Immunoluminometric assay (ILMA) for intact human proinsulin and its conversion intermediates

Abstract

We describe an immunoluminometric assay (ILMA) for determination of intact proinsulin and its conversion intermediates, 32,33-split and 65,66-split proinsulin, in human serum. After incubation of the serum samples with the IgG fraction of a guinea pig antiserum against human insulin coated to the surface of polystyrene beads, a sandwich complex was formed using a monoclonal antibody against human C-peptide labelled with acridinium ester as second antibody, yielding a detection limit of 0.11 pmol/l. Mean proinsulin concentration in the serum of 38 healthy fasting subjects was 7.3 pmol/l (S.D. ± 5 pmol/l, median 5 pmol/l, 95th percentile 15 pmol/l); maximum serum proinsulin after oral glucose stimulation never exceeded 40 pmol/l. Eighteen of 20 patients with confirmed insulinoma had proinsulin concentrations over 50 pmol/l (mean 261 pmol/l, S.D. ± 248 pmol/l, median 170 pmol/l, 95th percentile 663 pmol/l); serum proinsulin in two patients with completely enucleated B-cell adenoma declined to normal values after surgery.