Relationship between proinsulin and beta cell function in different states of glucose tolerance

Abstract

To measure proinsulin concentrations in different states of glucose tolerance and examine the relationship between proinsulin and beta cell function. Serum true insulin (TI), proinsulin (PI), immunoreactive insulin (IRI), C peptide (CP) and blood glucose (BG) levels were measured in the fasting state and during an oral glucose tolerance test (OGTT) in 32 individuals with normal glucose tolerance (NGT), 42 individuals with impaired glucose tolerance (IGT), and 54 individuals with newly diagnosed type 2 diabetes mellitus. All participants were also subdivided into nonobese [body mass index (BMI) <25 kg/m2] and obese (BMI ≥25 kg/m2) subgroups. The levels of TI, PI, IRI and CP were higher in obese patients compared with the corresponding NGT subgroup, whereas there was no difference in PI/TI. The levels of IRI, PI and PI/TI were higher in nonobese patients with type 2 diabetes mellitus than in the corresponding NGT subgroup. The levels of PI and TI increased in obese patients with NGT and IGT, whereas PI/TI did not change. In contrast, PI increased but TI did not in subjects with newly diagnosed diabetes mellitus, which led to an increase in PI/TI, and a decrease in beta cell function. Therefore, PI and the PI/TI ratio could offer markers for beta cell dysfunction in DM.