Two novel bradykinin-related peptides (Ala3,Thr6)-bradykinin and (Val1,Thr3,Thr6)-bradykinin, were identified by a systematic sequencing study of peptides in the defensive skin secretion of the yellow-bellied toad, Bombina variegata. These peptides are the first amphibian skin bradykinins to exhibit amino acid substitutions at the Pro3 position of the bradykinin nonapeptide. Previously reported bradykinins from other Bombina species were not detected. Respective precursor cDNAs, designated BVK-1 and BVK-2, respectively, were cloned from a skin library by 3¢-and 5¢-RACE reactions. BVK-1 contained an open-reading frame of 97 amino acids encoding a single copy of (Ala3,Thr6)-bradykinin and similarly, the open-reading frame of BVK-2 consisted of 96 amino acids encoding a single copy of (Val1,Thr3,Thr6)-bradykinin. Synthetic replicates of each novel bradykinin were found to be active on mammalian arterial and small intestinal smooth muscle preparations. The structural diversity of bradykinins in amphibian defensive skin secretions may be related to defence against specific predators.Keywords: amphibian; bradykinin; skin; precursor; cloning.The skins of anuran amphibians, in addition to mucous glands, contain highly specialized poison glands, which, in reaction to stress or attack, exude a complex noxious cocktail of biologically active molecules [1]. These secretions often contain a plethora of peptides among which bradykinin or structural variants have been identified [2][3][4][5].The kallikrein-kininogen system appears to be a common target for certain venom components of many invertebrates and vertebrates. Among snakes, venoms from members of the Crotalinae (pit vipers) contain low molecular mass peptides that inhibit angiotensin-converting enzyme -a major bradykinin inactivating protease -thus potentiating the activity of endogenous bradykinin [6]. These bradykinin-potentiating peptides were the leads in developing angiotensin-converting enzyme inhibitor drugs that are frontline pharmaceuticals in the treatment of hypertension.Peptides with angiotensin-converting enzyme inhibitory activity have also been isolated from scorpion venom [7,8]. Wasp venoms contain bradykinin-related peptides that are responsible for the reddening, oedema and intense pain associated with envenomation by these insects [9,10]. These molecular strategies appear to be related to either potentiation of the effects of endogenous bradykinin by inhibition of catabolic proteases (snakes and scorpions) or by local delivery of supraphysiological quantities of bradykinins (wasps and amphibians).Although canonical bradykinin (RPPGFSPFR) has been isolated from the European common frog (Rana temporaria), with a range of minor N-and C-terminally extended forms [11,12], other species of ranid frog have been found to contain structural variants. These include (Thr6)-bradykinin and (Thr6)-bradykinyl-IAPEIV in R. rugosa and (Val1, Thr6)-bradykinin and (Val1,Thr6)-bradykinyl-VAPAS in R. nigromaculata [13,14]. Additional structural variants such as phylloki...
