According to the previous method (Xie et al., 2013), the activity
of DES was determined by monitoring the release of sulfide from LCys
in a total volume of 3 ml containing 2.5 mM DL-dithiothreitol
(DTT), 0.8 mM L-Cys, 100 mM TriseHCl (pH 9.0), and enzyme
extract. The reaction was initiated by the addition of L-Cys. After
incubation at 37 C for 15 min, the reaction was terminated by
adding 300 ml of 30 mM FeCl3 dissolved in 1.2 N HCl and 300 ml
20 mM N,N-dimethyl-p-phenylenediamine dihydrochloride dissolved
in 7.2 N HCl. The formation of methylene blue was determined
at 670 nm. The DES enzymatic activity was calculated by
comparison to a standard curve of Na2S