[Proinsulin: physiology, measurement, and interest in clinical biology].

The proinsulin molecule results from the cleavage of pre-pro-insulin, produced in pancreatic beta cells. Its subsequent -cleavage allows the release of insulin, the key hormone of glycemia regulation and C-peptide in equimolar proportions. During fasting trial, insulinoma diagnosis relies on inadequately high insulin and C-peptide serum levels concomitant with an hypoglycemia. In this context, proinsulin assay can be interesting in the cases of discrepancy between the two parameters. In diabetes, endoplasmic reticulum stress and beta cells inflammation, lead to the secretion of misfolded proinsulin molecules. Thus, in type 2 diabetes, proinsulin/insulin ratio increases with the degree of insulin resistance. In type 1 diabetes, proinsulin/C-peptide ratio could predict the onset of diabetes in relatives. In our practice, serum pro-insulin determined using an Elisa immunoassay (Millipore®) during fasting trial can be complementary to C-peptide and insulin assays in relation to glycemia to label an hypoglycemia. In case of glucose intolerance and diabetes, proinsulin could thus be measured.