O-glucuronidation of the lung carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) by human UDP-glucuronosyltransferases 2B7 and 1A9

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone and its major metabolite, 4- (methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), are potent lung carcino gens in animals. UGT-mediated O-glucuronidation of NNAL is an important det oxification pathway for these carcinogens. To better characterize this path way in humans, we screened a series of UGT-overexpressing cell lines and ba culosome preparations for their ability to O-glucuronidate NNAL and examine d multiple human liver and lung specimens for NNAL-glucuronidating activity and their levels of expression of NNAL-glucuronidating UGTs. Human liver m icrosomal fractions exhibited significant levels of NNAL-glucuronidating ac tivity, with the NNAL-Gluc II diastereomer formed at a rate 3.4 times that observed for NNAL-Gluc I. As with liver microsomal fractions, NNAL-Gluc II was the major diastereomer formed by homogenates from UGT2B7-overexpressing HK293 cells or UGT2B7-overexpressing baculosomes; the major diastereomer f ormed by homogenates from UGT1A9-overexpressing V79 cells was NNAL-Gluc I. No significant O-glucuronidating activity of NNAL was detected in UGT1A1-, UGT1A4-, UGT1A6-, UGT2B4-, or UGT2B15-overexpressing HK293 or V79 cell homo genates, or in UGT1A1-, UGT1A3-, UGT1A7-, or UGT1A10-overexpressing baculos omes. Significant levels of UGT2B7 mRNA were detected by reverse transcript ase-polymerase chain reaction in human liver and at low levels in human lun g specimens. UGT1A9 mRNA was detected in liver but not in lung. These resul ts suggest that although both UGT2B7 and UGT1A9 play an important role in t he overall glucuronidation of NNAL in humans, UGT2B7 potentially plays an i mportant role in the detoxification of NNAL in the lung.