Endogenous H2S content was determined by the formation of methylene blue from dimethyl-p-phenylenediamine in H2SO4 according to the methods described previously ( Zhang et al., 2009), with some modifications. Samples (0.1 g) were ground and extracted in 5 ml 50 mM phosphate buffer solution (pH 6.8) containing 0.1 M EDTA and 0.2 M ascorbic acid (AsA). The homogenate was mixed with 0.5 ml 1 M HCl to release H2S, and H2S was then absorbed in a 1% (w/v) zinc acetate (0.5 ml) trap located at the bottom of the test tube. After 30 min of reaction, the trap was added with 0.3 ml 5 mM dimethyl-p-phenylenediamine dissolved in 3.5 mM H2SO4. Afterward, 0.3 ml 50 mM ferric ammonium sulfate in 100 mM H2SO4 was injected into the trap. The amount of H2S in zinc acetate traps was determined colorimetrically at 667 nm after leaving the mixture for 15 min at room temperature. Blanks were prepared by the same procedures without the zinc acetate solution and known concentrations of Na2S solution were used to make the calibration curve.