Comparative identification of prostanoid inducible proteins by LC-ESI-MS/MS and MALDI-TOF mass spectrometry

Protein identification by MS is well-established. Mixtures of proteins from cell extracts are separated by either one- or two-dimensional gel electrophoresis, and specific bands or spots are subjected to in-gel digestion and subsequent analysis by MS. The two most common types of ionization used in MS are electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). When ESI is used, the sample is typically analyzed by inline HPLC-ESI-MS/MS with fragmentation of individual digest peptides, followed by database comparison between theoretical and experimental fragmentation patterns. MALDI-MS analysis is based on peptide mass mapping, with mass measurements of the digest peptides searched against a database of theoretical digests. We give here the results of a comparison between ESI-ion trap and MALDI-TOF (time-of-flight) analysis of 11-deoxy,16,16-dimethyl prostaglandin E₂ (DDM-PGE₂) inducible proteins. Individual peptides identified by the two techniques differed, in general, but the resulting protein identification was the same. Slightly higher coverage of each protein was obtained by MALDI-TOF, but the MS/MS data were more definitive by requiring fewer peptides to assign a positive identification. Both methods effectively identified two proteins in the same gel band. The samples here are derived from a renal epithelial cell line (LLC-PK₁) established from the New Hampshire minipig, a species poorly represented in the current database, and strategies and limitations for analyzing such species are discussed.