Proinsulin in human serum: problems in measurement and interpretation.

The immunoreactivity of an extracted pancreatic human proinsulin standard in an indirect immunoradiometric assay was found to be at least one hundred times higher than that of biosynthetic human proinsulin. Limited tryptic digestion of the biosynthetic proinsulin increased its immunoreactivity in the assay and this was attributed to the production of partially cleaved proinsulin molecules which still retained the C-peptide moeity. This inference was confirmed by the finding that pure samples of 65/A1 and 32/33 split proinsulins reacted in the assay very similarly to the pancreatic proinsulin standard. The implications of these results are that the immunoassays of proinsulin using antisera to C-peptide may recognise the intact proinsulin molecule very poorly, if at all, and that the 'proinsulin' measured by such assays of human serum may be largely if not entirely intermediates of proinsulin cleavage.