1 Unit (U) = 1 μmol substrate converted per minute
Enzyme activity quantifies the catalytic power of an enzyme preparation. One International Unit (U) is defined as the amount of enzyme that converts 1 μmol of substrate per minute under specified conditions (pH, temperature, substrate concentration). The SI unit katal (1 kat = 1 mol/s) is rarely used in practice. Specific activity (U/mg protein) measures purity — it increases during purification as contaminating proteins are removed. Total activity (U) should remain constant or decrease through purification steps; a decrease indicates enzyme loss. Enzyme activity depends on temperature (Arrhenius behavior up to denaturation), pH (bell-shaped curve around optimal pH), substrate concentration (Michaelis-Menten kinetics), and the presence of inhibitors or activators.