A 0.5 mg sample of purified enzyme preparation hydrolyses 20 micromoles of substrate in 10 minutes under standard assay conditions. The specific activity of the preparation is:

• A 2 U/mg
• B 4 U/mg ✓
• C 20 U/mg
• D 40 U/mg

Explanation

One international unit (U) is the amount of enzyme converting 1 micromole of substrate per minute under defined conditions. This sample converts 20 micromoles in 10 minutes, that is 2 U total. Specific activity is units per milligram of protein: 2 U divided by 0.5 mg equals 4 U/mg. Option A results from forgetting to divide by the protein mass, and option D doubles the answer by mishandling the time factor.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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