Primary structure of insulin and a truncated C-peptide from an elasmobranchian fish, Torpedo marmorata

Insulin has been isolated from the pancreas of Torpedo marmorata, an elasmobranchian fish, and shown to contain 21 amino acid residues in the A-chain and 30 residues in the B-chain. The sequence of insulin has been strongly conserved within the class Elasmobranchii with only one substitution and one deletion in the A chain and one substitution in the B-chain compared with insulin from the spiny dogfish, Squalus acanthias. A second peptide, present in the pancreatic extracts in approximately equimolar concentration with insulin, was identified as a heptadecapeptide. The sequence of this peptide shows homology to the N-terminal region of anglerfish (Lophius americanus) C-peptide at six of 17 sites. The isolation of a truncated C-peptide suggests either that the sequence encoding the COOH-terminal region of T. marmorata C-peptide has been deleted from the preproinsulin gene or that a larger C-peptide has undergone a proteolytic cleavage in the central portion of the molecule during packaging in the secretory granules of the B cell.