Immunoprecipitation and liquid chromatographic–mass spectrometric determination of the peptide glucose-dependent insulinotropic polypeptides GIP1-42 and GIP3-42 from human plasma samples: New sensitive method to analyze physiological concentrations of pep

The gastrointestinal peptide glucose-dependent insulinotropic polypeptide (GIP1-42) is one of the incretin hormones regulating glucose-induced insulin secretion from the endocrine pancreas. GIP1-42 is a substrate of the circulating enzyme dipeptidyl peptidase IV, which removes the N-terminal peptide Tyr–Ala resulting in the inactive polypeptide GIP3-42. Hither to existing immunoassays do not enable a separate quantification of active and inactive forms, respectively. Therefore, we developed a highly specific and sensitive LC–MS assay for the identification and quantification of GIP1-42 and GIP3-42. Total GIP was immunoprecipitated from crude plasma samples using a C-terminally directed antibody. Thus, peptides were purified and concentrated prior to LC–MS analysis. The present immunoprecipitation–LC–MS assay enables the quantification of active and inactive GIP over a concentration range from 5 to 350 pmol/l in human plasma samples. Since this range covers the basal and postprandial levels of GIP, the method is applicable to the determination of concentration changes and changes in the ratio of active and inactive forms of GIP in human plasma.