Menthol Inhibits UGT2B10‐catalyzed Detoxification of the Potent Tobacco‐carcinogen, NNAL

Abstract

Menthol may be added to tobacco products as either naturally occurring L‐menthol or synthesized racemic DL‐menthol and is excreted as a menthol glucuronide (MG). The most potent tobacco carcinogen is 4‐(methylnitrosamino)‐1‐(3‐pyridyl)‐1‐butanone (NNK). NNK is extensively metabolized to enantiomers [(R) or (S)] of the equally carcinogenic chiral 4‐(methylnitrosamino)‐1‐(3‐pyridyl)‐1‐butanol (NNAL). Both (R)‐ and (S)‐NNAL have been shown to be carcinogenic. Detoxification of (R)‐ and (S)‐NNAL through glucuronidation on NNAL's pyridine ring nitrogen (NNAL‐N‐gluc) is primarily mediated by UGT2B10. Menthol has been shown to inhibit NNAL‐N‐gluc formation in human liver microsomes (HLM). The goal of the present study was to elucidate the impact of menthol on NNAL‐N‐gluc formation in vivo. In vitro inhibition assays using HLM or microsomes from UGT2B10 over‐expressing HEK293 cells were performed with racemic (R/S)‐NNAL as substrate with each menthol enantiomer added separately as inhibitor. In vivo data were gathered from analysis of MG, NNAL, and NNAL metabolites from 50 menthol smokers and 50 non‐menthol smokers. Samples were obtained from the Southern Community Cohort Study. Both L‐ and D‐menthol enantiomers were inhibitors of UGT2B10 in vitro with IC50 values of 236 and 202 μM, respectively. In vivo D‐MG levels, when present, were far below the levels of L‐MG observed in smokers of both menthol or non‐menthol cigarettes, with D‐MG comprising an average of 1.3% of total MG. Urinary L‐MG was detectable in 98 subjects and levels were not significantly different between menthol and non‐menthol smokers (p=0.50). When stratified by tertiles of urinary MG levels, NNAL‐N‐gluc levels were highest in smokers with the lowest MG levels, with NNAL‐N‐gluc levels inversely associated with MG levels in smokers (p<0.05). These data suggest that menthol inhibits a major detoxification pathway of the tobacco carcinogen, NNK. The relatively high levels of urinary menthol in smokers of both menthol and non‐menthol cigarettes raise the possibility that menthol branding may not be the best approach to studying the impact of menthol as a tobacco additive.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.