Enzyme unit

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The enzyme unit  ( U ) is a unit of enzyme activity that has been replaced by the katal (in Germany, the katal has been the legal unit in metrology since October 3, 2009). Since the numerical values ​​change when using the katal, the enzyme unit is still used in medicine and clinical chemistry .

One unit of enzyme corresponds to a micro- mol substrate turnover per minute under defined conditions:

The conditions (temperature, substrate concentration, pH value, etc.) should be chosen so that a maximum reaction rate is achieved. This makes corresponding additional information necessary and makes it difficult to compare information. A uniform temperature of at least 30 ° C is therefore often used as a basis.

Since catalytic activity and enzyme amount are directly related to the turnover number of the enzyme, information in U is often used as a measure of the enzyme amount. For example, commercially available enzymes for molecular genetics are often sold staggered according to enzyme units. For restriction endonucleases , however, these are usually defined by the complete digestion of 1 μg of genomic DNA of the λ phage in one hour under certain reaction conditions; if λ-DNA is not cut, the definition is based on another DNA, e.g. B. the adenovirus 2 genome.


  1. Third ordinance amending the Units Ordinance of September 25, 2009 ( Federal Law Gazette I p. 3169 ).
  2. International Union of Biochemistry: Report on the Commission on Enzymes of the International Union of Biochemistry , Pergamon Press, New York, 1961, p. 8.