Inhibition of mammalian thioredoxin reductase by black tea and its constituents: Implications for anticancer actions |
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| Authors: | Yatao Du Yunfei Wu Xueli Cao Wei Cui Huihui Zhang Weixi Tian Mingjuan Ji Arne Holmgren Liangwei Zhong |
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| Institution: | 1. College of Life Sciences, Graduate University of Chinese Academy of Sciences, 100049 Beijing, China;2. Beijing Key Laboratory of Plant Resource Research and Development, Beijing Technology and Business University, Beijing, China;3. College of Chemistry and Chemical Engineering, Graduate University of Chinese Academy of Sciences, 100049 Beijing, China;4. Medical Nobel Institute for Biochemistry, Department of Medical Biochemistry and Biophysics, Karolinska Institute, SE-171 77 Stockholm, Sweden |
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| Abstract: | Black tea is recently reported to have anti-carcinogenic effects through pro-oxidant property, but the underlying mechanisms remain unclear. Mammalian cytosolic thioredoxin reductase (TrxR1) is well -known for its anti-oxidation activity. In this study, we found that black tea extract (BTE) and theaflavins (TFs), the major black tea polyphenols, inhibited the purified TrxR1 with IC50 44 μg/ml and 21 ± 1 μg/ml, respectively. Kinetics of TFs exhibited a mixed type of competitive and non-competitive inhibition, with Kis 4 ± 1 μg/ml and Kii 26 ± 5 μg/ml against coenzyme NADPH, and with Kis 12 ± 3 μg/ml and Kii 27 ± 5 μg/ml against substrate DTNB. In addition, TFs inhibited TrxR1 in a time-dependent manner. In an equilibrium step, a reversible TrxR1-TFs complex (E * I) forms, which is followed by a slow irreversible first-order inactivation step. Rate constant of the inactivation was 0.7 min−1, and dissociation constant of E * I was 51.9 μg/ml. Treatment of NADPH-reduced TrxR1 with TFs decreased 5-(Iodoacetamido) fluorescein incorporation, a fluorescent thiol-reactive reagent, suggesting that Sec/Cys residue(s) in the active site may be involved in the binding of TFs. The inhibitory capacity of TFs depends on their structure. Among the TFs tested, gallated forms had strong inhibitory effects. The interactions between TFs and TrxR1 were investigated by molecular docking, which revealed important features of the binding mechanism of theaflavins. An inhibitory effect of BTE on viability of HeLa cells was observed with IC50 29 μg/ml. At 33 μg/ml of BTE, TrxR1 activity in HeLa cells was decreased by 73% at 22 h after BTE treatment. TFs inhibited cell viability with IC50 10 ± 4 μg/ml for HeLa cells and with IC50 20 ± 5 μg/ml for EAhy926 cells. The cell susceptibility to TFs was inversely correlated to cellular levels of TrxR1. The inhibitory actions of TFs on TrxR1 may be an important mechanism of their anti-cancer properties. |
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| Keywords: | Thioredoxin reductase Black tea Theaflavins Anticancer Pro-oxidant |
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