In vitro glucuronidation of 3'-azido-3'-deoxythymidine by human liver. Role of UDP-glucuronosyltransferase 2 form.

The glucuronidation of 3'-azido-3'-deoxythymidine (AZT) by human liver microsomes and human hepatocytes in culture has been studied in vitro to determine the UDP-glucuronosyltransferase (UDPGT) form conceivably involved in the AZT biotransformation process. The glucuronide of AZT was preliminarily identified through hydrolysis by beta-D-glucuronidase. Brij 58 was shown to be the best activator of AZT glucuronidation by human liver microsomes, as it increased the rate of glucuronide formation 3-fold. The UDPGT activities toward AZT measured in 29 different microsomal fractions was slightly variable among samples (79 to 268 nmol/hr/mg protein). The apparent KM value for AZT glucuronidation was about 5 mM. We sought to determine if various known UDPGT activities (i.e. p-nitrophenol UDPGT, 4-hydroxybiphenyl UDPGT, and DT1-UDPGT) in 18 microsomal samples were correlated with AZT-UDPGT activity. Experiments revealed that only 4-hydroxybiphenyl UDPGT activity was strongly correlated (r = 0.815, p less than 0.001) with AZT-UDPGT activity, whereas no correlation was found for the other UDPGT activities. To determine the isozyme conceivably involved in AZT glucuronidation, we studied the effect of various compounds on AZT glucuronidation. AZT glucuronidation was inhibited by numerous substrates of the UDPGT2, form: morphine (Ki = 1.8 mM), 4-hydroxybiphenyl (Ki = 0.92 mM), and ketoprofen (Ki = 0.75 mM), but also oxazepam, codeine, and chloramphenicol. p-Nitrophenol appeared to be an inhibitor, whereas acetaminophen had no effect. Bilirubin, aspirin, cimetidine, and acyclovir did not inhibit AZT glucuronidation. Since all the inhibitors tested except p-nitrophenol are known to be glucuronidated by the UDPGT2 form, our results strongly suggest the involvement of this isozyme in AZT glucuronidation.(ABSTRACT TRUNCATED AT 250 WORDS)