One U is defined as the amount of the enzyme that catalyzes the conversion of 1 micro mole of substrate per minute. The conditions also have to be specified: one usually takes a temperature of 25°C and the pH value and substrate concentration that yield the maximal substrate conversion rate.
The enzyme unit was adopted by the International Union of Biochemistry in 1964. Since the minute is not an SI unit, the enzyme unit is discouraged in favour of the katal, the unit recommended by the General Conference on Weights and Measures in 1978 and officially adopted in 1999. One katal is the amount of enzyme that converts 1 mole of substrate per second, so
- 1 U = 1/60 micro katal = 16.67 nano katal.
The enzyme unit should not be confused with the International Unit (IU), an unrelated measure of biologically active substances.