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 .
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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))