How this instrument works
Enzyme activity is reported in units (U), where one unit is internationally defined as the amount of enzyme that catalyzes the conversion of one micromole of substrate per minute under specified assay conditions. It's a measure of catalytic power, not mass or concentration directly — two enzyme preparations can weigh the same but have very different activity if one is purer or more catalytically efficient than the other, which is precisely why activity, not mass, is the number that actually matters for a reaction to proceed at a known rate.
A commercial or lab-prepared stock enzyme comes with a known specific activity — typically expressed as units per milligram of protein — printed on its label or determined during purification. Preparing a working solution with a specific target activity, in units per milliliter, in a specific final volume is a straightforward proportion once that stock activity is known: the mass of stock enzyme needed equals the desired activity times the final volume, divided by the stock's activity per unit mass.
This calculation shows up constantly at the bench whenever an assay protocol specifies a required activity level rather than a fixed mass — enzyme kinetics experiments, diagnostic assays, and industrial enzyme applications all typically call for a target activity in the reaction mixture, and it's the researcher's job to translate that target back into a weighable mass of whatever stock preparation happens to be on the shelf that day, at whatever specific activity that particular batch carries.
- Enter the desired enzyme activity, in units per mL, for your final working solution.
- Enter the desired final volume, in mL.
- Enter the stock enzyme's known activity, in units per mg.
- Read the mass of stock enzyme, in mg, needed to prepare that final volume at that target activity.
Worked example — preparing a working enzyme solution from stock
A protocol calls for 20 mL of a working enzyme solution at an activity of 10 units/mL, and the stock enzyme preparation on hand is rated at 5 units/mg. Required mass = (10 units/mL × 20 mL) ⁄ 5 units/mg = 200 units ⁄ 5 units/mg = 40 mg. Weighing out 40 mg of that stock preparation and bringing it to 20 mL with the appropriate buffer delivers exactly the target activity the protocol specifies.
Notice how the units cancel cleanly through the calculation — units/mL times mL gives total units needed, and dividing total units by the stock's units-per-mg gives milligrams: exactly the mass you'd weigh out on a balance. Tracking the units through each step like this is the fastest way to catch an error, such as accidentally entering a stock activity in units/mL instead of units/mg.
Questions
What is a unit (U) of enzyme activity?
One international unit (U) of enzyme activity is defined as the amount of enzyme that catalyzes the conversion of one micromole (μmol) of substrate per minute, under a specified set of assay conditions (temperature, pH, and substrate concentration). It's a measure of catalytic rate, not a measure of mass or protein concentration — the same mass of two different enzyme preparations can carry very different total unit counts depending on purity and catalytic efficiency.
What's the difference between enzyme activity and specific activity?
Activity (in units, or units/mL for a solution) describes the total catalytic capability present; specific activity (units per milligram of protein) normalizes that activity to the amount of protein present, and is a standard measure of an enzyme preparation's purity. A highly purified enzyme preparation has a higher specific activity than a crude extract containing the same total units, simply because less of its mass is non-catalytic contaminant protein.
Why can't I just weigh out a fixed mass of enzyme without calculating activity?
Because a fixed mass of enzyme doesn't guarantee a fixed catalytic rate — different batches, suppliers, or purification runs of the 'same' enzyme can carry meaningfully different specific activities. Basing a protocol on activity (units) rather than raw mass ensures the reaction proceeds at the intended rate regardless of which particular stock preparation, with its own specific activity, happens to be used.
What is a katal, and how does it relate to the unit (U)?
The katal (kat) is the SI-recognized unit of catalytic activity, defined as the amount of enzyme that converts one mole of substrate per second — a considerably larger unit than the traditional enzyme unit, since most enzyme assays are measured over minutes rather than seconds. One unit (U) equals about 16.67 nanokatals. Despite the katal being the formally recommended SI unit, the traditional unit (U) remains far more common in practice throughout biochemistry and clinical laboratory work.
Does temperature or pH affect the activity value used in this calculation?
Yes, indirectly — the stock activity you enter is only valid under the specific assay conditions (temperature, pH, substrate) it was measured under, since enzyme activity is highly sensitive to both. This calculator performs the dilution arithmetic correctly regardless of what those conditions are, but it's on you to make sure the stock activity figure you're using was determined, and will be used, under matching or comparable conditions — an activity measured at 37°C doesn't necessarily apply at room temperature.