Cryoconservation of Peptide Extracts From Trypsin Digestion of Proteins for Proteomic Analysis in a Hospital Biobank Facility

Abstract

[Abstract] We tested a semiautomated protocol for the proper storage and conservation in a hospital biobank of tryptic peptide extracts coming from samples with low and high protein complexity for subsequent mass spectrometry analysis. Low-complexity samples (serum albumin, serotransferrin. and alpha-S1-casein) were loaded in replicates in SDS-PAGE and subjected to standard in-gel trypsin digestion. For LC-MALDI-TOF/TOF analysis, purified β-galactosidase and human serum samples were in-solution digested following standard procedures and desalted with C18 stage-tips. In both cases, peptides extracts were aliquoted in individually 2D coded tubes, vacuum-dried, barcode-read, and stored in an automated -20 °C freezer in the Biobank facility. Samples were kept dried at -20 °C until the corresponding time-point of analysis, then reconstituted in the proper buffer and analyzed by either MALDI-TOF/TOF (peptide fingerprinting and MS/MS) or LC-MALDI-TOF/TOF following a highly reproducible pattern to ensure the reproducibility of the results. Protein identification was done with either Mascot or Protein Pilot as search engines using constant parameters. Over a period of 1 year we checked six different time points at days 0, 7, 30, 90, 180, and 365. We compared MS and MS/MS protein score, number of identified peptides, and coverage of the identified proteins. In the low complexity samples, the number of peptides detected gradually decreased over time, especially affecting the MS score. However, two of the three proteins - serum albumin and serotransferrin - were identified by both PMF and MS/MS at day 90. By day 180, only MS/MS identification in some replicates was possible. By LC-MS/MS, β-galactosidase and the most abundant serum proteins were identified with good scores at all time points even by day 365, with no detectable peptide loss or decrease in the fragmentation efficiency, although a progressive decrease in peptide intensity indicates that detection of low abundant proteins could not be optimal after very long periods of time. Our results encourage us to use the biobank facility in the future for long-term storage - up to 3 months - of dried peptide extracts.