Purification and structural characterization of insulin from a caecilian, Typhlonectes natans (Amphibia: Gymnophiona)

Despite the important position of amphibia in phylogeny, efforts at the structural characterization of amphibian neurohormonal peptides have largely been confined to the Anurans (frogs and toads). Insulin was purified from an extract of the pancreas of the caecilian, Typhlonectes natans. The primary structure of the peptide was established as:

A-chain: Gly-Ile-Val-Glu-Lys⁵-Cys-Cys-Leu-Ser-Thr¹⁰-Cys-Ser-Leu-Tyr-Glu¹⁵-Leu-Glu-Ser-Tyr-Cys²⁰-Asn

B-chain: Ile-Ala-Asn-Gln-His⁵-Leu-Cys-Gly-Ser-His¹⁰-Leu-Val-Glu-Ala-Leu¹⁵-Tyr-Leu-Val-Cys-Ala²⁰-Asp-Arg-Gly-Phe- Phe²⁵-Tyr-Thr-Pro-Lys-Ser³⁰

This amino acid sequence contains several unusual substitutions (Gln → Lys at A5, His → Leu at A8, Gln → Glu at A15, and Gly → Ala at B20) that are not present in other amphibian insulins. The structure of insulin appears to be less well conserved among the different orders of amphibia, compared with reptiles and birds.