Characterization of an insulin from the three-toed amphiuma (Amphibia: Urodela) with an N-terminally extended A-chain and high receptor-binding affinity

Insulin was isolated from an extract of the pancreas of a urodele, the three-toed amphiuma (Amphiuma tridactylum), and its primary structure established as Ala-Arg-Gly-Ile-Val-Glu-Gln-Cys-Cys-His¹⁰-Asn-Thr- Cys-Ser-Leu-Asn-Gln-Leu-Glu-Asn²⁰-Tyr-Cys-Asn for the A-chain and Ile-Thr-Asn-Gln-Tyr-Leu-Cys-Gly-Ser-His¹⁰-Leu-Val-Glu-Ala-Leu-Tyr-Leu-Val-Cys -Gly²⁰-Asp-Arg-Gly-Phe-Phe-Tyr-Ser-Pro-Lys for the B-chain. The N-terminus of the A-chain is extended by two amino acids (Ala-Arg) relative to all other known insulins suggesting an anomalous pathway of post-translational processing in the region of the C-peptide/A-chain junction of proinsulin. In common with chicken and Xenopus insulins, which contain a His(A8), amphiuma insulin was more potent (approx. 5-fold) than porcine insulin in inhibiting the binding of [¹²⁵I-Tyr(A14)]insulin to the soluble human insulin receptor from transfected 293EBNA cells (an adenovirus-transformed human kidney cell line). This result is consistent with previous data showing that insulin analogues extended at Gly(A1) by uncharged groups have reduced binding affinity whereas high affinity is preserved in analogues extended by basic amino acid residues.