Comparison of Internal Standard Approaches for SRM Analysis of Alpha-Synuclein in Cerebrospinal Fluid

Absolute protein quantification by selected reaction monitoring (SRM, also MRM) is an alternative to immunoassays, and the gold standard here is the addition of stable-isotope labeled (SIL) proteins (PSAQ). Cerebrospinal fluid (CSF) is the preferred source of biomarkers for neurological diseases, and recent improvements in mass spectrometry enable the quantification of disease-relevant proteins in CSF. We used alpha-synuclein SRM to investigate alternatives to the PSAQ approach in human CSF regarding precision and accuracy, including SIL peptides, winged SIL (WiSIL) peptides, and quantitative protein epitope signature tags (QPrESTs). All approaches yielded precise results in CSF with CV values <15% in several runs for all four measured peptides. PSAQ and QPrEST also showed good accuracy (deviation ≤15%), whereas SIL and WiSIL peptides yielded deviations up to 54% that greatly depended on the measured peptide. Total protein concentration in CSF did not affect precision and accuracy. Thus, our study indicates that all four approaches are suitable for relative quantification of alpha-synuclein in CSF. QPrESTs are a valuable alternative to PSAQ in terms of precision and accuracy, although SIL and WiSIL peptides can yield accurate results as well when peptides are selected consciously.