Purification and characterization of insulin from the Australian lungfish, Neoceratodus forsteri (Dipnoi)

The Australian lungfish Neoceratodus forsteri, a facultative air breather, is considered to be the most primitive of the extant Dipnoi and so occupies a uniquely important evolutionary position in the transition from fish to tetrapods. Insulin was isolated from an extract of the pancreas of N. forsteri and its primary structure established as: A-Chain, Gly-Ile-Val-Glu-Gln-Cys-Cys-His-Thr-Pro(10)-Cys-Ser-Leu-Tyr-Gln-Leu-G lu-Asn-Tyr-Cys(20)-Asn-Glu-Thr-Glu; B-Chain, Ala-Ala-Val-Asn-Gln-His-Leu-Cys-Gly-Ser(10)-His-Leu-Val-Glu-Ala-Leu- Tyr-Phe-Val-Cys(20)-Gly-Glu-Arg-Gly-Phe-Phe-Tyr-Leu-Pro- Lys(30)-Gly. This amino acid sequence is more similar to that of human insulin than to insulins from present-day amphibians. All the residues in human insulin that are considered to be important in receptor binding, dimerization, and hexamerization are conserved in lungfish insulin except for the substitution (Leu --> Phe) at the position corresponding to B17 in human insulin. Consistent with the assertion that the Dipnoi is a monophyletic group, insulins from N. forsteri and from the African lungfish Protopterus annectens contain extensions to the C-terminus of the A-chain and to the N-terminus of the B-chain that have not been found in other sarcopterygian species. However, the unusual amino acid substitutions found in insulin from P. annectens (e.g., GlyB21 --> Ala, GluB22 --> Asp, and ArgB23 --> Asn) are not present in N. forsteri insulin, suggesting that they occurred in the Protopterus lineage after divergence of the genera.