GRx1 was pretreated with sporidesmin, followed by incubation in the presence of 35 mM DTT for 1 h at room temperature, then subjected to mass spectral analysis. MALDI ions at m/z 11,613 +/− 2 and 443 were consistent with release of GRx1 and sporidesmin, respectively, each less a sulfur atom, as would be expected if two sulfurs had been eliminated from the adduct. Ions consistent with the presence of unreacted GRx1 (11,645) and SP-GRx1 adduct minus 64 Da (12,054), reflecting possible loss of two sulfur atoms, are also evident.