Protein purification

Protein purification separates a selected protein from other components of a sample while recovering material useful for a subsequent measurement or application. Different procedures exploit different properties. In affinity chromatography, a matrix-bound ligand selectively retains the target through a reversible interaction.^[ref]

An enzyme example separates recovery from enrichment. Suppose a starting fraction contains $10\mathrm{mg}$ total protein and $200$ measured activity units, while a collected fraction contains $2\mathrm{mg}$ and $120$ units. The specific activity rises from $200/10=20$ to $120/2=60$ units per milligram: a threefold increase. Recovery of activity is $120/200=0.60$, or $60\%$. These are constructed assay values, not reported experimental data.

The preparation can therefore be enriched even while some target activity is lost. That inference assumes the assay reports the same catalytic property under comparable conditions; activation or inhibition can change activity without an equivalent change in protein amount. Protein purification consequently needs a stated target, an identity or activity measurement, and a recovery criterion. A visually clean fraction alone does not establish that its protein remains functional.

Protein mass purity is the fraction of the total protein mass in a preparation contributed by the intended protein. With target mass $m_T$ and total protein mass $m_P$, the percentage is $100m_T/m_P$. The denominator includes other proteins, rather than the solvent or the entire mass of the solution.^[ref]

For a constructed preparation containing $0.84\mathrm{mg}$ target protein and $0.16\mathrm{mg}$ other proteins, mass purity is $84\%$. This does not say that $84\%$ of the target molecules are active, nor that $84\%$ 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 $8/(8+2)=80\%$. If, instead, the target signal per mass is twice the contaminant's, the inferred masses are proportional to 4 and 2, giving $4/6\approx 67\%$. 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.