Stereospecific Metabolism of the Tobacco-Specific Nitrosamine, NNAL

Among the most potent carcinogens in tobacco are the tobacco-specific nitrosamines (TSNAs), with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) being the most potent as well as one of the most abundant. NNK is extensively metabolized to the equally carcinogenic 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL). Of the two NNAL enantiomers, (S)-NNAL not only appears to be preferentially glucuronidated and excreted in humans but also exhibits higher stereoselective tissue retention in mice and humans and has been shown to be more carcinogenic in mice than its (R) counterpart. Due to the differential carcinogenic potential of the NNAL enantiomers, it is increasingly important to know which UGT enzyme targets the specific NNAL enantiomers for glucuronidation. To examine this, a chiral separation method was developed to isolate enantiomerically pure (S)- and (R)-NNAL. Comparison of NNAL glucuronides (NNAL-Glucs) formed in reactions of UGT2B7-, UGT2B17-, UGT1A9-, and UGT2B10-overexpressing cell microsomes with pure NNAL enantiomers showed large differences in kinetics for (S)- versus (R)-NNAL, indicating varying levels of enantiomeric preference for each enzyme. UGT2B17 preferentially formed (R)-NNAL-O-Gluc, and UGT2B7 preferentially formed (S)-NNAL-O-Gluc. When human liver microsomes (HLM) were independently incubated with each NNAL enantiomer, the ratio of (R)-NNAL-O-Gluc to (S)-NNAL-O-Gluc formation in HLM from subjects exhibiting the homozygous deletion UGT2B17 (*2/*2) genotype was significantly lower (p = 0.012) than that with HLM from wild-type (*1/*1) subjects. There was a significant trend (p = 0.015) toward a decreased (R)-NNAL-O-Gluc/(S)-NNAL-O-Gluc ratio as the copy number of the UGT2B17*2 deletion allele increased. These data demonstrate that variations in the expression or activity of specific UGTs may affect the clearance of specific NNAL enantiomers known to induce tobacco-related cancers.