4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is the primary tobacco-specific nitrosamine (TSNA) related to cancer. The major metabolic pathway for NNK is carbonyl reduction to 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) which, like NNK, is a potent lung and pancreatic carcinogen in rodents, with it estimated that 39-100% of NNK is converted to NNAL in cigarette smokers. NNAL is formed as both (R)- and (S)-NNAL enantiomers; while (S)-NNAL is more readily formed from NNK, it is also more readily glucuronidated and detoxified by the phase II UGTs than is (R)-NNAL in monkeys and probably in humans. Therefore, both (S)- and (R)-NNAL are likely to be important metabolites in the NNK carcinogenic process. In the present study, we demonstrated that while (S)-NNAL is the major form (>90%) produced using lung cytosolic fractions, lung microsomes formed primarily (R)-NNAL (>95%). Consistent with previous studies using placenta-fractionated cytosolic enzymes, we demonstrated that several cytosolic enzymes including the aldo-keto reductases (AKRs) 1C1, 1C2, and 1C4, as well as carbonyl reductase I (CBR1) were active against NNK, with all of them forming primarily (>90%) (S)-NNAL. This pattern is consistent with that observed for lung cytosolic fractions. Using HEK293 cells over-expressing microsomal enzymes HSD11β1, HSD11β2, HSD17β12, SDR5A1, SDR5A3, HSD17β1, HSD17β2 or HSD3β7, only HSD11β1 and HSD17β12, all of which were reported to be expressed in lung, exhibited activity against NNK. Most interestingly, both enzymes formed primarily (R)-NNAL (>80%), which is consistent with that observed in lung microsomes. Real-time PCR showed that HSD17β12 was expressed at a level that, on average, was three-fold higher than that observed for HSD11β1 in a screening of normal lung tissue specimens from five individuals. These data suggest that both cytosolic and microsomal enzymes are active against NNK and that HSD11β1 and HSD17β12 are the active microsomal reductases in human lung microsomes, contributing to the exclusive formation of (R)-NNAL

Citation Format: Joseph Ashmore, Ying Liu, Amity Peterson, Sabrina Fechtner, Gang Chen, Philip Lazarus. Characterization of human lung enzymes involved in the reduction of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2722. doi:10.1158/1538-7445.AM2015-2722