Capillary column chromatography improves sample preparation for mass spectrometric analysis: Complete characterization of human α-enolase from two-dimensional gels followingin situ proteolytic digestion
Two-dimensional polyacrylamide gel electrophoresis (2-DE) in combination with mass spectrometry is an extremely powerful tool for characterizing complex mixtures of proteins. In many cases, the success of this approach relies upon the ability to recover peptides at high concentrations and free of interfering artifacts from in-gel and/or on-membrane enzymatic digests. In previous studies, we demonstrated that capillary or microcolumn (< 350 microm ID) reversed-phase high performance liquid chromatography (RP-HPLC) is a powerful microseparation technique for proteins and peptides (Moritz, R. L. and Simpson, R. J., J. Chromatogr. 1992, 599, 119-130). Here we evaluate various capillary column RP-HPLC/mass spectrometric approaches for identifying and characterizing 2-DE resolved proteins. For these studies, stable and efficient 0.20 mm and 0.32 mm internal diameter (ID) fused-silica columns with hydrophilic polyvinylidene difluoride (PVDF) frits were fabricated and slurry packed with 7 microm spherical, 300 A pore size, C8 bonded phase silica particles. We show that capillary column chromatography is a rapid and efficient desalting/concentrating (ON/OFF) technique for sample cleanup prior to protein identification by peptide-mass fingerprinting using matrix-assisted laser desorption ionization (MALDI)-time-of-flight mass spectrometry. While marginally more peptide mass information can be obtained by stepped elution of the peptide mixture with increasing concentrations of organic solvent, best results were obtained by fractionation of the peptide mixture using a linear 60 min gradient. One salient feature of this study was the observation that, in contrast to the stepped elution and gradient approaches, the ionization of peptide T1 (m/z 2402.2 SGETEDTFIADLVV(PeCys)TGQIK) was almost completely suppressed using the ON/OFF approach. Maximal amino acid sequence coverage, a necessary prerequisite for complete characterization of a protein, was accomplished using a capillary column (0.2 mm ID) directly coupled with an electrospray ionization (ESI) ion-trap tandem mass spectrometer. For example, from an in situ tryptic digest of alpha-enolase isolated by 2-DE from the human breast carcinoma cell line MDA-MB231, 71% of the amino acid sequence was obtained. In addition to identifying two possible N-terminal acetylated alpha-enolase variants, Asn153Asp and Ile152Asp/Asn153Ile, the tandem mass spectrometric data revealed the presence of a number of process-induced modifications of alpha-enolase such as methionine oxidation and cysteine amidoethylation.