concentrations of H2O2 were calculated using a standard curve generated using H2O2 solutions of known concentrations. Preparation of H2O2 Hydrogen peroxide (30% v/v; Wako Chemical. Co., Ltd.) was diluted to a concentration of 100 mM in distilled water. The precise concentration of hydrogen peroxide was determined using the titanium oxide method [27], in which the molar coefficient of a titanium oxide-hydrogen peroxide complex is assumed to be 750 M21 cm21 at 405 nm. Briefly, 160 ml of hydrogen peroxide solution (prepared as described above) were added to a mixture of 30 ml of titanium sulfate and 50 ml of 20% (v/v) hydrogen sulfate. The resulting mixture was stirred at room temperature for 15 min, and the precise concentration of hydrogen peroxide was calculated from the absorbance at 405 nm. siRNA transfection in H4IIEC hepatocytes. 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