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The purification factor (also called enrichment) is a comparison of the specific activity at some point in the purification process to the specific activity at the beginning of the purification. The formula for purification factor is as follows:
For example, the protein concentration for the crude extract may be 1000 mg/mL and the enzyme activity might be 30,000 units/mL, leading to a specific activity of 30 units/mg. After the second step, the protein concentration may be 100 mg/mL and the enzyme activity could be 25,000 units/mL. So the specific activity would have improved from 30 units/mg to 250 units/mg. Although you may have lost some, you removed a lot of contaminating proteins so the specific activity improves from 30 units/mg to 250 units/mg. To express this as a purification factor: 250 units/mg = 8.33 So, the purity has improved by a factor of 8.33. Or, to say it a different way, the protein is 8.33 times more pure than it was when you started.
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