In chemistry and biochemistry in particular, specific activity is the amount of product formed by an enzyme in a given amount of time under given conditions per milligram of enzyme. Specific activity is equal to the rate of reaction multiplied by the volume of reaction divided by the mass of enzyme.
The rate of a reaction is the concentration of substrate disappearing (or product produced) per unit time (mol )
The enzyme activity is the moles converted per unit time (rate × reaction volume). Enzyme activity is a measure of quantity of enzyme present. The SI unit is the katal, 1 katal = 1 mol s-1, but this is an excessively large unit. A more practical value is 1 enzyme unit (EU) = 1 μmol min-1 (μ = micro, x 10-6).
The specific activity is the moles converted per unit time per unit mass of enzyme (enzyme activity / total mass of protein). The SI unit is katal kg-1, but a more practical unit is μmol mg-1 min-1. Specific activity is a measure of enzyme processivity, usually constant for a pure enzyme.
If the specific activity of 100% pure enzyme is known, then an impure sample will have a lower specific activity, allowing purity to be calculated.
The % purity is 100% × (specific activity of enzyme sample / specific activity of pure enzyme). The impure sample has lower specific activity because some of the mass is not actually enzyme.