DEVELOPMENT OF AN ISOTOPE-DILUTION ASSAY FOR PRECISE DETERMINATION OFINSULIN, C-PEPTIDE, AND PROINSULIN LEVELS IN NONDIABETIC AND TYPE-II DIABETIC INDIVIDUALS WITH COMPARISON TO IMMUNOASSAY

We describe the application of a stable isotope dilution assay (IDA) t o determine precise insulin, C-peptide, and proinsulin levels in blood by extraction from serum and quantitation by mass spectrometry using analogues of each target protein labeled with stable isotopes, Insulin and C-peptide levels were also determined by immunoassay, which gave consistently higher results than by IDA, the relative difference being larger at low concentrations. Insulin, C-peptide, and proinsulin leve ls were all shown by IDA to be higher in type II diabetics than in non diabetics, with mean values rising from 22 (+/- 2) to 92 (+/- 8), 335 (+/- 11) to 821 (+/- 24), and 6 (+/- 1) to 37 (+/- 3) PM, respectivel y, Interestingly, the ratio between IDA and immunoassay values for ins ulin levels increased from 1.3 in non-diabetics to 1.7 in type II diab etics. The ratio between proinsulin and insulin levels by IDA increase d from 0.24 in non-diabetics to 0.36 in type II diabetics, whereas the ratio between C-peptide and insulin levels by IDA decreased from 17.6 to 10.7. This disproportionate change in protein levels between diffe rent types of individuals has implications for the metabolism of insul in in the diabetics studied (type II) and suggests that C-peptide leve ls are not always a reliable guide as to pancreatic insulin secretion, In addition, levels of the 33-residue C-peptide (partially trimmed fo rm) were shown to be less than 10% that of the fully trimmed 31-residu e C-peptide levels, and we tested IDA in a clinical context by two pos t-pancreatic graft studies. IDA was shown to give direct, positive ide ntification of the target protein with unrivaled accuracy, avoiding ma ny of the problems associated with present methodology for protein det ermination.