Optimized sample-processing time and peptide recovery for the mass spectrometric analysis of protein digests

Abstract

Proteomics requires an optimized level of sample-processing, including a minimal sample-processing time and an optimal peptide recovery from protein digests, in order to maximize the percentage sequence coverage and to improve the accuracy of protein identification. The conventional methods of protein characterization from one-dimensional or two-dimensional gels include the destaining of an excised gel piece, followed by an overnight in-gel enzyme digestion. The aims of this study were to determine whether: (1) stained gels can be used without any destaining for trypsin digestion and mass spectrometry (MS); (2) tryptic peptides can be recovered from a matrix-assisted laser desorption/ionization (MALDI) target plate for a subsequent analysis with liquid chromatography (LC) coupled to an electrospray ionization (ESI) quadrupole ion trap MS; and (3) an overnight in-gel digestion is necessary for protein characterization with MS. These three strategies would significantly improve sample throughput. Cerebrospinal fluid (CSF) was the model biological fluid used to develop these methods. CSF was desalted by gel filtration, and CSF proteins were separated by two-dimensional gel electrophoresis (2DGE). Proteins were visualized with either silver, Coomassie, or Stains-All (counterstained with silver). None of the gels was destained. Protein spots were in-gel trypsin digested, the tryptic peptides were purified with ZipTip, and the peptides were analyzed with MALDI and ESI MS. Some of the samples that were spotted onto a wax-coated MALDI target plate were recovered and analyzed with ESI MS. All three types of stained gels were compatible with MALDI and ESI MS without any destaining. In-gel trypsin digestion can be performed in only 10–60 min for protein characterization with MS, the sample can be recovered from the MALDI target plate for use in ESI MS, and there was a 90% reduction in sample-processing time from overnight to ca. 3 h.