The immunological cross reactions of human, porcine and bovine proinsulins and C-peptides are described. In order to iodinate the C-peptides to high specific activity, tyrosine was coupled to the molecule using N-carboxytyrosyl anhydride. The tyrosylated bovine I-131-C-peptide was bound by bovine proinsulin antisera, but did not react with porcine proinsulin or insulin antisera, while the tyrosylated human-I-131-C-peptide did not react with high concentrations of these three antisera.

The cross reaction of human and porcine proinsulin with bovine proinsulin antisera was weak and resembled that of insulins of all three species. Similar results were observed with a porcine proinsulin antiserum. Removal of the insulin-binding antibodies from a proinsulin antiserumeliminated its reactivity with heterologous proinsulins.Using an assay system (bovine proinsulin antiserum: tyrosylated-I-131-C-peptide) which did not respond to insulin, human and porcine C-peptides reacted approximately 250 and 750 times less efficiently than the bovine molecule. These results indicate that the relatively large degree of variability in the amino acid sequence of the proinsulinconnecting segments gives rise to unique immunological determinants in the three species of proinsulin studied.

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