The enzyme unit is a unit for the amount of a particular enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

.

One U is defined as the amount of the enzyme that catalyzes

Catalysis

Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....

 the conversion of 1 micro

Micro

Micro is a prefix in the metric system denoting a factor of 10-6 . Confirmed in 1960, the prefix comes from the Greek , meaning "small".The symbol for the prefix is the Greek letter μ...

 mole

Mole (unit)

The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...

 of substrate

Substrate (biochemistry)

In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...

 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

International Union of Biochemistry and Molecular Biology

The International Union of Biochemistry and Molecular Biology is an international non-governmental organisation concerned with biochemistry and molecular biology...

 in 1964. Since the minute is not an SI

Si

Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

 unit, the enzyme unit is discouraged in favour of the katal

Katal

The katal is the SI unit of catalytic activity. It is a derived SI unit for expressing quantity values of catalytic activity of enzymes and other catalysts. Its use is recommended by the General Conference on Weights and Measures and other international organizations. It replaces the non-SI enzyme...

, the unit recommended by the General Conference on Weights and Measures

General Conference on Weights and Measures

The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures . It is one of the three organizations established to maintain the International System of Units under the terms of the Convention du Mètre of 1875...

 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

International unit

In pharmacology, the International Unit is a unit of measurement for the amount of a substance, based on biological activity or effect. It is abbreviated as IU, as UI , or as IE...

(IU), an unrelated measure of biologically active substances.