FAIMS Boosts Detection of Homocysteine-Modified Proteins in Mouse Tissues

Proteins carrying a chemically attached homocysteine molecule are notoriously hard to find. These modified proteins, known as N-Hcy-proteins, form when homocysteine-thiolactone, a reactive thioester form of the amino acid homocysteine, reacts with lysine residues. Because the modification is rare relative to the vast background of unmodified peptides, proteome-wide surveys of N-homocysteinylation have struggled with low detection rates. A new study reports that a gas-phase separation technique called high-field asymmetric waveform ion mobility spectrometry, or FAIMS, substantially raises the number of N-homocysteinylated peptides and proteins that mass spectrometry can identify, in mouse liver and brain samples.

The work, published as an open access original article in the journal Amino Acids on 3 August 2026, was carried out by Joanna Perła-Kaján, Bianka Świderska and Agata Malinowska. N-homocysteinylation matters because it is not a silent chemical footnote. According to the study’s abstract, the modification induces immunogenic, thrombogenic and amyloidogenic properties of proteins, meaning it can make proteins trigger immune responses, promote blood clotting, and encourage aggregation into amyloid-like structures. Understanding which proteins carry this mark, and where, is therefore considered important for unraveling the mechanisms of homocysteine toxicity, a topic relevant to human disease research.

The technical obstacle is abundance. Modified peptides occurring at very low levels drown in a sea of unmodified species, and co-eluting peptides can mask the fragment ions needed to localize the modification site. FAIMS acts as an online fractionation device between the chromatography column and the mass spectrometer. Ions travel through a carrier gas while an asymmetric electric field alternately pushes them toward opposing electrodes; because different ions compensate for this field to different degrees, only those within a chosen compensation voltage pass through at a given moment. This gas-phase filtering separates overlapping peptide populations before they reach the detector, allowing previously hidden species to be sequenced.

FAIMS is already established as a robust online fractionation method that improves identification of other post-translational modifications, the authors note, so they expected it to facilitate detection of N-homocysteinylated proteins and to help clarify their role in human disease. To test that expectation, the team performed extensive measurement optimization and then compared the yield of N-Hcy-protein and peptide identification across mouse liver and brain samples. The samples were analyzed in two states: native, meaning unmodified tissue as it was, and modified in vitro with homocysteine-thiolactone, which deliberately loads proteins with the modification to create a richer analytical target.

The results quantify the gain precisely. FAIMS increased the number of identified N-Hcy-Lys-peptides by 1.3 to 7-fold and the number of N-Hcy-proteins by 1.1 to 7-fold, depending on the experimental condition. The improvement applied across both tissue types and both native and in vitro-modified sample sets, making FAIMS a broadly useful addition rather than a niche trick for one preparation.

The study also examined a second, more chemical variable: how thiols are reduced and blocked before analysis. The researchers compared two standard pairs. The first combined dithiothreitol, a classical reducing agent, with iodoacetamide, which alkylates free cysteine thiols. The second paired tris(2-carboxyethyl)phosphine, a phosphine-based reductant, with methyl methanethiosulfonate, a thiol-blocking reagent. The yield of identified N-Hcy-sites was on average higher with the TCEP/MMTS thiol reduction and blocking method than with the DTT/IAA approach, an observation that could influence how future homocysteinylation mapping experiments are designed.

The overall harvest was substantial. The researchers identified 69 N-Hcy-proteins in vivo, meaning modified proteins detected in native mouse tissue, and 1198 N-Hcy-proteins in vitro, from the deliberately modified samples. The roughly seventeen-fold gap between the two numbers illustrates the core sensitivity problem of the field: even with FAIMS, the modification is far easier to detect when it has been artificially enriched than when it occurs naturally. Nevertheless, the in vivo list provides a direct catalog of proteins carrying homocysteine marks in liver and brain, tissues where homocysteine metabolism is active and where toxicity mechanisms are of particular interest.

The authors conclude that FAIMS is a valuable addition to the N-Hcy-proteome analysis workflow and that it facilitates mapping of N-Hcy-sites. To support reproducibility and reuse, the mass spectrometry data have been deposited in the ProteomeXchange repository under the identifier PXD062860, allowing other laboratories to reanalyze the spectra, test alternative search parameters, or benchmark their own homocysteinylation workflows against the same raw files.

For researchers studying homocysteine-linked pathology, the practical message is twofold. First, adding FAIMS fractionation to a liquid chromatography-tandem mass spectrometry pipeline can multiply the number of detectable N-homocysteinylated peptides and proteins without requiring new chemical reagents, because the gain comes from better gas-phase separation rather than from altered sample chemistry. Second, the choice of reduction and blocking chemistry is not neutral; the TCEP/MMTS combination outperformed DTT/IAA on average for site identification in this system. As a caveat, the reported gains come from mouse liver and brain samples analyzed under optimized conditions, and the study is an analytical methods paper rather than a disease-outcome study, so the findings speak to detection capability rather than to any clinical effect of homocysteine modification.

Subject of Research: Improving proteome-wide mapping of N-homocysteinylation in mouse liver and brain proteins using FAIMS ion mobility spectrometry

Article Title: High-field asymmetric waveform ion mobility spectrometry (FAIMS) improves proteome-wide N-homocysteinylation mapping in mouse liver and brain proteins

Article References: Perła-Kaján, J., Świderska, B., & Malinowska, A. (2026). High-field asymmetric waveform ion mobility spectrometry (FAIMS) improves proteome-wide N-homocysteinylation mapping in mouse liver and brain proteins. Amino Acids. https://doi.org/10.1007/s00726-026-03548-8

Image Credits: AI Generated

DOI: 10.1007/s00726-026-03548-8

Keywords: FAIMS, N-homocysteinylation, homocysteine, proteomics, mass spectrometry, post-translational modification, mouse liver, mouse brain, homocysteine-thiolactone, TCEP, MMTS, ProteomeXchange