Insulin and C-peptide (proinsulin) packaging: a quantitative immunocytochemical approach

C-peptide and insulin antigenic sites in rat pancreatic beta cells were both labeled by the protein A and immunogold techniques. Gold bead density in the secretory granules was correlated with granule size distribution. Our results demonstrate that the distribution patterns of antigenic sites for C-peptide (which also represents proinsulin sites) and insulin are similar when granule size distribution is correlated to the density of antigenic sites. It was found that the ratio of C-peptide to insulin in granules is independent of granule size. Examination of frequency histograms by a Kolmogorov-Smirnov analysis excluded the possibility that the antigenic sites of C-peptide and insulin are distributed in the same manner. Mean gold bead density for insulin antigen (97 +/- 24) was slightly higher than C-peptide density (64 +/- 18). The efficiency of labeling was found to be very low; less than 0.1% of the theoretically estimated antigenic sites were labeled.