Quantification of Folded and Misfolded Proinsulin Forms Using Nonreducing SDS-PAGE and Proinsulin-Specific Immunoblotting

We have observed that some proinsulin molecules in pancreatic islets and beta cell lines have incomplete or improper intramolecular disulfide bonds. These misfolded monomers can form intermolecular disulfide-linked complexes. Accurately quantifying the fraction of proinsulin molecules contained in these complexes is challenging. By proinsulin immunoblotting after nonreducing SDS-PAGE, the signal for disulfide-linked complexes can exceed the total proinsulin signal detected after reducing SDS-PAGE (i.e., overestimating the abundance of misfolded species due to antibody affinity differences). However, after modification of the SDS-PAGE and electrotransfer protocol, we have been able to more accurately estimate the fraction of proinsulin monomers folded to the native state, as well as misfolded proinsulin monomers and disulfide-linked complexes. Our improved technique offers the ability to obtain a more precise assessment of proinsulin misfolding under different environmental conditions in beta cells and normal islets and in diabetes. Key features • The protocol introduces a modification of a technique that enables clear separation and accurate quantification of native and non-native proinsulin monomers and aberrant disulfide-linked oligomers. • The protocol requires modifications to the standard SDS-PAGE and electrotransfer protocol to address quantitation inaccuracies that result from variations in antibody affinity. • Side-by-side comparison demonstrates that the standard immunoblotting method underestimates proinsulin monomers and overestimates the abundance of proinsulin disulfide-linked complexes. • This method is applicable to the study of recombinant proinsulin in heterologous cells, pancreatic β-cell lines, rodent or human pancreatic islets, and human iPSCs.