= 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 (biochemistry)> selectively retains the target through a reversible interaction.<Agarose Derivatives for Purification of Protein by Affinity Chromatography by Pedro Cuatrecasas>[opening text, selective interaction with an immobilized ligand]{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 (enzyme)> 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.
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