Circulating insulin: the proinsulin-like properties of “big” insulin in patients without islet cell tumors

Abstract When plasma is filtered on Sephadex G-50, insulin immunoreactivity is recovered in two peaks. “Big” insulin, the higher molecular weight component, and “little” insulin, the lower molecular weight component, have elution volumes that correspond to those of proinsulin-125I and insulin-125I respectively. When plasma was extracted with acid ethanol and filtered in 1.0 M acetic acid, the patterns and proportions of “big” and “little” insulin were indistinguishable from those obtained by filtration of whole plasma in neutral buffer. When “big” insulin was isolated from plasma and mixed with a tracer of porcine proinsulin-125I, trypsin converted the “big” insulin immunoreactivity to the gel filtration pattern of “little” insulin in the same way that it converted the proinsulin radioactivity. More than 90% of both “big” insulin and proinsulin were converted at optimal trypsin concentrations. Our present guinea pig anti-insulin serum failed to distinguish “big” from “little” but a porcine proinsulin anti-serum, under appropriate conditions of assay, reacted strongly with “big” insulin but not at all with “little.” When tested on isolated fat cells, “little” insulin had the same bioactivity as porcine insulin, whereas “big” insulin had the same low activity as porcine proinsulin. These studies suggest that “big” insulin represents either single-chain proinsulin and/or a proinsulin intermediate that has similar low bioactivity.