One U is defined as the amount of the enzyme that produces a certain amount of enzymatic activity, that is, the amount 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 = (1/000000)/60 × 1 kat = 1/60 µkat, and thus 1
- 1 U = 16.67 nkat:82
The enzyme unit should not be confused with the international unit (IU); although it is true that both measure chemical activity and that for many enzymes 1 U = 1 IU:30 (because for many enzymes the existing U was adopted as the newer IU), international units can be defined for the biologic activity of many kinds of substance besides enzymes (for example, vitamins and hormones).
- Nomenclature Committee of the International Union of Biochemistry (NC-IUB) (1979). "Units of Enzyme Activity". Eur. J. Biochem. 97 (2): 319–20. doi:10.1111/j.1432-1033.1979.tb13116.x.
- Principles of Biochemistry, page 94, 4th Edition, Lehninger
- Wharton, Christopher W.; Eisenthal, Robert (2013), Molecular Enzymology, Tertiary Level Biology, Springer Science and Business Media, ISBN 9781461585329.
- Bommarius, Andreas S.; Riebel-Bommarius, Bettina R. (2007), Biocatalysis: Fundamentals and Applications, John Wiley and Sons, ISBN 9783527606054.
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