ESI-MS analyses of selected hSOD1 variants.

Only variant proteins for which defined mass spectra were obtained are shown. The same hSOD1 variants were detected on the immunoblot in Figure 2. The corresponding ESI-MS spectra are shown in Figure 3. All hSOD1 variants were found with the N-terminal methionine cleaved off and acetylated alanine at position 2, as reported in the literature [41]. The occasionally attached sodium ions (+22.99 Da) most probably originated from the Strep-Tactin elution buffer which contained 150 mM NaCl. The buffer was not exchanged during sample concentration in order to avoid protein loss. In some of the protein preparations we found a known disulfide bond (S-S, −2 Da; between C57 and C146 [70]). 1All hSOD1 masses were calculated without N-terminal methionine, acetylated alanine at position 2 and with completely reduced cysteines.