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2. Specific Activity

Enzyme Activity 2 — Specific Activity

 
To obtain a relative measure of the purity of an enzyme preparation that one has assayed (as in section 1), one calculates its specific activity — the number of units per milligram of protein.

Specific Activity:  units/mg   
 
You should aim to do these calculations in your head or on a scrap of paper (i.e. without a calculator). The following logic may be helpful:
  • As specific activity is units of enzyme per mg protein, first calculate the mg protein in the sample.
  • The sample volumes are in µl, whereas the protein concentration is in mg/ml, so you need to divide the latter by 1000 to get mg/µl.
  • Then multiply by the volume in µl to get the number of mg protein in the sample.
  • Finally divide the number of units of enzyme by this number of mg protein to get the specific activity of the sample.
  • In summary, specific activity = enzyme units / (vol. in µl x (protein conc. in mg per ml / 1000))