The unit of enzyme activity
proposed by the International Union of Biochemistry in 1964. Specifically, it is the amount of enzyme that catalyses the conversion of one micromole of substrate per minute under the specified conditions of the assay
This unit is no longer recommended because the term does not indicate what physical quantity it refers to, and because the minute is not the SI unit of time.
PAC, 1994, 66, 2587
(Glossary of bioanalytical nomenclature - Part 1: General terminology, body fluids, enzymology, immunology (IUPAC Recommendations 1994))
on page 2593
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook