Last updated: July 2, 2026
Enzyme Activity Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
Enzyme activity measures how fast an enzyme converts substrate to product, expressed in units (U) where 1 U = 1 µmol/min. For spectrophotometric assays, activity (U/mL) = (ΔA/min × total reaction volume × dilution factor) ÷ (extinction coefficient × path length × sample volume). Specific activity (U/mg) = activity / protein concentration, and it increases as the enzyme is purified.
Enzyme activity in units per millilitre equals the absorbance rate in AU per minute times the total reaction volume times the dilution factor, all divided by the molar extinction coefficient times the path length times the sample volume. One enzyme unit equals one micromole of substrate converted per minute.
Key Takeaways
- Enzyme activity (U/mL) = (ΔA/min × V_total × dilution factor) ÷ (ε × path length × V_sample).
- 1 enzyme unit (U) = 1 µmol substrate converted per minute under defined assay conditions.
- Specific activity (U/mg) = activity (U/mL) ÷ protein concentration (mg/mL); it rises as the enzyme is purified.
- Always use the linear (steady-state) portion of the absorbance-vs-time curve for ΔA/min.
- The molar extinction coefficient ε must match the measured species (product or substrate) at the exact assay wavelength.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
Activity (U/mL) = (ΔA/min × V_total × DF) / (ε × l × V_sample)
Where:
- ΔA/min=Rate of absorbance change(AU·min⁻¹)
- V_total=Total assay (reaction) volume(mL)
- DF=Dilution factor(dimensionless)
- ε=Molar extinction coefficient(mM⁻¹·cm⁻¹)
- l=Optical path length(cm)
- V_sample=Enzyme sample volume added to assay(mL)
Worked Examples
LDH assay — NADH at 340 nm
Lactate dehydrogenase (LDH) activity measured by NADH consumption at 340 nm. A cell lysate sample is diluted 1:1 and assayed in a 3 mL cuvette.
- 1Measure the linear slope of the absorbance-vs-time curve: ΔA/min = 0.1 AU/min.
- 2Apply the Beer-Lambert formula: Activity = (0.1 × 3.0 × 1) / (6.22 × 1.0 × 0.1).
- 3Numerator: 0.1 × 3.0 × 1 = 0.3.
- 4Denominator: 6.22 × 1.0 × 0.1 = 0.622.
- 5Activity = 0.3 / 0.622 ≈ 0.4823 U/mL.
- 6Specific activity = 0.4823 U/mL ÷ 0.5 mg/mL ≈ 0.9646 U/mg.
Alcohol dehydrogenase — with 1:10 dilution
ADH activity measured by NADH production at 340 nm. The stock extract was diluted 10-fold before the assay.
- 1Record the slope of the linear phase: ΔA/min = 0.05 AU/min.
- 2Note the 10-fold dilution factor (DF = 10) used before the assay.
- 3Apply the formula: Activity = (0.05 × 1.5 × 10) / (6.22 × 1.0 × 0.05).
- 4Numerator: 0.05 × 1.5 × 10 = 0.75.
- 5Denominator: 6.22 × 1.0 × 0.05 = 0.311.
- 6Activity = 0.75 / 0.311 ≈ 2.4116 U/mL in the original sample.
Peroxidase — ABTS radical at 420 nm
Peroxidase activity using ABTS as a chromogenic substrate. Measured at 420 nm with a high extinction coefficient.
- 1Measure the linear absorbance rate: ΔA/min = 0.2 AU/min at 420 nm.
- 2Apply the formula with ε = 36.0 mM⁻¹cm⁻¹ for ABTS radical cation.
- 3Activity = (0.2 × 2.0 × 5) / (36.0 × 1.0 × 0.02).
- 4Numerator: 0.2 × 2.0 × 5 = 2.0.
- 5Denominator: 36.0 × 1.0 × 0.02 = 0.72.
- 6Activity = 2.0 / 0.72 ≈ 2.7778 U/mL.
- 7Specific activity = 2.7778 / 1.2 ≈ 2.3148 U/mg.
Introduction
Enzyme activity quantifies how much catalytic work an enzyme performs in a given time and volume. The standard unit is the enzyme unit (U): 1 U is defined as the amount of enzyme that converts 1 µmol of substrate per minute under defined assay conditions. Spectrophotometric assays measure the change in absorbance over time, then convert it to activity using the Beer-Lambert law: ΔA = ε × c × l. This calculator applies the full formula to give activity in U/mL and, if protein concentration is known, specific activity in U/mg — the standard metric for assessing enzyme purity. For enzyme kinetics constants (K_m, V_max), see the Michaelis-Menten calculator. The formula and unit conventions follow the IUPAC recommendations for enzyme kinetics and the NC-IUB/IUB Commission on Biochemical Nomenclature.
What is enzyme activity?
Enzyme activity is the rate at which an enzyme catalyses a chemical reaction under a specified set of conditions (temperature, pH, substrate concentration, and ionic strength). It is expressed in enzyme units (U) — sometimes called International Units (IU) — where 1 U = 1 µmol substrate converted per minute. Activity can be reported per unit volume (U/mL) to quantify a preparation, or per unit protein mass (specific activity, U/mg) to assess the proportion of active enzyme in a sample. As enzyme purity increases through purification steps, specific activity rises; a pure enzyme has a fixed, maximal specific activity.
1 U = 1 µmol substrate converted per minute (IUPAC/IUB definition).
Activity (U/mL) depends on the enzyme concentration and assay conditions.
Specific activity (U/mg) = activity / total protein — increases with purity.
The katal (kat) is the SI unit: 1 kat = 1 mol s⁻¹; 1 U = 16.67 nkat.
Always report the temperature, pH, and substrate concentration with activity data.
How the Beer-Lambert law gives enzyme activity
Most enzyme assays are spectrophotometric: a substrate or product absorbs light at a characteristic wavelength, and its concentration is tracked via absorbance (Beer-Lambert law: A = ε × c × l). Rearranging gives the change in concentration per minute: Δc = ΔA/(ε × l). Multiplying by the total assay volume converts to moles per minute (µmol/min = U). Dividing by the sample volume placed in the assay gives the volumetric activity (U/mL) in the original sample. If the sample was diluted before the assay, multiply by the dilution factor to back-calculate to the original sample concentration.
Use only the linear (steady-state) portion of the absorbance-vs-time curve when reading ΔA/min. A non-linear initial region often indicates lag phase, substrate depletion, or product inhibition.
How to calculate enzyme activity step by step
Follow these steps for a standard spectrophotometric enzyme assay:
Record the absorbance at the assay wavelength every 30 s for 3–5 minutes.
Plot absorbance vs. time and identify the linear (steady-state) phase.
Calculate the slope: ΔA/min (AU per minute).
Measure or look up the molar extinction coefficient ε of the product/substrate at that wavelength.
Enter the total reaction volume (buffer + substrate + enzyme), enzyme sample volume, path length, and dilution factor into this calculator.
Read off activity (U/mL). If you measured protein concentration with a Bradford or BCA assay, divide to get specific activity (U/mg).
Report the assay conditions (temperature, pH, substrate concentration) alongside the result.
Common extinction coefficients for enzyme assays
The extinction coefficient is the most important constant in the calculation. Using the wrong value for a cofactor or chromogenic substrate is the most frequent source of error. The table below lists widely used values verified against NIST and the literature.
| Compound | Wavelength (nm) | ε (mM⁻¹·cm⁻¹) | Typical assay |
|---|---|---|---|
| NADH / NADPH | 340 | 6.22 | Dehydrogenases (LDH, ADH, MDH) |
| NAD⁺ / NADP⁺ | 260 | 18.0 | Pyridine nucleotide assays |
| ABTS radical cation | 420 | 36.0 | Peroxidases, laccases |
| p-Nitrophenol (alkaline) | 405 | 18.2 | Phosphatases, esterases |
| Cytochrome c (reduced) | 550 | 21.1 | Cytochrome c oxidase |
| Hydrogen peroxide | 240 | 0.0436 | Catalase (direct) |
Specific activity and enzyme purity
Specific activity (U/mg total protein) rises as an enzyme is purified because inactive protein is removed while the active enzyme is retained. It is the primary metric for tracking a protein purification procedure. Dividing the specific activity of a pure preparation by the specific activity of the crude extract gives the fold purification, while the ratio of total units in the pure fraction to total units in the crude extract gives the yield (%) — both are reported in a standard purification table. Specific activity also allows comparison of activity between labs, independent of preparation concentration.
Specific activity (U/mg) = activity (U/mL) ÷ protein concentration (mg/mL).
Fold purification = specific activity at step n ÷ specific activity of crude extract.
Yield (%) = total units at step n ÷ total units in crude extract × 100.
A pure enzyme has a constant specific activity regardless of how much is loaded.
Practical tips and common mistakes
Accurate enzyme activity measurements require careful technique at every step. Common errors include reading the non-linear (burst or lag) phase of the curve, using the wrong extinction coefficient, forgetting to account for dilution, and failing to subtract a blank. For guidance on spectrophotometer calibration and microplate assay path-length correction, see the Worthington Enzyme Manual and LibreTexts Biochemistry.
Always run a no-enzyme blank to subtract any non-enzymatic background reaction, especially for ABTS and NBT-based assays which can oxidise spontaneously.
Enzyme units vs. katals (SI unit)
The International System of Units (SI) recommends the katal (kat) for catalytic activity: 1 kat = 1 mol s⁻¹. However, the older enzyme unit (U = 1 µmol min⁻¹) remains the dominant unit in biochemical literature and commercial enzyme specifications. The conversion is: 1 U = 16.67 nkat; 1 µkat = 60 U; 1 kat = 6 × 10⁷ U. Many journals now require authors to report both units. The katal is used in clinical chemistry, particularly for serum enzyme assays such as creatine kinase and alanine aminotransferase.
1 U (enzyme unit) = 1 µmol substrate min⁻¹.
1 katal (kat) = 1 mol substrate s⁻¹.
Conversion: 1 U = 16.67 nkat; 1 nkat = 0.06 U.
Clinical biochemistry increasingly uses µkat/L for serum enzyme activity.
Quick Reference Card
Enzyme Activity — Quick Reference
Quick reference • Enzyme Activity Calculator
Activity (U/mL) = (ΔA/min × V_total × DF) / (ε × l × V_sample)Valid range: Typically 0.01–10 U/mL for most laboratory enzyme assays; specific activity ranges from <1 U/mg (crude extract) to >100 U/mg (pure enzyme)
Common Values
⚠ Watch Out
- •Never use the non-linear burst or plateau phase — only the linear steady-state region for ΔA/min.
- •Subtract the no-enzyme blank absorbance rate before entering ΔA/min.
- •Check that extinction coefficient ε matches the wavelength AND the species being measured (product or substrate).
- •Microplate readers have shorter path lengths than standard cuvettes — correct for actual path length or use a path-length correction algorithm.
Pro Tips
- →Dilute the sample if ΔA/min > 0.1 AU/min to stay in the linear detector range; apply the dilution factor in the calculator.
- →Measure protein concentration (Bradford or BCA assay) from the same sample to enable specific activity calculation.
- →Run triplicate assays and report the mean ± SD; coefficient of variation should be < 5% for a well-optimised assay.
- →For coupled assays (e.g., hexokinase/G6PDH), verify that the coupling enzyme is not rate-limiting by checking excess.
FAQs
What is an enzyme unit (U)?
One enzyme unit (U) is defined as the amount of enzyme that catalyses the conversion of 1 micromole (µmol) of substrate per minute under specified conditions of temperature, pH, and substrate concentration. This definition was established by the International Union of Biochemistry (IUB) in 1961 and remains the most widely used unit in biochemistry.
Why do we divide by the molar extinction coefficient in the formula?
The extinction coefficient ε links absorbance to concentration via Beer-Lambert law (A = ε × c × l). Dividing the measured absorbance rate (ΔA/min) by ε converts it from absorbance units to a change in molar concentration per minute (µmol/mL/min = mM/min). Multiplying by the reaction volume then gives the total rate in µmol/min, which is the activity in enzyme units.
What is specific activity and why does it matter?
Specific activity is enzyme activity (U/mL) divided by the total protein concentration (mg/mL), giving units of U/mg. It measures what fraction of the protein is the enzyme of interest. Specific activity increases at each purification step as inactive protein is removed; a homogeneous, pure preparation has the highest specific activity. It is the standard way to compare enzyme quality between preparations and laboratories.
How do I measure ΔA/min correctly?
Record absorbance readings at regular intervals (every 30–60 seconds) immediately after adding the enzyme to start the reaction. Plot absorbance vs. time and identify the straight, linear portion — this is the steady-state phase where substrate is not yet limiting. Calculate the slope of this linear section (change in absorbance ÷ time in minutes). Avoid the initial non-linear burst or the later plateau where the reaction slows.
When should I use a dilution factor?
If the enzyme sample is too concentrated — producing a slope outside the linear range (ΔA/min > 0.1–0.15 AU/min for most photometers) or exhausting substrate too quickly — you should dilute it before the assay. Enter the dilution factor in the calculator so that the result is expressed per mL of the original, undiluted sample. For example, if you added 10 µL of a 1:10 dilution to the reaction, the dilution factor is 10.
What is the difference between the katal and the enzyme unit?
The enzyme unit (U) is the traditional biochemical unit: 1 U = 1 µmol substrate converted per minute. The katal (kat) is the SI unit: 1 kat = 1 mole converted per second. They are related by: 1 U = 16.67 nanokatal (nkat). Enzyme units remain more common in biochemistry literature and commercial data sheets, while katals are required by some clinical and SI-compliant journals.