units?
One unit corresponds to one micromole of substrate converted per minute under the specified conditions.
How do I handle path lengths other than 1 cm?
Enter the actual path length. Microplates often have effective path lengths less than 1 cm.
Should I blank against reference wells?
Yes. Subtract blank absorbance changes before entering delta absorbance to remove background contributions.
Can I use extinction coefficients in L·mol^-1·cm^-1?
Yes. The units are equivalent to M^-1 cm^-1; just ensure molarity units remain consistent.
What if the reaction is nonlinear?
Use the initial linear portion of the absorbance curve or apply nonlinear kinetic models separately.