Abstract

Glucuronidation of xenobiotics and endobiotics is catalyzed by a group of intrinsic membrane proteins of the endoplasmic reticulum of cells: the UDP-glucuronosyltransferases. Two isoforms with glucuronidation activity toward opioids have been purified and characterized from liver microsomes obtained from phenobarbital-treated Wistar rats. The proteins have been identified as the gene products of UGT2B1 and UGT1.1r. The purified proteins exhibited the same apparent KM values for morphine glucuronidation (2-3 mM). However, the purified UGT1.1r enzyme exhibited glucuronidation activity toward buprenorphine and bilirubin with high efficiency, but the UGT2B1 protein did not react with these compounds. Both purified enzymes glucuronidated chloramphenicol, 4-hydroxybiphenyl, chrysin, and ibuprofen. Flunitrazepam photoaffinity labeling was demonstrated for both enzymes, and naloxone, the opioid antagonist, antagonized the photoaffinity labeling reactions.