Enzymatic purity

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.