Protein mass purity is the fraction of the total protein mass in a preparation contributed by the intended protein. With target mass and total protein mass , the percentage is . The denominator includes other proteins, rather than the solvent or the entire mass of the solution.[ref]
For a constructed preparation containing target protein and other proteins, mass purity is . This does not say that of the target molecules are active, nor that of measured catalytic activity comes from them. Those are different quantities from a mass fraction.
An image-based estimate also requires a response model. Suppose densitometry gives target-band intensity 8 and contaminant intensity 2. If both stain equally per unit mass, the inferred purity is . If, instead, the target signal per mass is twice the contaminant's, the inferred masses are proportional to 4 and 2, giving . These invented response factors illustrate the assumption rather than a measured staining difference.
Thus an SDS-PAGE band pattern supports a mass-purity estimate when identification, detection and calibration are appropriate. Enzymatic purity and specific activity require additional assay information.
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