Enzyme Units and Concentrations

Enzyme concentrations may be represented in an identical manner as with nonca-talytic species, such as is in molar units or as mass per unit volume. The information required for this calculation is minimal the percentage purity of the enzyme preparation, the molecular weight of the enzyme, and the mass and volume of the solution prepared. Enzyme concentrations are rarely represented in this way, however, because such values say nothing about the catalytic power of the solution prepared. 

The concentration of an enzyme solution is most commonly given as the number of International Units (I.U.) per unit volume. The I.U. is defined as that quantity of enzyme required to consume one micromole of substrate per minute at a given temperature and pH, under conditions of substrate saturation  

Since 1 I.U. is usually a very large quantity of enzyme, concentrations are often given as milliunits (mU) or microunits (pU) per liter (L) or milliliter (mL). 

To convert these units to molar concentrations, it is necessary to know the turnover number, fccat, of the particular enzyme used. For a simple one-substrate 

Umax is obtained from the I.U. of enzyme activity present in a given volume (Vj) of enzyme solution  

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