Contribution of total and intact proinsulins to hyperinsulinism in subjects with obesity, impaired glucose tolerance or type 2 diabetes

The recent development of specific radioimmunoassays of insulin (Ins) and proinsulin (PI) led some authors to question the classical data on insulinosecretion in patients with abnormal glucose tolerance. The aim of this work was to determine the participation of intact proinsulin (iPI) and its split fragments to total insulinsecretion in obese subjects and in various stages of glucose intolerance determined by an oral glucose load according to the WHO recommendations. Five groups were constituted: non obese controls (C), obese subjects with normal glucose tolerance (O), non obese subjects with impaired glucose tolerance (I), obese subjects with impaired glucose tolerance (OI) and diabetic patients (O). The basal level of total Proinsulin (tPI) of D and OI was significantly higher than that of C, but the tPI/Ins ratio did not differ between the five groups. After glucose load, this ratio tended to be higher in D, but not significantly. No statistical difference between groups was observed for the iPI/Ins ratio. These results indicate that the determination of tPI is at least as informative or more than that of iPI. Furthermore, the proportionally constant participation of PI to insulin secretion observed in various stages of glucose intolerance suggests that the results obtained in the past with non specific insulin radioimmunoassays remain valid.