To investigate the presence of divalent metal ions in the purified SsTroA and assess the ability of SsTroA to bind metal ions, the recombinant protein was subjected to a test of inductively coupled plasma mass spectrometry (ICP-MS) analysis. The metal contents of SsTroA as isolated or after reconstitution were shown in Table 1 . The stoichiometry was found to be 0.9660.2 Zn 2+ /SsTroA monomer and 0.8960.2 Mn 2+ /SsTroA monomer, which suggests that recombinant SsTroA protein binds Zn 2+ and Mn 2+ roughly in a 1:1 ratio. The ICP-MS data provided support for one metal binding site per SsTroA monomer, as observed in TpTroA and Neisseria gonorrhoeae MntC [31, 33] . This led us to evaluate the metal interactions with the SsTroA protein in more details.