ELISA for human proinsulin.

A micro enzyme-linked-immunosorbent-assay (ELISA) for monitoring circulating human proinsulin (hPI) was developed. A micro test plate was coated with guinea pig anti-insulin antibody. As labelling system peroxidase-labelled F(ab1)2-fragments of a guinea pig anti-human-C-peptide was used. The detection limit in buffer (95% level) was 0.6 pmol/l corresponding to 0.06 fmol/incubation well and to 1.2 pmol/l in serum, since samples were diluted 50%. Standard operating range was from 0-160 pmol/l. Interassay variation was 9% estimated from two human control materials (assayed within the range 6-9 pmol/l and 9-14 pmol/l, respectively). Insulin in samples did not interfere in concentrations below 400 pmol/l. Human C-peptide, porcine, and bovine proinsulins did not cross-react even at 10 000 pmol/l. In 38 healthy fasting subjects a reference range less than 1.2-13 pmol/l with a median of 4.1 pmol/l was found. Serum from total pancreatectomised patients showed values below the detection limit. The value from a patient with an insulinoma was 263 pmol/l. When stored at -20 degrees C human proinsulin appeared stable in serum or plasma for at least 9 mth. This ELISA, although among the most sensitive immunoassays for human proinsulin, is still not sensitive enough to measure the concentrations expected in samples from IDDM patients in the fasting state. In spite of this the method is useful in characterising beta-cell function in stimulated situations, as well as in the diagnosis of insulinoma.