Enzyme Activity Calculator
Convert absorbance rates into enzyme units and convert between activity units.
U/mL = (ΔA/min) ÷ (ε × l), where ε is in mM⁻¹ cm⁻¹ and l is the path length in cm.
Activity per mL
0.016077U/mL
Total activity
0.016077
U
Specific activity
—
U/mg
nkat
0.26795
nkat
µkat
2.680e-4
µkat
Enter protein concentration to get specific activity.
Learn more
What it does
Enzyme activity expresses how much substrate an enzyme converts per unit time, and it is reported either as total units for a whole sample or as specific activity per milligram of protein. The two most common unit systems are the enzyme unit (U), defined as 1 µmol of substrate per minute, and the katal, the SI unit equal to 1 mol per second. This calculator bridges absorbance-based kinetic assays with both systems so you can report results the way your field expects.
How it works
In rate mode the activity per milliliter is U/mL = (ΔA/min) ÷ (ε × l), where ε is the molar extinction coefficient in mM⁻¹ cm⁻¹ and l is the path length in cm. Using ε in mM⁻¹ cm⁻¹ gives a result directly in µmol/mL/min, which is U/mL, with no extra factor of 1000. The total units are U/mL × reaction volume (mL), and the specific activity is U/mL ÷ protein concentration (mg/mL). In unit mode the conversions rest on 1 U = 1 µmol/min = 16.667 nkat = 0.016667 µkat and 1 katal = 6×10⁷ U, with 1 mU = 10⁻³ U.
Worked example
With ΔA/min = 0.1, ε = 6.22 mM⁻¹ cm⁻¹ for NADH, path length 1 cm, reaction volume 1 mL and protein concentration 0.5 mg/mL, the activity is 0.01608 U/mL and the total activity is 0.01608 U. The specific activity is 0.03215 U/mg, equivalent to 0.2680 nkat or 0.0002680 µkat. As a check, 1 U = 16.667 nkat and 100 nkat = 6.0 U.
When to use it
Use rate mode when you measure product formation by following absorbance on a spectrophotometer or plate reader, for example NADH oxidation at 340 nm or pNA release at 410 nm. Use the specific activity to compare enzyme purity between preparations, and the total units to size a reaction or dosing step. Use unit mode when a paper or kit reports activity in katals and you need it in U, or vice versa, so values across methods become comparable.
FAQ
- How do I convert between U and katal?
- 1 U equals 1 µmol of substrate per minute, which is 1.6667×10⁻⁸ katal, so 1 katal = 6×10⁷ U. In practice that means 1 U = 16.667 nkat = 0.016667 µkat. Multiply your U value by 16.667 to get nkat, or divide a katal value by 6×10⁷ to get U.
- How do I measure ΔA/min?
- Run the assay in kinetics mode on a spectrophotometer or plate reader and record absorbance at a fixed wavelength over time. Fit a straight line to the early, linear part of the progress curve and take its slope as ΔA/min. Read only within the linear range, because substrate depletion, product inhibition or enzyme instability bend the curve later.
- What is the difference between total units and specific activity?
- Total units describe the activity of the whole sample (U/mL × volume), so they scale with how much material you have. Specific activity is U per mg of protein and measures how pure or concentrated the enzyme is. A cleaner preparation has higher specific activity even if total units stay the same.
- Which ε value should I use?
- Use the coefficient for the wavelength and product you measure. Common presets are 6.22 mM⁻¹ cm⁻¹ for NADH/NADPH (340 nm), 8.8 for pNA (410 nm) and 14.15 for TNB from Ellman's reagent (412 nm). Enter it directly in mM⁻¹ cm⁻¹; the formula already treats that unit as µmol/mL/min.