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Abstract
Bradykinin-related peptides (BRPs) are significant components of the defensive skin secretions of many anuran amphibians, and these secretions represent the source of the most diverse spectrum of such peptides so far encountered in nature. Of the many families of bioactive peptides that have been identified from this source, the BRPs uniquely appear to represent homologues of counterparts that have specific distributions and receptor targets within discrete vertebrate taxa, ranging from fishes through mammals. Their broad spectra of actions, including pain and inflammation induction and smooth muscle effects, make these peptides ideal weapons in predator deterrence. Here, we describe a novel 12-mer BRP (RVALPPGFTPLR-RVAL-(L1, T6, L8)-bradykinin) from the skin secretion of the Fujian large-headed frog (Limnonectes fujianensis). The C-terminal 9 residues of this BRP (-LPPGFTPLR) exhibit three amino acid substitutions (L/R at Position 1, T/S at Position 6 and L/F at Position 8) when compared to canonical mammalian bradykinin (BK), but are identical to the kinin sequence present within the cloned kininogen-2 from the Chinese soft-shelled turtle (Pelodiscus sinensis) and differ from that encoded by kininogen-2 of the Tibetan ground tit (Pseudopodoces humilis) at just a single site (F/L at Position 8). These data would imply that the novel BRP is an amphibian defensive agent against predation by sympatric turtles and also that the primary structure of the avian BK, ornithokinin (RPPGFTPLR), is not invariant within this taxon. Synthetic RVAL-(L1, T6, L8)-bradykinin was found to be an antagonist of BK-induced rat tail artery smooth muscle relaxation acting via the B2-receptor.
Highlights
Bradykinins (BKs) and related peptides (BRPs) are among the most abundant and structurally-diverse group of pharmacologically-active peptides present in anuran amphibian defensive skin secretions [1,2]
Bradykinin-related peptides (BRPs) of amphibian skin secretions are synthesised within the granular gland cells, and their biosynthetic precursors possess the typical organisation observed for other skin secretion peptides with the active
Further bioinformatic analyses of kininogen sequences archived in the NCBI database (March, 2014) with this in mind and focusing on their bradykinin-encoding domains revealed the presence of a third variant BRP in birds—(Thr6)-BK—once considered the archetypal BRP in crocodilians and testudines [2]
Summary
Bradykinins (BKs) and related peptides (BRPs) are among the most abundant and structurally-diverse group of pharmacologically-active peptides present in anuran amphibian defensive skin secretions [1,2]. Following from the initial discovery of canonical mammalian bradykinin (BK) in the skin of the European brown frog, Rana temporaria, many site-substituted, truncated and/or N- and C-terminally-extended peptides have been isolated from the skins/skin secretions of representative species from the families, Ranidae, Hylidae, Bombinatoridae and Leiopelmatidae, with ranid frogs having the most diverse range of BRPs [3,4,5,6,7,8,9] It has become increasingly clear, following the application of skin peptide biosynthetic precursor-encoding cDNA cloning techniques to this field of research, that there is a high degree of variability in mature BRP primary structures, propeptide convertase processing sites and copy numbers of BRP-encoding domains and their locations, within such precursor proteins. C-terminal nonapeptide sequence (-LPPGFTPLR) of this novel peptide produced several results of general interest to molecular evolutionary biologists, which will be discussed
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