Host-defence peptides from the glandular secretions of amphibians: structure and activity

Kasstasin: A novel potent vasoconstrictor peptide from the skin secretion of the African red-legged running frog, Kassina maculata

Amphibian skin secretions are established sources of bioactive peptides. Here we describe the isolation, structural and pharmacological characterisation of a novel vasoconstrictor peptide from the skin secretion of the African hyperoliid frog, Kassina maculata, which exhibits no structural similarity to any known class of amphibian skin peptide. The peptide consists of 21 amino acid residues, FIKELLPHLSGIIDSVANAIK, and is C-terminally amidated. The provisional structure was obtained by MS/MS fragmentation using an Orbitrap mass spectrometer and L/I ambiguities were resolved following molecular cloning of biosynthetic precursor-encoding cDNA. A synthetic replicate of the peptide was found to possess weak antimicrobial and haemolytic activities but was exceptionally effective in constricting the smooth muscle of rat tail artery (EC(50) of 25 pM). In reflection of its exceptional potency in constricting rat arterial smooth muscle, the peptide was named kasstasin, a derivation of Kassina and "stasis" (stoppage of flow). These data illustrate the continuing potential of amphibian skin secretions to provide novel natural peptide templates for biological evaluation.

Molecular cloning of novel antimicrobial peptide genes from the skin of the Chinese brown frog, Rana chensinensis

One species of the Chinese brown frog, Rana chensinensis, is widely distributed in north-central China. In this study, a cDNA library was constructed to clone the antimicrobial peptides' genes from the skin of R. chensinensis. Twenty-three prepropeptide cDNA sequences encoding twelve novel mature antimicrobial peptides were isolated and characterized. Six peptides belonged to three known families previously identified from other Ranid frogs: temporin (4 peptides), brevinin-2 (1 peptide), and palustrin-2 (1 peptide). The other six peptides showed little similarity to known antimicrobial peptides. According to the amino acid sequences, with or without α-helix structure, and either hydrophilic or hydrophobic, these were organized into four new families: chensinin-1 (3 peptides), chensinin-2 (1 peptide), chensinin-3 (1 peptide), and chensinin-4 (1 peptide). Five peptides from different families were chemically synthesized, and their antimicrobial, cytolytic, and hemolytic activities were evaluated. Of these, brevinin-2CE showed strongest antimicrobial activities against both the Gram-positive and Gram-negative bacteria with a slight hemolysis. Temporin-1CEe and palustrin-2CE also displayed a slight hemolysis, but they had different activities to prokaryotic cells. Temporin-1CEe showed higher antimicrobial activity against Gram-positive bacteria than Gram-negative bacteria, whereas it was contrary to palustrin-2CE. Chensinin-1 CEb and chensinin-3CE only had moderate antimicrobial activity against microorganisms. In addition, the brevinin-2 peptides from different brown frogs were analyzed to reveal the taxonomy and phylogenetic relationships of R. chensinensis.

Histidine-containing host-defence skin peptides of anurans bind Cu2+. An electrospray ionisation mass spectrometry and computational modelling study

Anuran peptides which contain His, including caerin 1.8 (GLFKVLGSVAKHLLPHVVPVIAEKL-NH(2)), caerin 1.2 (GLLGVLGSVAKHVLPHVVPVIAEHL-NH(2)), Ala(15) maculatin 1.1 (GLFGVLAKVAAHVVAIEHF-NH(2)), fallaxidin 4.1 (GLLSFLPKVIGHLIHPPS-OH), riparin 5.1 (IVSYPDDAGEHAHKMG-NH(2)) and signiferin 2.1 (IIGHLIKTALGMLGL-NH(2)), all form MMet(2+) and (M + Met(2+)-2H(+))(2+) cluster ions (where Met is Cu, Mg and Zn) following electrospray ionisation (ESI) in a Waters QTOF 2 mass spectrometer. Peaks due to Cu(II) complexes are always the most abundant relative to other metal complexes. Information concerning metal(2+) connectivity in a complex has been obtained (at least in part) using b and y fragmentation data from ESI collision-induced dissociation tandem mass spectrometry (CID MS/MS). Theoretical calculations, using AMBER version 10, show that MCu(2+) complexes with the membrane active caerin 1.8, Ala(15) maculatin 1.1 and fallaxidin 4.1 are four-coordinate and approximating square planar, with ligands including His and Lys, together with the carbonyl oxygens of particular backbone amide groups. When binding can occur through two His, or one His and one Lys, the His/Lys ligand structure is the more stable for the studied systems. The three-dimensional (3D) structures of the complexes are always different from the previously determined structures of the uncomplexed model peptides (using 2D nuclear magnetic resonance (NMR) spectroscopy in membrane-mimicking solvents like trifluoroethanol/water).

Mass spectrometric study of bradykinin-related peptides (BRPs) from the skin secretion of Russian ranid frogs

Amphibian skin secretion is known to contain biologically active peptides. Bradykinins and related peptides (BRPs) can be found in these animals, while frogs from the genus Rana are considered to be leaders in the levels and variety of these peptides. A reasonable rationalization of this fact is that bradykinins are efficient defense compounds against predators. Forty-four various BRPs have been identified in the skin secretions of five ranid frog species (R. ridibunda, R. lessonae, R. esculenta, R. temporaria, R. arvalis) from the Zvenigorod region (Moscow district, Russia). Some of these peptides are already known, but the novel ones constitute a significant portion. An interesting group of novel peptides was isolated from R. lessonae. These are bradykinin analogues bearing a tyrosine residue in the 5th or 8th position. [Arg(0), Trp(5), Leu(8)]bradykinin and [Thr(6), Leu(8)]bradykinin that had been isolated from fish and avian species, respectively, were also detected in the frog secretion, supporting the predator defense hypothesis. Furthermore, a novel group of BRPs named 'lessonakinins' was discovered in R. lessonae and R. esculenta. All of them include the [Arg(0), Trp(5), Leu(8)]bradykinin sequence and have some structural resemblance to the precursor of this peptide cloned by Chen and coworkers recently. However, the C-terminal part of the lessonakinins does not match the sequence predicted by Chen, demonstrating possible incompleteness of information obtained by cDNA cloning.

Antimicrobial peptides from amphibians

Increased prevalence of multi-drug resistance in pathogens has encouraged researchers to focus on finding novel forms of anti-infective agents. Antimicrobial peptides (AMPs) found in animal secretions are components of host innate immune response and have survived eons of pathogen evolution. Thus, they are likely to be active against pathogens and even those that are resistant to conventional drugs. Many peptides have been isolated and shown to be effective against multi-drug resistant pathogens. More than 500 AMPs have been identified from amphibians. The abundance of AMPs in frog skin is remarkable and constitutes a rich source for design of novel pharmaceutical molecules. Expression and post-translational modifications, discovery, activities and probable therapeutic application prospects of amphibian AMPs will be discussed in this article.

Importance of residue 13 and the C-terminus for the structure and activity of the antimicrobial peptide aurein 2.2

Previous studies on aurein 2.2 and 2.3 in DMPC/DMPG and POPC/POPG membranes have shown that bilayer thickness and phosphatidylglycerol content have a significant impact on the interaction of these peptides with membrane bilayers. Further examination with the DiSC(3)5 assay has indicated that aurein 2.2 induces greater membrane leakage than aurein 2.3 in Staphylococcus aureus C622. The only difference between these peptides is a Leu to Ile mutation at residue 13. To better understand the importance of this residue, the structure and activity of the L13A, L13F, and L13V mutants were investigated. In addition, we investigated a number of peptides with truncations at the C-terminus to determine whether the C-terminus, which contains residue 13, is crucial for antimicrobial activity. Solution circular dichroism results demonstrated that the L13F mutation and the truncation of the C-terminus by six residues resulted in decreased helical content, whereas the L13A or L13V mutation and the truncation of the C-terminus by three residues showed little to no effect on the structure. Oriented circular dichroism results demonstrated that only an extensive C-terminal truncation reduced the ability of the peptide to insert into lipid bilayers. (31)P NMR spectroscopy showed that all peptides disorder the headgroups. The implications of these results in terms of antimicrobial activity and the ability of these peptides to induce leakage in S. aureus are discussed. The results suggest that the presence of the 13th residue in aurein 2.2 is important for structure and activity, but the exact nature of residue 13 is less important as long as it is a hydrophobic residue.

Skin peptides of different life stages of Ewing's tree frog

In frogs, an important mechanism of skin innate immunity against invading microbial pathogens is secretion of antimicrobial peptides from the specialized granular glands. Since these glands develop fully in skin dermis after completion of metamorphosis, they are small and immature in skin of larvae (tadpoles). Skin secretions vary among different life stages. Antimicrobial activity and peptide composition of natural mixture of skin peptides of three different life stages of New Zealand Ewing's Tree Frog (Litoria ewingii), tadpoles, metamorphs and adults were analyzed. The peptide mixtures were collected from skin secretions and analyzed for activity against the standard reference bacterium, Escherichia coli (ATCC 25922). Their peptide components were analyzed using liquid chromatography mass spectrometry (LC-MS). The peptide mixture from adults and metamorphs contained the species-specific antimicrobial peptide uperin 7.1 and inhibited the growth of E. coli (ATCC 25922). In contrast, the peptide mixture of tadpoles did not inhibit the growth of E. coli (ATCC 25922). This peptide mixture did not contain uperin 7.1 but had peptides whose molecular masses did not correspond to molecular masses of any known frog antimicrobial peptides.

Skin peptide and cDNA profiling of Australian anurans: genus and species identification and evolutionary trends

Host defense peptides of 35 species of Australian frogs from the hylids Cyclorana and Litoria, and the myobatrachids Crinia, Limnodynastes and Uperoleia have been identified. The biological activities of the majority of these peptides have been determined and include hormones, neuropeptides, opioids, immunomodulators, membrane active peptides [including antimicrobial, anticancer, antiviral (enveloped viruses like HIV and Herpes) and antifungal peptides], neuronal nitric oxide synthase inhibitors, pheromones and individual peptides with other specific activities. The host defense peptide skin profile can be diagnostic at both the species and higher taxonomic levels; for example, species of Crinia, Litoria and Uperoleia each produce quite different types of peptides. Species of Cyclorana and Limnodynastes are more difficult to characterize by skin peptides alone: species of both genera produce similar peptides with no apparent activity. The skin peptide profiles of frogs from the genera Crinia, Litoria and Uperoleia may be used together with morphological and cognate methods, to differentiate between sub-species and even different population clusters of the same species. Nucleotide sequencing of cDNAs of precursors (pre-pro peptides) of bioactive peptides from the skin glands of various species of the genus Litoria show that the majority of these peptides originated from a single ancestor gene before the break away of Australia from Gondwana. The exceptions are the caerulein neuropeptides {e.g. caerulein [pEQDY(SO(3)H)TGWMDF(NH(2))]} which have a different origin to that of other Litoria peptides. Disulfide containing peptides from skin glands of species of Crinia show a different evolutionary route to peptides from species of Litoria.

Evidence for convergent evolution in the antimicrobial peptide system in anuran amphibians

Amphibians are characterized in part by their highly specialized and glandular skin that enables key physiological functions such as cutaneous respiration and defense against a variety of micro- and macroscopic predators via toxic components (e.g., alkaloids and bufodienolids), biogenic amines, neuropeptides and antimicrobial peptides (AMPs). To date, DNA sequence information regarding AMP genes in anurans is restricted to only a few anuran families and largely to "higher frogs" (Neobatrachia). Here, we analyze the DNA information for the signal sequences of the AMP precursors in anuran amphibians available to the end of 2009 in an explicit phylogenetic framework to characterize the evolution of this large, diverse gene family. Comparison of cDNA sequences suggests that there are at least three different motifs within the signal peptide sequence of the AMP-precursor corresponding to the evolutionary lineages Neobatrachia, Bombinatoridae (Bombina spp.) and Pipidae (Xenopus laevis). The signal sequences are strongly conserved within each lineage (as previously noted for Neobatrachia), but highly divergent between them. Together with the lack of a linear relationship between the degree of sequence divergence and evolutionary time, we hypothesize that the anuran AMP system has evolved convergently on at least three occasions. However, additional sampling, especially among the largely poorly sampled non-neobatrachian lineages, is required to confirm this hypothesis and could reveal the existence of additional signal sequence motifs.

Matrix-assisted laser desorption/ionization-post source decay fragmentation of cystine- containing amphibian peptides with novel cysteine tags

Long disulphide-containing peptides brevinins 1E and 2Ec from the skin secretion of the frog Rana ridibunda were reduced and alkylated with ten novel and three known derivatizing agents. Nine of novel reagents are maleimide derivatives. The peptides were also reduced with DTT directly onto the MALDI target without alkylation. Modified samples were subjected to MALDI-PSD study. Procedures, fragmentation patterns, fragment ion signal abundances and sequence coverage for two peptides modified with thirteen tags (or on-plate reduced) are described. The fast on-plate procedure for reduction/alkylation was applied to Rana ridibunda crude secretion, providing intensive signals of derivatized peptides. The corresponding ions may be used for the MS/MS sequencing procedure.

A revisão do Código Florestal Brasileiro: impactos negativos para a conservação dos anfíbios

Nos últimos meses está em efervescente discussão uma proposta para que um novo código florestal (Projeto de Lei nº 1876/99) substitua o vigente instaurado por meio da Lei Federal nº 4771/65 e modificado recentemente por Medidas Provisórias. Este novo código proposto, todavia, propõe alterações legais que devem afetar negativamente as populações naturais de anfíbios do Brasil. O declínio de anfíbios deve gerar impactos negativos tanto para a população (humana) nacional, como para a comunidade internacional. Entre os possíveis efeitos dessa lei, em consequência da perda de diversidade de anfíbios, podemos citar o aumento nos custos de produção agrícola, perda de matéria prima para produção de remédios, descontrole ecológico, eutrofização de corpos d'água, encarecimento do custo do tratamento de água para abastecimento humano, aumento de pragas agrícolas e aumento de doenças transmitidas por insetos vetores. Isto tudo é bastante preocupante ainda mais se levarmos em conta a própria perda da biodiversidade de anfíbios, o grupo de vertebrados terrestres mais ameaçado do planeta. Sendo assim, ressaltamos a necessidade de que, caso um novo código florestal seja elaborado, este tenha embasamento também em questões técnicas e científicas que impreterivelmente afetam, não só a conservação da natureza, mas também a economia, saúde e bem estar das populações humanas.

Characterization of a peptide family from the skin secretion of the Middle East tree frog Hyla savignyi by composition-based de novo sequencing

A new tryptophyllin-like peptide family was found in the skin secretion of the tree frog Hyla savignyi. Peptides were characterized by database-independent sequencing strategies and specific ion fragmentation features were investigated. Skin secretions from specimens of Hyla savignyi were collected by mild electrical stimulation. Peptides were separated by reversed-phase nano-high-performance liquid chromatography (nanoHPLC) and mass spectra were acquired online by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Peptides were characterized by manual de novo sequencing and by composition-based sequencing (CBS), appearing mostly as C-terminal free acids and as their acid amide analogs. Amide peptides yielded lower intensities of y-type ions after collision-induced dissociation (CID) than their acid analogs. A mechanism of internal b-ion formation (positive ion mode) and of CO(2) elimination (negative ion mode) is proposed. We also exemplified phenomena such as the proline effect and formation of non-direct sequence ions after sequence rearrangements. The occurrence of rearrangement products, of internal ions and of the proline effect made the CID spectra highly complex. CBS analysis nevertheless resulted in successful and highly reliable sequence analysis.

Isolation and sequence analysis of peptides from the skin secretion of the Middle East tree frog Hyla savignyi

Novel peptides were identified in the skin secretion of the tree frog Hyla savignyi. Skin secretions were collected by mild electrical stimulation. Peptides were separated by reversed-phase high-performance liquid chromatography. Mass spectra were acquired by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS), and fragment ion spectra were obtained after collision-induced dissociation and electron capture dissociation. Peptides were analyzed by manual de novo sequencing and composition-based sequencing (CBS). Sequence analyses of three so far undescribed, structurally unrelated peptides are presented in this paper, having the sequences DDSEEEEVE-OH, P*EEVEEERJK-OH, and GJJDPJTGJVGGJJ-NH(2). The glutamate-rich sequences are assumed to be acidic spacer peptides of the prepropeptide. One of these peptides contains the modified amino acid hydroxyproline, as identified and localized by high-accuracy FTICR-MS. Combination of CBS and of experience-based manual sequence analysis as complementary and database-independent sequencing strategies resulted in peptide identification with high reliability.

The contribution of skin antimicrobial peptides to the system of innate immunity in anurans

Cationic peptides with the propensity to adopt an amphipathic α-helical conformation in a membrane-mimetic environment are synthesized in the skins of many species of anurans (frogs and toads). These peptides frequently display cytolytic activities against a range of pathogenic bacteria and fungi consistent with the idea that they play a role in the host's system of innate immunity. However, the importance of the peptides in the survival strategy of the animal is not clearly understood. It is a common misconception that antimicrobial peptides are synthesized in the skins of all anurans. In fact, the species distribution is sporadic suggesting that their production may confer some evolutionary advantage to the organism but is not necessary for survival. Although growth inhibitory activity against the chytrid fungus Batrachochytrium dendrobatidis, responsible for anuran population declines worldwide, has been demonstrated in vitro, the ability of frog skin antimicrobial peptides to protect the animal in the wild appears to be limited and there is no clear correlation between their production by a species and its resistance to fatal chytridiomycosis. The low potency of many frog skin antimicrobial peptides is consistent with the hypothesis that cutaneous symbiotic bacteria may provide the major system of defense against pathogenic microorganisms in the environment with antimicrobial peptides assuming a supplementary role in some species.

Novel natural peptides from Hyla arborea schelkownikowi skin secretion

Hyla arborea schelkownikowi is one of the leaf frog species inhabiting the southern territories of Russia and the former USSR. This frog species is a member of the Hylidae Rafinesque, 1815 batrachians family. The present study deals with the previously uninvestigated peptidome of the Hyla arborea schelkownikowi skin secretion. Nano-electrospray ionization Fourier transform mass spectrometry (nanoESI-FTMS) of the skin secretion, in the intact form and after acetylation, was selected as the general method of analysis. Electron-capture dissociation (ECD) and collision-induced dissociation (CID) fragmentation were both employed, while de novo sequencing was performed by manual interpretation of the MS data. The suppression of the cyclization of b-ions in the mass spectrometer by the acetylation reaction proved to be very efficient for the de novo sequencing of short peptides. Ten skin peptides were found and all of them, except for bradykinin, had not previously been reported. Six of the peptides belong to the tryptophyllins and related peptides, while three peptides are similar to the aureins.

Anti-proliferative and cytotoxic activity of pentadactylin isolated from Leptodactylus labyrinthicus on melanoma cells

Nowadays, the emergence of resistance to the current available chemotherapeutic drugs by cancer cells makes the development of new agents imperative. The skin secretion of amphibians is a natural rich source of antimicrobial peptides (AMP), and researchers have shown that some of these wide spectrum molecules are also toxic to cancer cells. The aim of this study was to verify a putative anticancer activity of the AMP pentadactylin isolated for the first time from the skin secretion of the frog Leptodactylus labyrinthicus and also to study its cytotoxic mechanism to the murine melanoma cell line B16F10. The results have shown that pentadactylin reduces the cell viability of B16F10 cells in a dose-dependent manner. It was also cytotoxic to normal human fibroblast cells; nevertheless, pentadactylin was more potent in the first case. The studies of action mechanism revealed that pentadactylin causes cell morphology alterations (e.g., round shape and shrinkage morphology), membrane disruption, DNA fragmentation, cell cycle arrest at the S phase, and alteration of mitochondrial membrane potential, suggesting that B16F10 cells die by apoptosis. The exact mechanism that causes reduction of cell viability and cytotoxicity after treatment with pentadactylin is still unknown. In conclusion, as cancer cells become resilient to death, it is worthwhile the discovery of new drugs such as pentadactylin that induces apoptosis.

Myxinidin, a novel antimicrobial peptide from the epidermal mucus of hagfish, Myxine glutinosa L

Fish epidermal mucus contains innate immune components that provide a first line of defense against various infectious pathogens. This study reports the bioassay-guided fractionation and characterization of a novel antimicrobial peptide, myxinidin, from the acidic epidermal mucus extract of hagfish (Myxine glutinosa L.). Edman sequencing and mass spectrometry revealed that myxinidin consists of 12 amino acids and has a molecular mass of 1,327.68 Da. Myxinidin showed activity against a broad range of bacteria and yeast pathogens at minimum bactericidal concentration (MBC) ranging from 1.0 to 10.0 microg/mL. Screened pathogens, Salmonella enterica serovar Typhimurium C610, Escherichia coli D31, Aeromonas salmonicida A449, Yersinia ruckeri 96-4, and Listonella anguillarum 02-11 were found to be highly sensitive to myxinidin at the MBC of 1.0-2.5 microg/mL; Staphylococcus epidermis C621 and yeast (Candida albicans C627) had an MBC of 10.0 microg/mL. The antimicrobial activity of myxinidin was found to be two to 16 times more active than a potent fish-derived antimicrobial peptide, pleurocidin (NRC-17), against most of the screened pathogens. The microbicidal activity of myxinidin was retained in the presence of sodium chloride (NaCl) at concentrations up to 0.3 M and had no hemolytic activity against mammalian red blood cells. These results suggest that myxinidin may have potential applications in fish and human therapeutics.

The host-defence skin peptide profiles of Peron's Tree Frog Litoria peronii in winter and summer. Sequence determination by electrospray mass spectrometry and activities of the peptides

Positive and negative ion electrospray mass spectrometry together with Edman sequencing (when appropriate) has been used to sequence the host-defence peptides secreted from skin glands of the tree frog Litoria peronii. The peptide profiles are different in winter and summer. In winter, the frog produces small amounts of the known caerin 1.1 [GLLSVLGSVAKHVLPHVVPVIAEHL-NH(2)] (a wide-spectrum antibiotic) and caerin 2.1 [GLVSSIGRALGGLLADVVKSKQPA-OH], a narrow-spectrum antibiotic and an inhibitor of neuronal nitric oxide synthase. The major peptides produced throughout the year are the pGlu-containing peroniins 1.1 to 1.5 (e.g. peroniin 1.1 [pEPWLPFG-NH(2)], a smooth muscle contractor from 10(-7) M), and caerulein [pEQDY(SO(3)H)TGWMDF-NH(2)], a known and potent smooth muscle contractor from 10(-10) M. There are also some precursors to the peroniin 1 peptides, only detected in the skin secretion in summer, which are inactive and appear to be all (or part) of the spacer peroniin 1 peptides, e.g. peroniin 1.1b [SEEEKRQPWLPFG-NH(2)]. There are three members of the Litoria peronii Group of tree frogs classified in Australia, namely, L. peronii, L. rothii and L.tyleri. A comparison of the skin peptide profiles of L. peronii with those reported previously for L. rothii suggests that either these two species of tree frog are not as closely related as determined previously on morphological grounds, or that skin peptide divergence in tree frogs of this Group is more extensive than in others that have been studied.

Purification, characterization and cloning of two novel tigerinin-like peptides from skin secretions of Fejervarya cancrivora

While investigating the innate defense of brackish water-living amphibian and its comparison with freshwater-living amphibians, two novel 12-residue antimicrobial peptides were purified from the skin secretions of the crab-eating frog, Fejervarya cancrivora which typically inhabits brackish water of mangrove forests of Southeast Asia. These two antimicrobial peptides, tigerinin-RC1 and -RC2 share significant structural similarity with tigerinins found in the skin of Indian frog, Hoplobatrachus tigerinus. cDNAs encoding tigerinin-RC1 and -RC2 were also cloned from the skin cDNA library of F. cancrivora. Tigerinin-RC precursors are composed of 71 amino acid residues including a signal peptide, acidic spacer peptide, which are very similar to other amphibian antimicrobial peptide precursors and mature tigerinin-RC. The current results confirmed that both amphibians inhabiting freshwater and brackish water share the same antimicrobial peptide family to exert innate defense. Furthermore, the current work was also the first report of precursor and cDNA cloning of the tigerinin antimicrobial peptide family.

Negative ion fragmentations of deprotonated peptides. The unusual case of isoAsp: a joint experimental and theoretical study. Comparison with positive ion cleavages

The following peptides have been examined in this study: GLDFG(OH), caeridin 1.1 [GLLDGLLGLGGL(NH(2))], 11 Ala citropin 1.1 [GLFDVIKKVAAVIGGL(NH(2))], Crinia angiotensin [APGDRIYVHPF(OH)] and their isoAsp isomers. It is not possible to differentiate between Asp- and isoAsp-containing peptides (used in this study) using negative ion electrospray mass spectrometry. This is because the isoAsp residue cleaves to give the same fragment anions as those formed by delta and gamma backbone cleavage of Asp. The isoAsp fragmentations are as follows: RNHCH(CO(2)H)(-)CHCONHR' --> [RNH(-)(HO(2)CCH=CHCONHR')] --> RNH(-)+HO(2)CCH=CHCONHR' and RNHCH(CO(2)H)(-)CHCONHR' --> [RNH(-)(HO(2)CCH=CHCONHR'] --> (-)O(2)CCH=CHCONHR'+RNH(2). Calculations at the HF/6-31+G(d)//AM1 level of theory indicate that the first of these isoAsp cleavage processes is endothermic (by +115 kJ mol(-1)), while the second is exothermic (-85 kJ mol(-1)). The barrier to the highest transition state is 42 kJ mol(-1). No diagnostic cleavage cations were observed in the electrospray mass spectra of the MH(+) ion of the Asp- and isoAsp-containing peptides (used in this study) to allow differentiation between these two amino acid residues.

Mass spectrometric study of peptides secreted by the skin glands of the brown frog Rana arvalis from the Moscow region

A high-performance liquid chromatography nano-electrospray ionization Fourier transform mass spectrometry (HPLC/nanoESI-FTMS) approach involving recording of collision-activated dissociation (CAD) and electron-capture dissociation (ECD) spectra of an intact sample and two its modifications after performic oxidation and reduction followed by carboxamidomethylation helps to establish peptide profiles in the crude secretion of frog species at mid-throughput level, including de novo sequencing. The proposed derivatization procedures allow increasing of the general sequence coverage in the backbone, providing complementary information and, what is more important, reveal the amino acid sequence in the cystine ring ('rana box'). Thus purely mass spectrometric efficient sequencing becomes possible for longer than usual proteolytic peptides. Seventeen peptides belonging to four known families were identified in the secretion of the European brown frog Rana arvalis inhabiting the Moscow region in Russia. Ranatuerins, considered previously a unique feature of the North American species, as well as a new melittin-related peptide, are worth special mention. The developed approach was previously successfully used for the identification of peptides in the skin secretion of the Caucasian green frog Rana ridibunda.

Skin peptides from anurans of the Litoria rubella Group: sequence determination using electrospray mass spectrometry. Opioid activity of two major peptides

Many species of frogs of the genus Litoria secrete bioactive peptides from their skin glands. These peptides are normally host-defence compounds and may have one, or more of the following activities; smooth muscle contraction, analgesic, antimicrobial, antiviral, lymphocyte proliferator (immunomodulator) and neuronal nitric oxide synthase (nNOS) inactivation. Two frog species of the Litoria rubella Group that have been studied before, namely, Litoria electrica and Litoria rubella, are different from other species of the genus Litoria in that they produce small peptides that show neither membrane, lymphocyte nor nNOS activity. In this study we have used electrospray mass spectrometry together with Edman sequencing to identify eight skin peptides of the third member of this Group, Litoria dentata: surprisingly, none of these peptides show activity in our biological screening program. However, two major peptides (FPWL-NH(2) and FPWP-NH(2)) from L. electrica and L. rubella are opioids at the micromolar concentration.

The role of amphibian antimicrobial peptides in protection of amphibians from pathogens linked to global amphibian declines

Amphibian species have experienced population declines and extinctions worldwide that are unprecedented in recent history. Many of these recent declines have been linked to a pathogenic skin fungus, Batrachochytrium dendrobatidis, or to iridoviruses of the genus Ranavirus. One of the first lines of defense against pathogens that enter by way of the skin are antimicrobial peptides synthesized and stored in dermal granular glands and secreted into the mucus following alarm or injury. Here, I review what is known about the capacity of amphibian antimicrobial peptides from diverse amphibians to inhibit B. dendrobatidis or ranavirus infections. When multiple species were compared for the effectiveness of their in vitro antimicrobial peptides defenses against B. dendrobatidis, non-declining species of rainforest amphibians had more effective antimicrobial peptides than species in the same habitat that had recently experienced population declines. Further, there was a significant correlation between the effectiveness of the antimicrobial peptides and resistance of the species to experimental infection. These studies support the hypothesis that antimicrobial peptides are an important component of innate defenses against B. dendrobatidis. Some amphibian antimicrobial peptides inhibit ranavirus infections and infection of human T lymphocytes by the human immunodeficiency virus (HIV). An effective antimicrobial peptide defense against skin pathogens appears to depend on a diverse array of genes expressing antimicrobial peptides. The production of antimicrobial peptides may be regulated by signals from the pathogens. However, this defense must also accommodate potentially beneficial microbes on the skin that compete or inhibit growth of the pathogens. How this delicate balancing act is accomplished is an important area of future research.

Leptoglycin: a new Glycine/Leucine-rich antimicrobial peptide isolated from the skin secretion of the South American frog Leptodactylus pentadactylus (Leptodactylidae)

Antimicrobial peptides are components of innate immunity that is the first-line defense against invading pathogens for a wide range of organisms. Here, we describe the isolation, biological characterization and amino acid sequencing of a novel neutral Glycine/Leucine-rich antimicrobial peptide from skin secretion of Leptodactylus pentadactylus named leptoglycin. The amino acid sequence of the peptide purified by RP-HPLC (C(18) column) was deduced by mass spectrometric de novo sequencing and confirmed by Edman degradation: GLLGGLLGPLLGGGGGGGGGLL. Leptoglycin was able to inhibit the growth of Gram-negative bacteria Pseudomonas aeruginosa, Escherichia coli and Citrobacter freundii with minimal inhibitory concentrations (MICs) of 8 microM, 50 microM, and 75 microM respectively, but it did not show antimicrobial activity against Gram-positive bacteria (Staphylococcus aureus, Micrococcus luteus and Enterococcus faecalis), yeasts (Candida albicans and Candida tropicalis) and dermatophytes fungi (Microsporum canis and Trichophyton rubrum). No hemolytic activity was observed at the 2-200 microM range concentration. The amino acid sequence of leptoglycin with high level of glycine (59.1%) and leucine (36.4%) containing an unusual central proline suggests the existence of a new class of Gly/Leu-rich antimicrobial peptides. Taken together, these results suggest that this natural antimicrobial peptide could be a tool to develop new antibiotics.

Effect of membrane composition on antimicrobial peptides aurein 2.2 and 2.3 from Australian southern bell frogs

The effects of hydrophobic thickness and the molar phosphatidylglycerol (PG) content of lipid bilayers on the structure and membrane interaction of three cationic antimicrobial peptides were examined: aurein 2.2, aurein 2.3 (almost identical to aurein 2.2, except for a point mutation at residue 13), and a carboxy C-terminal analog of aurein 2.3. Circular dichroism results indicated that all three peptides adopt an alpha-helical structure in the presence of a 3:1 molar mixture of 1,2-dimyristoyl-sn-glycero-3-phosphocholine/1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPC/DMPG), and 1:1 and 3:1 molar mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPC/POPG). Oriented circular dichroism data for three different lipid compositions showed that all three peptides were surface-adsorbed at low peptide concentrations, but were inserted into the membrane at higher peptide concentrations. The (31)P solid-state NMR data of the three peptides in the DMPC/DMPG and POPC/POPG bilayers showed that all three peptides significantly perturbed lipid headgroups, in a peptide or lipid composition-dependent manner. Differential scanning calorimetry results demonstrated that both amidated aurein peptides perturbed the overall phase structure of DMPC/DMPG bilayers, but perturbed the POPC/POPG chains less. The nature of the perturbation of DMPC/DMPG bilayers was most likely micellization, and for the POPC/POPG bilayers, distorted toroidal pores or localized membrane aggregate formation. Calcein release assay results showed that aurein peptide-induced membrane leakage was more severe in DMPC/DMPG liposomes than in POPC/POPG liposomes, and that aurein 2.2 induced higher calcein release than aurein 2.3 and aurein 2.3-COOH from 1:1 and 3:1 POPC/POPG liposomes. Finally, DiSC(3)5 assay data further delineated aurein 2.2 from the others by showing that it perturbed the lipid membranes of intact S. aureus C622 most efficiently, whereas aurein 2.3 had the same efficiency as gramicidin S, and aurein 2.3-COOH was the least efficient. Taken together, these data show that the membrane interactions of aurein peptides are affected by the hydrophobic thickness of the lipid bilayers and the PG content.

Hylin a1, the first cytolytic peptide isolated from the arboreal South American frog Hypsiboas albopunctatus ("spotted treefrog")

RP-HPLC fractionation of the electrically stimulated skin secretion of the arboreal South American frog Hypsiboas albopunctatus ("spotted treefrog") led to the isolation of a cytolytic C-terminally amidated peptide. This novel peptide, named hylin a1 (Hy-a1), consists of 18 amino acid residues (IFGAILPLALGALKNLIK-NH(2)). The alpha-helical structure of the synthetic hylin a1 peptide was confirmed by CD spectroscopy in the presence of 60% (v/v) TFE. The synthetic peptide displayed broad-spectrum antimicrobial activity against Gram-negative and Gram-positive bacteria including Escherichia coli, Staphylococcus aureus, Enterococcus faecalis, Bacillus subtilis and Pseudomonas aeruginosa and also against fungi (Candida albicans, C. krusei, C. parapsilosis and Cryptococcus neoformans). Hylin a1 was also able to disrupt human erytrocytes (HC(50)=18 microM). Similarity analysis using PSI-BLAST revealed 50-44% of identity to maximins Hv, H16, H15 and H10 from Bombina maxima and also to hylins b1 and b2 (Hy-b1 and Hy-b2) from Hypsiboas lundii (synonym: Hyla biobeba).

Buforins: histone H2A-derived antimicrobial peptides from toad stomach

Antimicrobial peptides (AMPs) constitute an important component of the innate immune system in a variety of organisms. Buforin I is a 39-amino acid AMP that was first isolated from the stomach tissue of the Asian toad Bufo bufo gargarizans. Buforin II is a 21-amino acid peptide that is derived from buforin I and displays an even more potent antimicrobial activity than its parent AMP. Both peptides share complete sequence identity with the N-terminal region of histone H2A that interacts directly with nucleic acids. Buforin I is generated from histone H2A by pepsin-directed proteolysis in the cytoplasm of gastric gland cells. After secretion into the gastric lumen, buforin I remains adhered to the mucous biofilm that lines the stomach, thus providing a protective antimicrobial coat. Buforins, which house a helix-hinge-helix domain, kill a microorganism by entering the cell without membrane permeabilization and thus binding to nucleic acids. The proline hinge is crucial for the cell penetrating activity of buforins. Buforins also are known to possess anti-endotoxin and anticancer activities, thus making these peptides attractive reagents for pharmaceutical applications. This review describes the role of buforins in innate host defense; future research paradigms; and use of these agents as human therapeutics.

De novo sequencing of peptides secreted by the skin glands of the Caucasian Green Frog Rana ridibunda

Amphibian skin glands are known to secrete various types of bioactive peptides. The array of these peptides is specific for every frog species. The present research deals with the identification of peptides isolated from the skin secretion of the Marsh frog R. ridibunda inhabiting the Kolkhida Canyon of the Caucasian region. The research is based on comprehensive high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS) analysis of intact and chemically modified peptides. In particular, an oxidation procedure was applied directly to the crude skin secretion to open S--S loops whereas N-terminal acetylation was additionally carried out for one individual peptide. Sequences were determined by manual interpretation of electron capture dissociation (ECD) and collisionally induced dissociation (CID) tandem mass spectra. A total of 29 peptides were identified in the skin secretion of the Caucasian Marsh frog. The peptide profile is represented with disulfide-containing peptides belonging to the brevinin, esculentin and ranatuerin families, neuropeptides of the bradykinin and bombesin families. Two identified peptides belonging to the ranatuerins are the first peptides of this family discovered in the skin secretions of European frogs. Ten of the identified peptides coincide with those reported earlier for the European Edible frog. Another ten are identical to those found in R. ridubunda from the Moscow region. This fact verifies the described method as being an efficient analytical tool to compare intra- and interspecific variabilities.

Binding studies of nNOS-active amphibian peptides and Ca2+ calmodulin, using negative ion electrospray ionisation mass spectrometry

Amphibian peptides which inhibit the formation of nitric oxide by neuronal nitric oxide synthase (nNOS) do so by binding to the protein cofactor, Ca2+calmodulin (Ca2+CaM). Complex formation between active peptides and Ca2+CaM has been demonstrated by negative ion electrospray ionisation mass spectrometry using an aqueous ammonium acetate buffer system. In all cases studied, the assemblies are formed with a 1:1:4 calmodulin/peptide/Ca2+ stoichiometry. In contrast, the complex involving the 20-residue binding domain of the plasma Ca2+ pump C20W (LRRGQILWFRGLNRIQTQIK-OH) with CaM has been shown by previous two-dimensional nuclear magnetic resonance (2D NMR) studies to involve complexation of the C-terminal end of CaM. Under identical conditions to those used for the amphibian peptide study, the ESI complex between C20W and CaM shows specific 1:1:2 stoichiometry. Since complex formation with the studied amphibian peptides requires Ca2+CaM to contain its full complement of four Ca2+ ions, this indicates that the amphibian peptides require both ends of the CaM to effect complex formation. Charge-state analysis and an H/D exchange experiment (with caerin 1.8) suggest that complexation involves Ca2+CaM undergoing a conformational change to a more compact structure.

Membrane interactions of antimicrobial peptides from Australian frogs

The membrane interactions of four antimicrobial peptides, aurein 1.2, citropin 1.1, maculatin 1.1 and caerin 1.1, isolated from Australian tree frogs, are reviewed. All four peptides are amphipathic alpha-helices with a net positive charge and range in length from 13 to 25 residues. Despite several similar sequence characteristics, these peptides compromise the integrity of model membrane bilayers via different mechanisms; the shorter peptides exhibit a surface interaction mechanism while the longer peptides may form pores in membranes.

Antimicrobial peptides from the skins of North American frogs

North America is home to anuran species belonging to the families Bufonidae, Eleutherodactylidae, Hylidae, Leiopelmatidae, Ranidae, and Scaphiopodidae but antimicrobial peptides have been identified only in skin secretions and/or skin extracts of frogs belonging to the Leiopelmatidae ("tailed frogs") and Ranidae ("true frogs"). Eight structurally-related cationic alpha-helical peptides with broad-spectrum antibacterial activity, termed ascaphins, have been isolated from specimens of Ascaphus truei (Leiopelmatidae) occupying a coastal range. Characterization of orthologous antimicrobial peptides from Ascaphus specimens occupying an inland range supports the proposal that this population should be regarded as a separate species A. montanus. Ascaphin-8 shows potential for development into a therapeutically valuable anti-infective agent. Peptides belonging to the brevinin-1, esculentin-1, esculentin-2, palustrin-1, palustrin-2, ranacyclin, ranatuerin-1, ranatuerin-2, and temporin families have been isolated from North American ranids. It is proposed that "ranalexins" represent brevinin-1 peptides that have undergone a four amino acid residue internal deletion. Current taxonomic recommendations divide North American frogs from the family Ranidae into two genera: Lithobates and Rana. Cladistic analysis based upon the amino acid sequences of the brevinin-1 peptides provides strong support for this assignment.

The fallaxidin peptides from the skin secretion of the Eastern Dwarf Tree Frog Litoria fallax. Sequence determination by positive and negative ion electrospray mass spectrometry: antimicrobial activity and cDNA cloning of the fallaxidins

The glandular skin secretion of the Eastern Dwarf Tree Frog Litoria fallax contains nine peptides named fallaxidins. The sequences of these peptides were elucidated using a combination of positive and negative electrospray mass spectrometry together with Edman sequencing. Among these peptides are: (i) fallaxidins 1.1 and 2.1 which have the sequences YFPIPI-NH2 and FWPFM-NH2. The activities of these peptides are unknown, but it has been shown that they are not smooth muscle active, opioids or antimicrobially active, nor do they effect proliferation of lymphocytes; (ii) two weakly active antibiotics, fallaxidins 3.1 and 3.2 (e.g. fallaxidin 3.1, GLLDLAKHVIGIASKL-NH2), and a moderately active antibiotic fallaxidin 4.1 (GLLSFLPKVIGVIGHLIHPPS-OH). Fallaxidin 4.1 has an unusual sequence for an antibiotic, containing three Pro residues together with a C-terminal CO2H group. cDNA cloning has confirmed the identity of the nine isolated peptides from L. fallax, together with five additional peptides not detected in the peptide profile. The pre-regions of the nine preprofallaxidins are conserved and similar to those of the caerin peptides from L. caerulea and L. splendida, suggesting that the fallaxidin and caerin peptides, although significantly different in sequence, originated from a common ancestor gene.

Solution NMR structures of the antimicrobial peptides phylloseptin-1, -2, and -3 and biological activity: the role of charges and hydrogen bonding interactions in stabilizing helix conformations

Phylloseptins are antimicrobial peptides of 19-20 residues which are found in the skin secretions of the Phyllomedusa frogs that inhabit the tropical forests of South and Central Americas. The peptide sequences of PS-1, -2, and -3 carry an amidated C-terminus and they exhibit 74% sequence homology with major variations of only four residues close to the C-terminus. Here we investigated and compared the structures of the three phylloseptins in detail by CD- and two-dimensional NMR spectroscopies in the presence of phospholipid vesicles or in membrane-mimetic environments. Both CD and NMR spectroscopies reveal a high degree of helicity in the order PS-2> or =PS-1>PS-3, where the differences accumulate at the C-terminus. The conformational variations can be explained by taking into consideration electrostatic interactions of the negative ends of the helix dipoles with potentially cationic residues at positions 17 and 18. Whereas two are present in the sequence of PS-1 and -2 only one is present in PS-3. In conclusion, the antimicrobial phylloseptin peptides adopt alpha-helical conformations in membrane environments which are stabilized by electrostatic interactions of the helix dipole as well as other contributions such hydrophobic and capping interactions.

Development of an assay for testing the antimicrobial activity of skin peptides against the amphibian chytrid fungus (Batrachochytrium dendrobatidis) using Xenopus laevis

This report describes the preliminary characterization of a bioassay for testing the antimicrobial activity of amphibian skin peptides against the chytrid fungus, Batrachochytrium dendrobatidis. Peptide secretions from Xenopus laevis were induced by subcutaneous injections of norepinephrine. Partially purified secretions were quantified and incubated at various dilutions with 10(7) cells/mL of freshly isolated zoospores for 7 days. Peptide bioactivity was measured as cell growth inhibition over the incubation period. The concentration that inhibited growth by 80% or greater (IC(80)), based on the linear portion of the growth curve, averaged 457+/-158 microg/mL. Growth curve slopes of best-fit line equations for individual samples were less variable within control groups than the average IC(80) value, and are viewed as a more reliable indicator of peptide mixture bioactivity. This assay may be useful for evaluating the impact of environmental chemicals on amphibian host resistance to potentially lethal skin infections.

Disulfide-containing peptides from the glandular skin secretions of froglets of the genus Crinia: structure, activity and evolutionary trends

The skin secretions of Crinia signifera, C. riparia and C. deserticola contain bioactive disulfide-containing peptides. Signiferin 1 (RLCIPYIIPC-OH) from C. signifera and C. deserticola) contracts smooth muscle at a concentration of 10(-9) M, and effects proliferation of lymphocytes at 10(-6) M. In contrast, riparin 1.1 (RLCIPVIFC-OH) and riparin 1.2 (FLPPCAYKGTC-OH) from C. riparia show lymphocyte activity but do not contract smooth muscle. The lymphocyte and smooth muscle activities involve CCK2R. 3D structures of signiferin 1 and riparin 1.1 have been established using 2D NMR methods: these studies show significant differences in the shapes of the disulfide rings and with the orientations of the N-terminal residues. cDNA cloning establishes that the pre sections of the precursor pre-pro-riparin 1.4-1.6 peptides are different from the conserved pre regions of disulfide-containing antimicrobial peptides from species of the genus Rana found in the northern hemisphere and caerin antimicrobial peptides isolated from Australian tree frogs of the genus Litoria. This suggests that (i) either that riparins 1 have converged to similar structure and function to the ranid and hyloid prepropeptides which were lost initially from the myobatrachid lineage, or (ii) the prepropeptides in all three groups were derived from a single ancestral form that has remained relatively conserved in the hyloid and ranoid lineages but has undergone substantial divergent evolution in the myobatrachids.

Oxidation versus carboxamidomethylation of S-S bond in ranid frog peptides: pro and contra for de novo MALDI-MS sequencing

Five natural peptides isolated from ranid skin secretions of European frog species of Rana ridibunda and Rana arvalis (molecular masses 3516, 2674, 2636, 1874, and 1810 Da) were studied by MALDI-TOF/TOF to compare two procedures of disulfide bond cleavage: (1) performic oxidation and (2) reduction/carboxamidomethylation. The processes are relevant for the elucidation of the amino acid sequence inside the seven-member cystine ring at the C-terminus. The results clearly demonstrated that oxidation of the disulfide bond led to notably higher abundances of b- and y-ions, corresponding to the C-terminal peptide bonds, than reduction/carboxamidomethylation. This conclusion is true for all five peptides studied. Besides that, the oxidation procedure is simpler than carboxamidomethylation, as it is a one-step process with no purification required. The oxidation is more reproducible. The results were similar each time the peptide was subjected to the process. It was successfully applied to all five peptides while reduction/carboxamidomethylation failed in the case of brevinin-1Ra, despite all variations of reaction conditions.

When dinner is dangerous: toxic frogs elicit species-specific responses from a generalist snake predator

In arms races between predators and prey, some evolved tactics are unbeatable by the other player. For example, many types of prey are inedible because they have evolved chemical defenses. In this case, prey death removes any selective advantage of toxicity to the prey but not the selective advantage to a predator of being able to consume the prey. In the absence of effective selection for postmortem persistence of the toxicity then, some chemical defenses probably break down rapidly after prey death. If so, predators can overcome the toxic defense simply by waiting for that breakdown before consuming the prey. Floodplain death adders (Acanthophis praelongus) are highly venomous frog-eating elapid snakes native to northern Australia. Some of the frogs they eat are nontoxic (Litoria nasuta), others produce gluelike mucus when seized by a predator (Limnodynastes convexiusculus), and one species (Litoria dahlii) is dangerously toxic to snakes. Both the glue and the toxin degrade within about 20 min of prey death. Adders deal with these prey types in different and highly stereotyped ways: they consume nontoxic frogs directly but envenomate and release the other taxa, waiting until the chemical defense loses its potency before consuming the prey.

Effect of cholesterol on the interaction of the amphibian antimicrobial peptide DD K with liposomes

DD K is an antimicrobial peptide previously isolated from the skin of the amphibian Phyllomedusa distincta. The effect of cholesterol on synthetic DD K binding to egg lecithin liposomes was investigated by intrinsic fluorescence of tryptophan residue, measurements of kinetics of 5(6)-carboxyfluorescein (CF) leakage, dynamic light scattering and isothermal titration microcalorimetry. An 8 nm blue shift of tryptophan maximum emission fluorescence was observed when DD K was in the presence of lecithin liposomes compared to the value observed for liposomes containing 43 mol% cholesterol. The rate and the extent of CF release were also significantly reduced by the presence of cholesterol. Dynamic light scattering showed that lecithin liposome size increase from 115 to 140 nm when titrated with DD K but addition of cholesterol reduces the liposome size increments. Isothermal titration microcalorimetry studies showed that DD K binding both to liposomes containing cholesterol as to liposomes devoid of it is more entropically than enthalpically favored. Nevertheless, the peptide concentration necessary to furnish an adjustable titration curve is much higher for liposomes containing cholesterol at 43 mol% (2 mmol L(-1)) than in its absence (93 micromol L(-1)). Apparent binding constant values were 2160 and 10,000 L mol(-1), respectively. The whole data indicate that DD K binding to phosphatidylcholine liposomes is significantly affected by cholesterol, which contributes to explain the low hemolytic activity of the peptide.