. The membrane filtration (50,000 cut-off) step was carried out due to the expected molecular size (about 70 kDa) of recombinant SIase and the removal of ammonium sulfate from the enzyme solution. The protein solution was applied onto a Ni–NTA affinity column (Qiagen, Hilden, Germany). Before eluting recombinant SIase, Ni–NTA affinity column was washed with washing buffer (50 mM NaH2PO4, 300 mM NaCl, and 20 mM imidazole, pH 8.0), and the recombinant SIase was eluted with elution buffer (50 mM NaH2PO4, 300 mM NaCl, and 250 mM imidazole,pH 8.0). The eluant’s purity was checked by sodium dodecylsulfate–polyacrylamide gel electrophoresis (SDS–PAGE).