Enzyme assay

An enzyme assay measures the catalytic activity of an enzyme by following consumption of a substrate or appearance of a reaction product under specified conditions. The signal may be chemical concentration directly or a calibrated optical readout, such as absorbance or fluorescence.

For example, if product concentration increases from 0 to 6 micromolar in the first two minutes, a linear initial interval gives a rate of 3 micromolar per minute. A no-enzyme control tests whether the signal also changes without the catalyst. Comparing rates requires the same temperature, pH, substrate concentration and detection conditions; raw brightness alone is not an activity unit.

The initial interval matters because later measurements can be affected by substrate depletion, product accumulation or loss of enzyme activity. Assay-development guidance therefore recommends establishing the linear initial-rate region before comparing inhibitors or enzyme preparations.^[ref]

Enzymatic purity is the fraction of the activity observed in a particular enzyme assay attributable to the intended enzyme. It concerns the origin of the measured reaction signal. A preparation can have low protein mass purity but high enzymatic purity if the other proteins contribute no activity in that assay.^[ref]

A constructed counterexample shows why mass purity is insufficient. Suppose a mixture contains $99\mu \mathrm{g}$ target and $1\mu \mathrm{g}$ contaminating enzyme. In the chosen assay, let their activities be 1 and 100 units per microgram respectively. The two contributions are 99 and 100 units. Target mass purity is $99\%$, but its fraction of the signal is $99/199\approx 49.7\%$.

Separating those contributions needs experimental evidence. Comparing an active target with an otherwise matched inactive-target preparation can reveal activity surviving without target catalysis. Substrate choices, inhibitor responses and independently purified preparations provide complementary tests.^[ref]

The conclusion remains assay-specific: a contaminant may be silent for one substrate yet active for another. Specific activity reports total activity per protein mass; it does not by itself identify which enzyme produced that activity.

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]