Correlation between UDP-Glucuronosyltransferase Genotypes and 4-(Methylnitrosamino)-1-(3-Pyrldyl)-1-Butanone Glucuronidation Phenotype in Human Liver Microsomes

The nicotine-derived tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, is one of the most potent and abundant procarcinogens found in tobacco and tobacco smoke, and glucuronidation of its major metabolite, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), is an important mechanism for 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone detoxification. Substantial interindividual variability in urinary NNAL glucuronide formation has been observed in smokers and tobacco chewers. To determine whether genetic variations may play a role in this interindividual variability, NNAL-glucuronidating activities were analyzed in 78 human liver microsomal specimens and compared with the prevalence of missense polymorphisms in the two major NNAL-glucuronidating enzymes UGT1A4 and UGT2B7. In vitro assays using liver microsomal specimens from individual subjects demonstrated a 70- and 50-fold variability in NNAL-N-Gluc and NNAL-O-Gluc formation, respectively, and a 20-fold variability in the ratio of NNAL-N-Gluc: NNAL-O-Gluc formation. Microsomes from subjects with a homozygous polymorphic UGT1A4^24Thr/UGT1A4^24Thr genotype exhibited a significantly higher (P < 0.05) level of NNAL-N-Gluc activity compared with microsomes from subjects with the wild-type UGT1A4^24Pro/UGT1A4 ^24Pro genotype, and a significantly higher (P < 0.05) number of subjects with liver microsomes having high NNAL-N-Gluc formation activity contained the UGT1A4^24Thr/UGT1A^24Thr genotype. Microsomes from subjects with the homozygous polymorphic UGT2B7 ^268Tyr/UGT2B7^268Tyr genotype exhibited a significantly lower level (P < 0.025) of NNAL-O-Gluc activity when compared with microsomes from subjects with the wild-type UGT2B7^268His/UGT2B7 ^268His genotype, and a significantly (P < 0.05) higher number of subjects with liver microsomes having low NNAL-O-Gluc formation activity contained the UGT2B7^268Tyr/UGT2B7^268Tyr genotype. These data suggest that the UGT1A4 codon 24 and UGT2B7 codon 268 polymorphisms may be associated with altered rates glucuronidation and detoxification of NNAL in vivo.