The radiometric assay of microsomal UDP-glucuronyltransferase using 125I-labelled phenolphthalein

A rapid and sensitive method is described for assaying UDP-glucuronyl-transferase (EC 2.4.1.17) activity. The substrate is the relatively apolar phenolphthalein, labelled with 125I on one of its two phenolic rings leaving a free hydroxyl group for glucuronidation. Extraction with ethyl acetate leaves the glucuronide in the aqueous reaction medium which is then counted. Less than 10 mug microsomal protein is required and the glucuronidation of 10 ng 125I-labelled phenolphthalein (0.03 nmoles) is easily detected.