![]() Serial dilutions of an enzyme solution will have different enzyme activity values, but identical specific activity values because in calculating specific activity the numerator (units/ml) and denominator (mg/ml) are affected equally by sample dilution.Īlthough specific activity is very different from activity, the calculation of specific activity nonetheless is dependent on the activity value, and therefore the stated specific activity value will also be dependent on the enzyme unit definition. Specific activity is an important measure of enzyme purity and values for different batches of a pure enzyme should be the same, within normal experimental error. ![]() Specific activity values are therefore quoted as units/mg or nmol/min/mg (if unit definition B is applied). Specific enzyme activity (usually stated simply as ‘specific activity’) is the number of enzyme units per ml divided by the concentration of protein in mg/ml. ![]() Since activity relates to concentration it follows that two vials of enzyme can contain the same number of units (in total) but have different activities (concentrations). Again there cannot be any confusion if activity is expressed in terms of nmol per min per ml rather than units per ml. Thus activity values expressed in units are also subject to an illusory 1000-fold ‘increase’ if one switches from unit definition A to unit definition B. What Is ‘Enzyme Activity’Īctivity is quoted in units per ml (U/ml), in other words nmol per min per ml (if unit definition B has been adopted). As long as the unit definitions are provided you can transform the stated number of units into nmol per min, which is unambiguous and allows valid comparisons to be made.įor clarity in your own work you may prefer to use ‘nmol per min’ (or ‘umol per min’), though if constant repetition is required then clearly the much shorter term ‘unit’ has its attractions. You may also see enzyme units expressed as milli-unit (or mU) which simply means a thousandth of a unit, irrespective of how the unit has been defined.Ĭlearly the actual amount of an enzyme in a tube is not altered simply by changing the unit definition, but care is required when comparing the activities of samples from different suppliers. 1 unit of enzyme according to definition A would equate to 1000 units according to definition B! Note that the change in definition has a profound effect on the stated number of units i.e. The following non-standard definition is commonly used:ġ unit (U) is the amount of enzyme that catalyses the reaction of 1 nmol of substrate per minute (definition B). In most R&D settings, 1 umol of substrate is actually quite a lot of material and other definitions may be preferred to avoid expressing quantities in fractions of units. A standard unit definition is given below:ġ unit (U) is the amount of enzyme that catalyses the reaction of 1 umol of substrate per minute (definition A). Definitions of Enzyme UnitsĮnzymology would be less complicated if everyone used the same unit definition. Finally, we explain how enzyme activity values are calculated and we give a simple overview of kinetic equations. Some tips on setting up assay controls and subtracting blanks are discussed. We also consider standard curves and whether you should plot concentration or absolute amount of product on the x axis. This guide explains these key concepts in simple terms and discusses enzyme assay design and the importance of operating in the ‘linear range’. There is often much confusion over the meaning of ‘enzyme units’, ‘enzyme activity’ and ‘specific enzyme activity’.
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