Development of an isotope dilution mass spectrometry assay for the quantification of insulin based on signature peptide analysis

An isotope dilution mass spectrometry (IDMS) method that involves peptide-based protein analysis was developed to accurately quantify insulin. In this study, a signature peptide (GFFYTPK) obtained from tryptic digestion of insulin was selected as a surrogate for insulin. Then, the optimal conditions for signature peptide analysis through mass spectrometry detection and enzymatic digestion were determined. The analytical performance of this method was assessed and validated using porcine insulin-certified reference material. The linear range of the insulin calibration curve ranged from 0.05 similar to 2 mass ratios, with recoveries ranging from 96.15 to approximately 101.15%. The limit of detection was 0.19 ng/mL, and the limit of quantification was 0.63 ng/mL. The quantitative results corresponded well with a certified value that was obtained from measuring a porcine insulin reference material with amino acid-based IDMS. In addition, the target peptide GFFYTPK can be found in other species of insulin. This method was also applied for the quantification of human insulin-certified reference material. Finally, we applied the method to quantify the concentrations of simulated serum insulin. These findings suggested that this signature peptide-based IDMS approach can accurately quantify insulin levels, can assign a certified value to insulin reference materials, and has the potential to quantify serum insulin with traceable measurements.