The amount of an enzyme in a given solution or tissue extract can be assayed quantitatively in terms of the catalytic effect it produces.

It is necessary to know the overall equation of the reaction catalyzed, an analytical procedure for determining the disappearance of the substrate or the appearance of the reaction products., whether the enzyme requires cofactors such as metal ions or coenzymes, the dependence of the enzyme activity on substrate concentration, i.e., the Km value, the optimum pH, and a temperature zone in which the enzyme is stable and has high activity.

Enzymes are assayed at their optimum pH, at optimum temperature, usually within the range 25 to 38ºC, and with a near-saturating concentration of substrate. Under these conditions the initial reaction rate is usually proportional to enzyme concentration, at least over a given range of enzyme concentration.