When an enzyme has been purified, it is possible to express the amount of enzyme in tissues or plasma as the number of moles of enzyme protein present. However, what is more important is not how much of the enzyme protein is present in the cell, but how much catalytic activity there is — how much substrate can be converted to product in a given time. Therefore, amounts of enzymes are usually expressed in units of activity.
The SI unit of catalysis is the katal = 1 mole of substrate converted per second. However, enzyme activity is usually expressed as the number of micromoles (|mol) of substrate converted (or of product formed) per minute. This is the standard unit of enzyme activity, determined under specified optimum conditions for that enzyme, at 30 °C. This temperature is a compromise between mammalian biochemists, who would work at body temperature (37 °C for human beings) and microbiological biochemists, who would normally work at 20 °C.
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