The MALDI-TOF Mass Spectrometric View of the Plasma Proteome and Peptidome

Abstract

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and the related technique, surface-enhanced laser desorption/ionization (SELDI)-TOF MS, are being applied widely to analyze serum or plasma specimens for potential disease markers. Reports on the basic principles and applications of MALDI-TOF MS were reviewed and related to information on abundance and masses of major plasma proteins. MALDI-TOF MS is a particle-counting method that responds to molar abundance, and ranking of plasma proteins by molar abundance increases the rank of small proteins relative to traditional ranking by mass abundance. Detectors for MALDI-TOF MS augment the bias for detecting smaller components by yielding stronger signals for an equivalent number of small vs large ions. Consequently, MALDI-TOF MS is a powerful tool for surveying small proteins and peptides comprising the peptidome or fragmentome, opening this new realm for analysis. It is complementary to techniques such as electrophoresis and HPLC, which have a bias for detecting larger molecules. Virtually all of the potential markers identified by MALDI-TOF MS to date represent forms of the most abundant plasma proteins. Analyses of serum or plasma by MALDI-TOF MS provide new information mainly about small proteins and peptides with high molar abundance. The spectrum of observed proteins and peptides suggests value for applications such as assessment of cardiovascular risk, nutritional status, liver injury, kidney failure, and systemic immune responses rather than early detection of cancer. Extending analysis by MALDI-TOF MS to lower abundance components, such as markers for early-stage cancers, probably will require more extensive specimen fractionation before analysis.