What is the conventional unit for enzyme activity expressed as μmol per minute?

• A. IU (μmol/min)
• B. Katal (mol/s)
• C. Unit (μmol/min)
• D. Katal (μmol/s)

Answer: (A)

Enzyme activity is a rate—how much substrate is converted per unit time. In clinical chemistry, the standard way to express this rate is with the International Unit (IU), defined as the amount of enzyme that catalyzes the transformation of 1 μmol of substrate per minute under specified conditions. So expressing enzyme activity as μmol per minute aligns directly with this definition, and using IU provides a standardized way to report that rate across labs.

The SI unit for enzyme activity is the katal (mol per second), but in practice that is not the conventional unit used in clinical reports for enzyme activity, especially when the rate is given in μmol/min. Using a generic “Unit” is vague unless it’s distinctly defined, whereas IU has a precise, internationally understood meaning.