[A method for determination of proinsulin levels in serum using both insolubilized anti-insulin antibody and anti-C-peptide antibody].

A method is described for determination of proinsulin levels in serum. The principle of the assay is that proinsulin reacts with both anti-insulin and anti-C-peptide antibodies. The assay procedure is as follows; Anti-insulin antibody fixed to bacterial cell wall and insolubilized is incubated with test serum to form a complex of proinsulin-anti-insulin antibody (solid phase), followed by washing twice with buffer to eliminate free C-peptide. Then, glycin-HCl buffer is added to dissociate the bound proinsulin. After centrifugation, the supernatant is neutralized with NaOH and proinsulin in it is measured using RIA kit for CPR assay. The assay is simple, sensitive and reproducible. Neither insulin nor C-peptide contained in test serum influences the proinsulin levels determined by this assay. The mean +/- S.D. of the fasting serum proinsulin levels of healthy donors was 7.0 +/- 2.6 PM/l. A patient with insulinoma showed extremely high serum proinsulin level, which decreased to the normal range after extirpation of insulinoma.