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The enzyme unit U,by its definition,is the amount of enzyme that catalyses the transformation of one micromole of substrate per minute under standard assay conditions.However,in practical use,it is identical with micromole or microgram per minute,which deviates from the essence of the definition as a measure of amount of substance.Such usage appears in many publications that involve enzyme kinetics and conveys obscure signification.In this article,we solved the confusion by regarding the unit U as a measure of amount of enzyme having a specified catalytic activity just like the basic SI unit mole (symbol mol)defined as the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kg of carbon 12.We introduced the concept of unit rate,ru,which is defined as the rate of a reaction mediated by 1 U enzyme,in calculating the enzyme units in an amount of enzyme so as to avoid the inconsistence of the expression with the definition of enzyme unit.The method can also be applied to the case of katal,the recommended SI unit of enzyme activity.