quantitative systems description parameter enzyme activity A measure of the amount of active enzyme present, expressed under specified conditions. This is often expressed as micromol per min (also known as enzyme unit, U), rather than the less practical official SI unit, Katal (1 mol per second). Enzyme activity normally refers to the natural substrate for the enzyme, but can also be given for standardised substrates such as gelatin, where it is then referred to as GDU (Gelatin Digesting Units).