Expressed human UGT1.4 protein catalyzes the formation of quaternary ammonium-linked glucuronides

In humans, the metabolism of a number of tertiary amine-containing pharmacological agents to quaternary ammonium-linked glucuronides, catalyzed by UDP-glucuronosyltransferase (UGT), represents a unique and important metabolic pathway for these compounds. A full-length cDNA-encoding human UGT1.4 (the so-called "minor" human bilirubin UGT) was inserted into the expression vector pREP9 and transfected into human embryonic kidney 293 cells, and stable transfectants were obtained after geneticin selection. As expected, the expressed protein had low catalytic activity toward bilirubin. However, expressed human UGT1.4 protein exhibited glucuronidation activity toward tertiary amine substrates, such as imipramine, cyproheptadine, tripelennamine, and chlorpromazine, which form quaternary ammonium-linked glucuronides. Carcinogenic primary amines (beta-naphthylamine, benzidine, and 4-aminobiphenyl) also reacted with the expressed UGT1.4 protein at rates approximately 10-fold higher than the rates for quaternary ammonium glucuronide formation. Although a number of other UGT gene products are capable of catalyzing the glucuronidation of primary amine substrates, expressed human UGT1.4 protein is the only UGT isoform that has been shown to conjugate tertiary amine substrates, forming quaternary ammonium-linked glucuronides.