Peptides 2000 Sep 01;219:1355-60. doi: 10.1016/s0196-9781(00)00278-3.

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

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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: GIVEQCCHSS(10)CTLLQLETYC(20) N and B-Chain: FSNQR LCGSH(10) LVEALHLVCG(20) DRGFFYYPKA(30). 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 (HSQGTFTSDY(10) SKYLDSRRAQ(20) DFVQWLMNTK(30)QSGGLSS) 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 Lys(30)-Arg(31) 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.

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Species referenced: Xenopus laevis
Genes referenced: gcg ins