Previous studies have shown that NAD(P)H quinone oxidoreductase 1 (NQO1) plays an important role in the detoxification of menadione (2-methyl-1,4-naphthoquinone, also known as vitamin K3) However, menadiol (2-methyl-1,4-naphthalenediol) formed from menadione by NQO1-mediated reduction continues to be an unstable substance, which undergoes the reformation of menachone with concomitant formation of reactive oxygen species (ROS) Hence, we focused on the roles of phase II enzymes, with particular attention to UDP-glucuronosyltransferases (UGTs), in the detoxification process of menadione In this study, we established an HEK293 cell line stably expressing NQO1 (HEK293/NQO1) and HEK293/NQO1 cell lines with doxycycline (DOX)-regulated expression of UGT1A6 (HEK293/NQO1/UGT1A6) and UGT1A10 (FIEK293/NQO1/UGT1A10), and evaluated the role of NQ01 and UGTs against menadione-induced cytotoxicity Our results differed from those of previous studies HEK293/NQO1 was the most sensitive cell line to menadione cytotoxicity among cell lines established in this study. These phenomena were also observed in HEK293/NQO1/UGT1A6 and HEK293/NQ01/UGT1A10 cells in which the expression of UGT was suppressed by DOX treatment On the contrary, HEK293/NQO1/UGT1A6 and HEK293/NQ01/UGT1A10 cells without DOX treatment were resistant to menadione-induced cytotoxicity These results demonstrated that NQ01 is not a detoxification enzyme for menadione and that UGT-mediated glucuronidation of menadiol is the most important detoxification process (C) 2009 Elsevier Inc All rights reserved