Glucuronide conjugation of tertiary amine xenobiotics represents a unique and important metabolic pathway for these compounds in humans. In this study, we show that human UDP-glucuronosyltransferase 1.4 protein, stably expressed in human embryonic kidney 293 cells, catalyzes the N-glucuronidation of primary, secondary, and tertiary amine substrates. In addition, the substrate specificity of the expressed enzyme toward many hydroxylated and carboxylic acid-containing compounds was examined. Of the hydroxylated compounds tested, only sapogenins gave glucuronidation rates comparable with those observed for amine substrates. The apparent KM and Vmax values for sapogenins were such that the efficiency of glucuronidation (Vmax/KM) for these compounds was higher than that determined for amine substrates. Human UDP-glucuronosyltranferase 1.4 also catalyzes the glucuronidation of monoterpenoid alcohols and simple phenolic compounds. The enzyme kinetic values determined for these substrates suggested that this enzyme may have relatively limited significance for the conjugation of these classes of compounds. Of the endobiotics tested, androstanediol and progestins were glucuronidated at high rates by expressed human UDP-glucuronosyltransferase 1.4 protein. The glucuronidation efficiency for 5alpha-pregnane-3beta,20alpha-diol was comparable with that determined for the sapogenins. Because UDP-glucuronosyltransferases are integral membrane proteins, the effects of different detergents on the catalytic activity of the expressed enzyme were determined. The results show that detergents (such as Lubrol PX, Emulgen 911, and Triton X-100) are inhibitory for the quaternary ammonium-linked glucuronidation of chlorpromazine and imipramine catalyzed by expressed human UDP-glucuronosyltransferase 1.4. In contrast, CHAPS and nonanoyl-N-methylglucamide are less inhibitory toward the glucuronidation of these compounds. The results suggest that human UDP-glucuronosyltransferase 1.4 may be an important enzyme for the detoxication of environmentally derived amines and sapogenins and for the conjugation of progestins.