Characterization of insulin and atypically processed proglucagon-derived peptides from the Surinam toad Pipa pipa (Anura:Pipidae)

Electrospray mass spectrometry was used to identify insulin, glucagon and two peptides related to glucagon-like peptide-1 (GLP-1) in an extract of the pancreas of the Surinam toad, Pipa pipa, a species belonging to the same family as the African clawed frog, Xenopus laevis. Purification and characterization of the peptides established the primary structure of Pipa insulin as A-chain: GIVEQCCHSS¹⁰CTLLQLETYC²⁰ N and B-Chain: FSNQR LCGSH¹⁰ LVEALHLVCG²⁰ DRGFFYYPKA³⁰. This amino acid sequence contains several substitutions (B5 His → Arg, B16 Tyr → His, A12 Ser → Thr, A14 Tyr→ Leu, A18Asn → Thr) of residues that have otherwise been quite strongly conserved during vertebrate evolution. Pipa glucagon comprises 37 amino acid residues (HSQGTFTSDY¹⁰ SKYLDSRRAQ²⁰ DFVQWLMNTK³⁰QSGGLSS) and the 29 amino-acid-residue peptide was not identified in the extract. In Xenopus and mammalian preproglucagons, the glucagon-29 sequence is followed by Lys-Arg which functions as a recognition site for a prohormone convertase. We propose that a point mutation in the gene encoding Pipa preproglucagon has transformed the Lys³⁰-Arg³¹ processing site into Lys-Gln with the result that the site in no longer recognized by the processing enzyme. In contrast, Pipa GLP-32 and GLP-37 are of the same molecular size as the corresponding peptides from Xenopus. Copyright (C) 2000 Elsevier Science Inc.