Measurement of Circulating Human Proinsulin Concentrations Using a Proinsulin-specific Antiserum

Antibodies have been raised against biosynthetic human proinsulin that show < 1% cross-reactivity with human insulin and C-peptide. A sensitive (IC₅₀ 0.16 pmol/ml; minimum detectable concentration 0.004 pmol/ml) radioimmunoassay has been developed using this antiserum and ¹²⁵I-proinsulin that will measure proinsulin-like immunoreactivity in human serum without the need for prior separation of insulin or C-peptide. In healthy, fasted subjects (N = 23), the serum proinsulin concentration was 0.015 ± 0.001 pmol/ml (mean ± SEM). In six healthy subjects, serum proinsulin rose to 250% of basal after 120 min in response to 100 g oral carbohydrate, but to only 130% after 60 min following 25 g oral carbohydrate. The proinsulin/total immunoreactive insulin ratio and the proinsulin/C-peptide ratio fell sharply after both high and low carbohydrate loads. Endogenous human serum proinsulin-like immunoreactivity released into the circulation after 100 g carbohydrate was eluted from a Mono Q high-performance, ion-exchange column with the same retention time as biosynthetic human proinsulin. Treatment of biosynthetic proinsulin with trypsin under mild conditions led to a decrease in proinsulin-like immunoreactivity concomitant with an increase in C-peptide and insulin-like immunoreactivity, indicating that the proinsulin-specific antiserum did not preferentially recognize intermediates of proinsulin cleavage.