Micro-heterogeneities in native and recombinant proteins studied by electrospray ionization and FAB-MS coupled with micro-column liquid chromatography

A home-made electrospray ionization (ESI) source, which generated multiply charged gas-phase ions of proteins under atmospheric pressure, was coupled with a tandem quadrupole mass spectrometer. A detection limit of 18 fmol (1.8 × 10-14 mol) and a precision of 0.01% in the molecular weight measurement were attained using hen-egg lysozyme as a typical protein.ESI mass spectrometry (MS) was applied to the prompt identification of the amino acid substitutions in recombinant human lysozymes (HLY). However, the direct analysis of HLY was prevented by the co-existing inorganic salts added to retain its biological activities. On-line coupling of ESI-MS with micro-column liquid chromatography (ESI-LC/MS) was developed to perform direct molecular weight measurements of salt-containing proteins at the picomole level. LC/MS interfacing by ESI required to alleviate the sensitivity losses brought about by the high electric conductivity and the high surface tension of the LC eluent.ESI-MS and ESI-LC/MS were successfully applied to the characterization of the post-translational modifications found in recombinant wheat germ agglutinin (WGA2). These methods along with fast atom bombardment (FAB) MS and FAB LC/MS were also applied to the characterization of natural variants of β-casein (β-CA). The possibility and limitation of the present methods for the analysis of complex proteins were discussed