Stable Isotope-Labeled Protein Internal Standards

Mass spectrometry-based protein assays impart increased specificity and more rapid development times versus traditional methods, such as ELISA. Coupled with immunoaffinity enrichment, LC-MS/MS is becoming a powerful tool for the quantitation of proteins in plasma. Such methods typically rely on synthetic stable isotope labeled (SIL) peptide internal standards to correct for instrumental variability. For more accurate protein quantitation by LC-MS/MS, experimental variations throughout the entire sample preparation workflow, including protein fractionation, immunoaffinity enrichment, and enzymatic digestion, must be accounted for. An ideal way of improving assay reproducibility is to add a full-length stable isotope labeled recombinant protein, that is equivalent to the native target protein, to the sample at the initial stage of the assay workflow. We have developed a set of stable-isotope-labeled monoclonal antibodies expressed in CHO cells as well as SIL versions of several clinically-relevant human proteins expressed in E. coli, such as IGF1, and in mammalian HEK293 cells, such as Thyroglobulin (manufactured as a Certified Reference Material). We will present data to demonstrate that the use of full-length SIL proteins and antibodies as internal standards allows for more accurate and rapid quantitation of biotherapeutic antibodies and clinically-relevant human protein biomarkers in plasma by LC-MS/MS.