(IU) a unit of enzyme activity equal to the amount of enzyme that catalyzes the conversion of one micromole of substrate or coenzyme per minute under specified conditions (temperature, pH, and substrate concentration) of the assay method. Abbreviated U. any of several arbitrary units that have been adopted by international bodies to express the quantities of certain vitamins (A, C, D, and thiamine hydrochloride), hormones (androgen, chorionic gonadotropin, estradiol benzoate, estrone, insulin, progesterone, and prolactin), and drugs (digitalis and penicillin).