Constrained selected reaction monitoring: Quantification of selected post-translational modifications and protein isoforms

•SRM/MRM assays can quantify PTMs and specific isoforms as well as whole proteins.•Constraints on the choice of signature peptides can result in low SRM signals.•SRM signals can be enhanced by tuning the MS instrument or fragmenting product ions.•Enriching the sample or selecting an alternative protease can increase SRM signals. Selected reaction monitoring (SRM) is a mass spectrometry method that can target signature peptides to provide for the detection and quantitation of specific proteins in complex biological samples. When quantifying a protein, multiple peptides are generated using a specific protease such as trypsin, thereby allowing a choice of signature peptides with robust signals. In contrast, signature peptide selection can be constrained when the goal is to monitor a specific post-translational modification (PTM) or protein isoform, as the signature peptide must include the amino acid residue(s) of PTM attachment or sequence variation. This can force the selection of a signature peptide with a weak SRM response or one that is confounded by high background. In this article, we discuss steps that can be optimized to maximize peptide selection and assay performance of constrained SRM assays, including tuning instrument parameters, fragmenting product ions, using a different protease, and enriching the sample. Examples are provided for phosphorylated or citrullinated peptides and protein isoforms.