During protein purification, what term denotes enzyme activity per unit mass of total protein, used to quantify purification progress?

Introduction / Context:
Protein purification workflows track both yield and purity. A key progress indicator is activity normalized to total protein, which allows comparison across fractions independent of volume or concentration differences.

Given Data / Assumptions:

• Assaying a catalytic protein across purification steps.
• Measuring total activity (units) and total protein (mg).

Concept / Approach:
Specific activity = activity units per milligram of total protein (U/mg). As purification proceeds, total protein usually decreases while the fraction that is the target enzyme rises, so specific activity should increase if the procedure is effective.

Step-by-Step Solution:

Verification / Alternative check:
Fold-purification can be calculated by dividing the specific activity at a given step by that of the crude extract; trends should align with SDS-PAGE band enrichment.

Why Other Options Are Wrong:

• Relative activity: ambiguous without normalization to protein mass.
• Purity ratio: non-standard term in enzymology.
• All of these: only specific activity is the accepted definition.
• Turnover frequency per cell: not a purification metric.

Common Pitfalls:
Confusing yield (total units) with purity (specific activity), or comparing activities without protein normalization.

Final Answer:
Specific activity.