Understand the relationship
The reasoning behind the result
An activity unit belongs to stated assay conditions
An enzyme unit U represents conversion of one micromole per minute under the specified assay conditions. It is an activity quantity, not a mass or a molecule count. A product-specific unit defined by a supplier may use another assay or endpoint and cannot automatically be treated as this conventional U.
A concentration rate must first be multiplied by reaction volume in litres. A reading of 100 µmol/L/min in a 2 mL assay is 0.2 µmol/min, or 0.2 U in the added enzyme aliquot. Reaction volume and enzyme aliquot volume serve different roles and must not be interchanged.
Undo dilution before expanding to the preparation
cactivity,original=(Uassay/Valiquot)×DF; Utotal=cactivity,original×Vpreparation
Dividing assay activity by the volume of diluted enzyme preparation added gives its U/mL. Multiplying by the original-to-diluted concentration factor returns U/mL of the original preparation. Its whole volume then determines total activity.
This scaling assumes the sampled material represents that preparation and that the assay response scales with the enzyme aliquot over the applicable range. Assay inhibition, matrix differences or nonlinear enzyme dependence can break that assumption.
Specific activity, yield and enrichment answer different questions
Specific activity=Utotal/mprotein; activity recovery=100Ustage/Ubaseline; enrichment=(U/mg)stage/(U/mg)baseline
Total activity counts the activity represented by the whole preparation. Specific activity divides that activity by all measured protein in the same preparation. A stage can lose total activity while increasing activity per milligram; the output therefore reports recovery and enrichment separately.
Recovery above 100% remains visible and does not prove that new enzyme was created; changing inhibitors, activation, assay conditions or measurement error can affect the comparison. Specific activity is not a purity percentage unless an independently justified pure-enzyme reference and comparable conditions are established elsewhere. A zero baseline makes ratio comparisons undefined.
Follow the numbers
Activity recovery and enrichment can move in opposite directions
- A crude assay gives 0.1 U from a 0.1 mL aliquot diluted tenfold. Original activity is 0.1/0.1×10=10 U/mL. Across 10 mL, total activity is 100 U; with 100 mg protein, specific activity is 1 U/mg.
- A purified assay gives 0.4 U from 0.1 mL diluted twofold, or 8 U/mL. Across 5 mL this is 40 U; with 10 mg protein, specific activity is 4 U/mg.
- Activity recovery is 40/100×100=40%, while specific-activity enrichment is 4/1=4-fold.
The stage retained 40% of recorded activity while increasing activity per milligram fourfold; neither figure alone states purity.
Quick guide
How to use this calculator
- Choose whether the measured rate is already an amount rate or a concentration rate.
- For concentration rates, enter assay reaction volume; it is different from the enzyme aliquot added to that reaction.
- Enter original preparation volume and its total protein mass for each record.
- Inspect the dilution/volume ledger, total activity and specific activity. Compare later rows with the first only when assay conditions and accounting are comparable.
Calculation method
Calculation and interpretation
Trace assay activity back through dilution and aliquot size before comparing whole preparations or purification stages.
1 U=1 µmol/min under the assay conditions; original U/mL=assay U×dilution/aliquot mL; total U=(U/mL)×preparation mL; specific activity=total U/total protein mg.
Worked example
Activity recovery and enrichment can move in opposite directions
The stage retained 40% of recorded activity while increasing activity per milligram fourfold; neither figure alone states purity.
1 U=1 µmol/min under the assay conditions; original U/mL=assay U×dilution/aliquot mL; total U=(U/mL)×preparation mL; specific activity=total U/total protein mg.
Supported inputs
Precision and limits
An experimental model, not a biological conclusion
These calculations do not establish enzyme identity, purity, active fraction, assay validity or inhibition mechanism. Temperature, pH, substrate, cofactors and preparation must match the experiment behind the entered values.
Keep measurements and mechanisms separate
A time-window slope is not automatically an initial velocity; Km is not generally an equilibrium dissociation constant. No clinical interpretation, dose or treatment decision follows from these outputs.
Activity scaling assumptions
Use nonnegative measured conversion rates in the conventional µmol/min definition. Signed trend analysis belongs in the time-window tool. No assay-specific supplier unit, active-site abundance or enzyme purity percentage is inferred.
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