Enzymatic purity
= 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.<Validating Identity, Mass Purity and Enzymatic Purity of Enzyme Preparations by John E. Scott and Kevin P. Williams>[Definitions, Enzymatic Purity or Activity Purity]{ref}
A constructed counterexample shows why mass purity is insufficient. Suppose a mixture contains $99\,\mathrm{\mu g}$ target and $1\,\mathrm{\mu g}$ 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 $99/199\approx49.7\%$.
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.<Validating Identity, Mass Purity and Enzymatic Purity of Enzyme Preparations by John E. Scott and Kevin P. Williams>[Validating Enzymatic Purity; Enzyme Dead Mutant or Mock Parallel Purification]{ref}
The conclusion remains assay-specific: a contaminant may be silent for one substrate yet active for another. <Specific activity (enzyme)> reports total activity per protein mass; it does not by itself identify which enzyme produced that activity.