MALDI spectra of GRx1 reacted with iodoacetamide (A), and GRx1 first reacted with sporidesmin, then with iodoacetamide (B). IAM-treated GRx1 contained a major ion m/z 11,930 +/− 3 consistent with alkylation of all five cysteines. MALDI analysis of the products of reaction of GRx1 with sporidesmin, then treated with IAM, only showed major ions corresponding to the SP-GRx1 adduct (m/z 12054) and GRx1 modified by carboxamidomethylation of a single cysteine (calculated m/z 11,702).