In SI, a derived unit of enzyme activity. One katal is that catalytic activity which will raise the rate of reaction by one mole per second in a specified assay system.¹ Symbol, “kat”. The katal was adopted by Resolution 12 of the 21st CGPM in October 1999,² on the recommendation of the International Federation of Clinical Chemistry and Laboratory Medicine. In its resolution, the CGPM recommended that “when the katal is used, the measurand be specified by reference to the measurement procedure; the measurement procedure must identify the indicator reaction.”

The katal is not used to express a rate of reaction itself, which should be expressed in moles per second.

Prior to the katal's adoption by the CGPM, researchers had been using a similar unit for approximately 30 years. The International Union of Biochemistry adopted a unit in 1964 which was generally nameless (though sometimes called an “international unit”) but had the symbol “U.” That earlier enzyme unit used minutes rather than seconds as its unit of time, which was not in keeping with SI (minutes are not an SI unit) or with the usual way of expressing rate constants in chemical kinetics.

In 1966, the International Union of Pure and Applied Chemistry and the International Federation of Clinical Chemistry recommended adoption of a unit to be called the catal (symbol, cat), such that 1 catal was equal to the catalytic amount of a system which catalyses as many cycles per second of a stated reaction scheme as there are atoms in 0.012 kilograms of the pure nuclide ¹²C.³

When the katal was first proposed,⁴ it was conceived as a unit of rate of reaction, but the 1978 Recommendation distinguishes between catalytic activity and rate of reaction.

The katal is too large for most purposes and it is usually encountered as the microkatal, nanokatal or picokatal.

The CGPM is exceedingly reluctant to add new names for derived units to the SI, since there is almost no end to the number of derived units that might be named. In the case of the katal, the argument that human safety required it carried the day:

The case for its introduction was based on the need to introduce it from the viewpoint of human safety; for nurses particularly have been finding it difficult to compound and convey quantities involving combinations of three or more SI units. On this basis the CCU forwarded a recommendation for its adoption to the CIPM.⁵

1. International Union of Biochemistry, Nomenclature Committee.
Units of enzyme activity: Recommendations 1978.
European Journal of Biochemistry, volume 97, pages 319-320. (1979)

2. For the text of the resolution, download the pdf file www.bipm.org/utils/en/pdf/CGPM/CGPM21-EN.pdf and go to page 334.

3. R. Dybkær and K. Jørgensen.
Quantities and Units in Clinical Chemistry, IUPAC Recommendation 1966.
Copenhagen: Munksgaard, 1967.

4. IUPAC-IUB Commission on Biochemical Nomenclature.
Enzyme Nomenclature, Recommendations 1972.
Amsterdam: Elsevier, 1973.

5. IU14. IUPAC Interdivisional Committee on Nomenclature and Symbols (IDCNS).
Report to the 1999 IUPAP General Assembly,
Report on recent Committee activities on behalf of IUPAP by Brian W Petley September 1998.


R. Dybkær and K. Jørgensen.
Quantities and Units in Clinical Chemistry.
Copenhagen: Munksgaard, 1969.

Page 28. Another edition: Baltimore: Williams and Wilkins.

R. Dybkær.
The special name ”katal“ for the SI derived unit, mole per second, when expressing catalytic activity.
Metrologia, volume 37, number 6, pages 671-676 (2000).

R. Dybkaer.
Unit “katal” for catalytic activity. (IUPAC technical report)
Pure and Applied Chemistry, vol. 73, no. 6, pages 927-931 (2001)

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