 Specific activity (enzyme)

Specific activity is measured enzymatic activity divided by the total protein mass in the preparation. It describes activity per mass, commonly reported in assay units per milligram, and is distinct from total activity or the fraction of the preparation consisting of the intended enzyme.^[ref]

For a constructed protein purification example, a fraction with $240$ activity units and $12\mathrm{mg}$ total protein has specific activity $20\mathrm{U}\mathrm{m}{\mathrm{g}}^{-1}$. A later fraction with $150$ units and $3\mathrm{mg}$ gives $50\mathrm{U}\mathrm{m}{\mathrm{g}}^{-1}$: enrichment is $50/20=2.5$-fold, while activity recovery is $150/240=62.5\%$. The denominator includes other proteins, not just the target.

The comparison requires compatible enzyme assay conditions. A higher value can reflect removal of inactive material, but can also follow activation or removal of an inhibitor. Moreover, a contaminating enzyme may perform the measured reaction. High specific activity therefore supports an assessment of a preparation without independently establishing its molecular identity or proving that all measured activity comes from the desired catalyst.^[ref]