1. Amino acid sequence diversity of pancreatic polypeptide among the amphibia
J M Conlon, J E Platz, N Chartrel, H Vaudry, P F Nielsen Gen Comp Endocrinol. 1998 Nov;112(2):146-52. doi: 10.1006/gcen.1998.7153.
It has been suggested that the amino acid sequence of pancreatic polypeptide (PP) may provide a useful molecular marker with which to study evolutionary relationships between tetrapods but few PP sequences from amphibia are available to test this hypothesis. PPs have been purified from the pancreata of five species belonging to the different orders of amphibians. Their amino acid sequences were established as: APSEPEHPGD10 NASPDELAKY20 YSDLWQYITF30 VGRPRY for the lesser siren, Siren intermedia (Caudata); GPTEPIHPGK10 DATPEELTKY20 YSDLYDYITL30 VGRSRW for the caecilian, Typhlonectes natans (Gymnophiona); and TPSEPQHPGD10 QASPEQLAQY20 YSDLWQYITF30 VTRPRF for the cane toad, Bufo marinus (Anura). The structure of Rana sylvatica PP is the same as that of Rana catesbeiana PP whereas PP from the green frog Rana ridibunda contains one substitution (His6 --> Gln). The data provide further support for the conclusion that the amino acid sequence of PP has been poorly conserved during evolution with only 17 residues invariant among the eight species of amphibia yet studied and only 8 residues (Pro5, Pro8, Gly9, Ala12, Leu24, Tyr27, Arg33, and Arg35) invariant among all tetrapods. A maximum parsimony analysis based upon the amino acid sequence of PP and using the sequence of frog PYY as outgroup to polarize the in-group taxa generates a consensus phylogenetic tree in which the Amniota and Amphibia form two distinct clades. However, such a tree does not permit valid conclusions to be drawn regarding branching order within the Amphibia.
2. Isolation and structural characterization of proglucagon-derived peptides, pancreatic polypeptide, and somatostatin from the urodele Amphiuma tridactylum
E S Cavanaugh, P F Nielsen, J M Conlon Gen Comp Endocrinol. 1996 Jan;101(1):12-20. doi: 10.1006/gcen.1996.0003.
The expression of the preproglucagon gene in vertebrates is markedly species- and tissue-dependent. Three peptides derived from the posttranslational processing of preproglucagon were isolated from an extract of the pancreas of the urodele Amphiuma tridactylum (threetoed amphiuma). The primary structures of the peptides indicated identity with glucagon (HSQGTFTSDY10 SKYLDNRRAQ20 DFIQWLMST), glucagon-like peptide-1 (GLP-1) (HADGTLTSDI10 SSFLEKQATK20 EFIAWLVSGR30 GRRQ), and glucagon-like peptide-2 (GLP-2) (HADGSFTSDI10 NKVLDTIAAK20 EFLNWLISTK30 VTE). Thus, in a urodele, as in the bullfrog but in contrast to the chicken and all nontetrapod species yet studied, pancreatic preproglucagon mRNA encodes a GLP-2 sequence. The amino acid sequence of glucagon has been better conserved during evolution of tetrapods (3 substitutions between amphiuma and human) than the sequences of either GLP-1 (7 substitutions) or GLP-2 (15 substitutions). Pancreatic polypeptide was also isolated from the extract and its primary structure (APKEPEHPGD10 DASPEQLEKY20 YQDLFQYIIF30 ITRPRY.NH2) indicates that the amino acid sequence of this peptide has been very poorly conserved, even among the amphibia. Amphiuma pancreatic somatostatin is identical to mammalian somatostatin-14.
3. Insulin and proglucagon-derived peptides from the horned frog, Ceratophrys ornata (Anura:Leptodactylidae)
A M White, S M Secor, J M Conlon Gen Comp Endocrinol. 1999 Jul;115(1):143-54. doi: 10.1006/gcen.1999.7300.
Insulin and peptides derived from the processing of proglucagon have been isolated from an extract of the pancreas of the South American horned frog, Ceratophrys ornata (Leptodactylidae). Ceratophrys insulin is identical to the insulin previously isolated from the toad, Bufo marinus (Bufonidae). Ceratophrys glucagon was isolated in two molecular forms with 29- and 36-amino acid residues in approximately equal amounts. Glucagon-29 is identical to glucagon from B. marinus and from the bullfrog, Rana catesbeiana (Ranidae) and contains only 1 amino acid substitution (Thr29 --> Ser) compared with glucagon from Xenopus laevis (Pipidae). Glucagon-36 comprises glucagon-29 extended from its C-terminus by Lys-Arg-Ser-Gly-Gly-Met-Ser. This extension is structurally dissimilar to the C-terminal octapeptide of mammalian oxyntomodulin and resembles more closely that found in C-terminally extended glucagons isolated from fish pancreata. Ceratophrys glucagon-like peptide-1 (GLP-1) (His-Ala-Asp-Gly-Thr-Tyr-Gln-Asn-Asp-Val10-Gln-Gln-Phe-Leu-Glu- Glu-Lys-Ala-Ala-Lys20-Glu-Phe-Ile-Asp-Trp-Leu-Ile-Lys-Gly- Lys30-Pro-Lys-Lys-Gln-Arg-Leu-Ser) contains 3 amino acid substitutions compared with the corresponding peptide from B. marinus, 8 substitutions compared with GLP-1 from R. catesbeiana, and between 4 and 11 substitutions compared with the three GLP-1 peptides identified in X. laevis proglucagon. GLP-2 was not identified in the extract of Ceratophrys pancreas. The data indicate that, despite its importance in the regulation of glucose metabolism, the primary structure of GLP-1 has been very poorly conserved during evolution, even among a single order such as the Anura.
