MRM/SRM (Multiple/Selected Reaction Monitoring)

Targeted LC-MS/MS approach using a triple-quadrupole (QQQ) instrument to monitor pre-selected precursor → fragment ion transitions with high sensitivity and reproducibility.

Role in this research

• Not currently used for HSA isoform profiling (TD approach preferred — intact protein captures multi-PTM isoforms)
• Potential future use: clinical deployment of a minimal HSA biomarker panel using MRM of HSA tryptic peptides carrying specific PTMs
• Absolute quantification possible with isotope-labeled peptide standards (SIS peptides)

Advantages over TD for clinical deployment

• Triple-quad instruments widely available in clinical labs
• Very high sensitivity for specific targeted peptides
• Regulatory precedent (CLIA/FDA-approved MRM assays exist)

Limitation for HSA isoforms

• Peptide-level: cannot directly capture multi-PTM co-occurrences on the same molecule
• ⚠️ A peptide with +162 Da (glycation) cannot be assigned to a specific intact isoform (e.g., HSA+GLYC vs HSA+CYS+GLYC) without knowing the intact protein mass

See also

• Bottom-up proteomics — MRM is a targeted extension of the BU workflow
• SWATH — data-independent alternative for untargeted but reproducible quantification
• Top-down proteomics — intact protein approach preferred for multi-PTM isoform resolution