Profiling of intact blood proteins by matrix‐assisted laser desorption/ionization mass spectrometry without the need for freezing – Dried serum spots as future clinical tools for patient screening

Abstract

Rationale To open up new ways for matrix‐assisted laser desorption/ionization mass spectrometry (MALDI‐MS)‐based patient screening, blood serum is the most preferred specimen because of its richness in patho‐physiological information and due to ease of collection. To overcome deleterious freeze/thaw cycles and to reduce high costs for shipping and storage, we sought to develop a procedure which enables MALDI‐MS protein profiling of blood serum proteins without the need for serum freezing. Methods Blood sera from patients/donors were divided into portions which after pre‐incubation were fast frozen. Thawed aliquots were deposited on filter paper discs and air‐dried at room temperature. Intact serum proteins were eluted with acid‐labile detergent‐containing solutions and were desalted by employing a reversed‐phase bead system. Purified protein solutions were screened by MALDI‐MS using standardized instrument settings. Results MALDI mass spectra from protein solutions which were eluted from filter paper discs and desalted showed on average 25 strong ion signals (mass range m/z 6000 to 10,000) from intact serum proteins (apolipoproteins, complement proteins, transthyretin and hemoglobin) and from proteolytic processing products. Semi‐quantitative analysis of three ion pairs: m/z 6433 and 6631, m/z 8205 and 8916, as well as m/z 9275 and 9422, indicated that the mass spectra from either pre‐incubated fast‐frozen serum or pre‐incubated dried serum spot eluted serum contained the same information on protein composition. Conclusions A workflow that avoids the conventional cold‐chain and yet enables the investigation of intact serum proteins and/or serum proteolysis products by MALDI‐MS profiling was developed. The presented protocol tremendously broadens the clinical application of MALDI‐MS and simultaneously allows a reduction in the costs for storage and shipping of serum samples. This will pave the way for clinical screening of patients also in areas with limited access to health care systems, and/or specialized laboratories.