Host defence peptides from the skin glands of the Australian blue mountains tree-frog Litoria citropa. Solution structure of the antibacterial peptide citropin 1.1

Surface behaviour and peptide–lipid interactions of the antibiotic peptides, Maculatin and Citropin

Surface behaviour of Maculatin 1.1 and Citropin 1.1 antibiotic peptides have been studied using the Langmuir monolayer technique in order to understand the peptide-membrane interaction proposed as critical for cellular lysis. Both peptides have a spontaneous adsorption at the air-water interface, reaching surface potentials similar to those obtained by direct spreading. Collapse pressures (Pi(c), stability to lateral compression), molecular areas at maximal packing and surface potentials (DeltaV) obtained from compression isotherms of both pure peptide monolayers are characteristic of peptides adopting mainly alpha-helical structure at the interface. The stability of Maculatin monolayers depended on the subphase and increased when pH was raised. In an alkaline environment, Maculatin exhibits a molecular reorganization showing a reproducible discontinuity in the Pi-A compression isotherm. Both peptides in lipid films with the zwitterionic palmitoyl-oleoyl-phosphatidylcholine (POPC) showed an immiscible behaviour at all lipid-peptide proportions studied. By contrast, in films with the anionic palmitoyl-oleoyl-phosphatidylglycerol (POPG), the peptides showed miscible behaviour when the peptides represented less than 50% of total surface area. Additional penetration experiments also demonstrated that both peptides better interact with POPG compared with POPC monolayers. This lipid preference is discussed as a possible explanation of their antibiotic properties.

Host-defence peptides of Australian anurans: structure, mechanism of action and evolutionary significance

Host-defence peptides secreted from the skin glands of Australian frogs and toads, are, with a few notable exceptions, different from those produced by anurans elsewhere. This review summarizes the current knowledge of the following classes of peptide isolated and characterized from Australian anurans: neuropeptides (including smooth muscle active peptides, and peptides that inhibit the production of nitric oxide from neuronal nitric oxide synthase), antimicrobial and anticancer active peptides, antifungal peptides and antimalarial peptides. Other topics covered include sex pheromones of anurans, and the application of peptide profiling to (i). recognize particular populations of anurans of the same species and to differentiate between species, and (ii). investigate evolutionary aspects of peptide formation.

The solution structure of frenatin 3, a neuronal nitric oxide synthase inhibitor from the giant tree frog,Litoria infrafrenata

The peptide frenatin 3 is a major component of the skin secretion of the Australian giant tree frog, Litoria infrafrenata. Frenatin 3 is 22 amino acids in length, and shows neither antimicrobial nor anticancer activity. It inhibits the production of nitric oxide by the enzyme neuronal nitric oxide synthase at a micromolar concentration by binding to its regulatory protein, Ca2+ calmodulin, a protein known to recognize and bind amphipathic alpha-helices. The solution structure of frenatin 3 has been investigated using NMR spectroscopy and restrained molecular dynamics calculations. In trifluoroethanol/water mixtures, the peptide forms an amphipathic alpha-helix over residues 1-14 while the C-terminal eight residues are more flexible and less structured. The flexible region may be responsible for the lack of antimicrobial activity. In water, frenatin 3 exhibits some alpha-helical character in its N-terminal region.

Solid-state NMR study of antimicrobial peptides from Australian frogs in phospholipid membranes

Antimicrobial peptides, isolated from the dorsal glands of Australian tree frogs, possess a wide spectrum of biological activity and some are specific to certain pathogens. These peptides have the capability of disrupting bacterial membranes and lysing lipid bilayers. This study focused on the following amphibian peptides: (1) aurein 1.2, a 13-residue peptide; (2) citropin 1.1, with 16 residues; and (3) maculatin 1.1, with 21 residues. The antibiotic activity and structure of these peptides have been studied and compared and possible mechanisms by which the peptides lyse bacterial membrane cells have been proposed. The peptides adopt amphipathic alpha-helical structures in the presence of lipid micelles and vesicles. Specifically 15N-labelled peptides were studied using solid-state NMR to determine their structure and orientation in model lipid bilayers. The effect of these peptides on phospholipid membranes was determined by 2H and 31P solid-state NMR techniques in order to understand the mechanisms by which they exert their biological effects that lead to the disruption of the bacterial cell membrane. Aurein 1.2 and citropin 1.1 are too short to span the membrane bilayer while the longer maculatin 1.1, which may be flexible due to the central proline, would be able to span the bilayer as a transmembrane alpha-helix. All three peptides had a peripheral interaction with phosphatidylcholine bilayers and appear to be located in the aqueous region of the membrane bilayer. It is proposed that these antimicrobial peptides have a "detergent"-like mechanism of membrane lysis.

nNOS inhibition, antimicrobial and anticancer activity of the amphibian skin peptide, citropin 1.1 and synthetic modifications. The solution structure of a modified citropin 1.1

A large number of bioactive peptides have been isolated from amphibian skin secretions. These peptides have a variety of actions including antibiotic and anticancer activities and the inhibition of neuronal nitric oxide synthase. We have investigated the structure-activity relationship of citropin 1.1, a broad-spectrum antibiotic and anticancer agent that also causes inhibition of neuronal nitric oxide synthase, by making a number of synthetically modified analogues. Citropin 1.1 has been shown previously to form an amphipathic alpha-helix in aqueous trifluoroethanol. The results of the structure-activity studies indicate the terminal residues are important for bacterial activity and increasing the overall positive charge, while maintaining an amphipathic distribution of residues, increases activity against Gram-negative organisms. Anticancer activity generally mirrors antibiotic activity suggesting a common mechanism of action. The N-terminal residues are important for inhibition of neuronal nitric oxide synthase, as is an overall positive charge greater than three. The structure of one of the more active synthetic modifications (A4K14-citropin 1.1) was determined in aqueous trifluoroethanol, showing that this peptide also forms an amphipathic alpha-helix.

Nonmammalian vertebrate antibiotic peptides

With the exception of cyclostomes, all vertebrates share the common immune strategy of adaptive, highly specific immunity, based on the products of recombination-activating genes and recombined noninherited receptors for antigens. In addition, they have retained ancient vectors of innate immunity, such as antimicrobial peptides, which are widespread in all eukaryotic organisms and show a high degree of structural homology across most animal taxa. Recently, these substances have become the objects of intensive study for their outstanding bioactive properties with the aim to be applied as very efficient antibiotics, antimicrobials, and even cancerostatics in clinical practice.

Interaction of antimicrobial peptides from Australian amphibians with lipid membranes

Solid-state NMR and CD spectroscopy were used to study the effect of antimicrobial peptides (aurein 1.2, citropin 1.1, maculatin 1.1 and caerin 1.1) from Australian tree frogs on phospholipid membranes. 31P NMR results revealed some effect on the phospholipid headgroups when the peptides interact with DMPC/DHPC (dimyristoylphosphatidylcholine/dihexanoylphosphatidylcholine) bicelles and aligned DMPC multilayers. 2H NMR showed a small effect of the peptides on the acyl chains of DMPC in bicelles or aligned multilayers, suggesting interaction with the membrane surface for the shorter peptides and partial insertion for the longer peptides. 15N NMR of selectively labelled peptides in aligned membranes and oriented CD spectra indicated an alpha-helical conformation with helix long axis approximately 50 degrees to the bilayer surface at high peptide concentrations. The peptides did not appear to insert deeply into PC membranes, which may explain why these positively charged peptides preferentially lyse bacterial rather than eucaryotic cells.

Amphibian peptides that inhibit neuronal nitric oxide synthase. Isolation of lesuerin from the skin secretion of the Australian Stony Creek frog Litoria lesueuri

Two neuropeptides have been isolated and identified from the secretions of the skin glands of the Stony Creek Frog Litoria lesueuri. The first of these, the known neuropeptide caerulein 1.1, is a common constituent of anuran skin secretions, and has the sequence pEQY(SO3)TGWMDF-NH2. This neuropeptide is smooth muscle active, an analgaesic more potent than morphine and is also thought to be a hormone. The second neuropeptide, a new peptide, has been named lesueurin and has the primary structure GLLDILKKVGKVA-NH2. Lesueurin shows no significant antibiotic or anticancer activity, but inhibits the formation of the ubiquitous chemical messenger nitric oxide from neuronal nitric oxide synthase (nNOS) at IC(50) (16.2 microm), and is the first amphibian peptide reported to show inhibition of nNOS. As a consequence of this activity, we have tested other peptides previously isolated from Australian amphibians for nNOS inhibition. There are three groups of peptides that inhibit nNOS (IC(50) at microm concentrations): these are (a) the citropin/aurein type peptides (of which lesueurin is a member), e.g. citropin 1.1 (GLFDVIKKVASVIGGL-NH(2)) (8.2 microm); (b) the frenatin type peptides, e.g. frenatin 3 (GLMSVLGHAVGNVLG GLFKPK-OH) (6.8 microm); and (c) the caerin 1 peptides, e.g. caerin 1.8 (GLFGVLGSIAKHLLPHVVPVIAEKL-NH(2)) (1.7 microm). From Lineweaver-Burk plots, the mechanism of inhibition is revealed as noncompetitive with respect to the nNOS substrate arginine. When the nNOS inhibition tests with the three peptides outlined above were carried out in the presence of increasing concentrations of Ca(2+) calmodulin, the inhibition dropped by approximately 50% in each case. In addition, these peptides also inhibit the activity of calcineurin, another enzyme that requires the presence of the regulatory protein Ca(2+) calmodulin. It is proposed that the amphibian peptides inhibit nNOS by interacting with Ca(2+)calmodulin, and as a consequence, blocks the attachment of this protein to the calmodulin domain of nNOS.

Backbone cleavages of [M - H](-) anions of peptides. Cyclisation of citropin 1 peptides involving reactions between the C-terminal [CONH](-) residue and backbone amide carbonyl groups. A new type of beta cleavage: a joint experimental and theoretical study

This paper reports the study of backbone cleavages in the collision-induced negative-ion mass spectra of the [M - H](-) anions of some synthetic modifications of the bioactive amphibian peptide citropin 1 (GLFDVIKKVASVIGGL-NH(2)). The peptides chosen for study contain no amino acid residues which could effect facile side-chain cleavage, i.e. Ser (-CH(2)O, side-chain cleavage) and Asp (-H(2)O) are replaced by Ala or Lys. We expected that such peptides should exhibit standard and pronounced peaks due to alpha cleavage ions (and to a lesser extent beta cleavage ions) in their collision-induced negative-ion spectra. This expectation was realised, but the spectra also contained peaks formed by a new series of cleavage anions. These are produced following cyclisation of the C-terminal CONH(-) moiety at carbonyl functions of amide groups along the peptide backbone; effectively transferring the NH of the C-terminal CONH(-) group to other amino acid residues. We have called the product anions of these processes beta' ions, in order to distinguish them from standard beta ions. Some beta' ions also fragment directly to some other beta' ions of smaller mass. The reaction coordinates of alpha,beta and beta' backbone processes have been calculated at the HF/6-31G*//AM1 level theory for simple model systems. The initial cyclisation step of the beta' sequence is barrierless and exothermic. Subsequent steps have a maximum barrier of +40 kcal mol(-1), with the overall reaction being endothermic by some 30 kcal mol(-1) at the level of theory used. These calculations take no account of the complexity of the conformationally flexible peptide system, and it is surprising that each of the two reacting centres can 'find' each other in such a large system.

Amphipathic, alpha-helical antimicrobial peptides

Gene-encoded antimicrobial peptides are an important component of host defense in animals ranging from insects to mammals. They do not target specific molecular receptors on the microbial surface, but rather assume amphipathic structures that allow them to interact directly with microbial membranes, which they can rapidly permeabilize. They are thus perceived to be one promising solution to the growing problem of microbial resistance to conventional antibiotics. A particularly abundant and widespread class of antimicrobial peptides are those with amphipathic, alpha-helical domains. Due to their relatively small size and synthetic accessibility, these peptides have been extensively studied and have generated a substantial amount of structure-activity relationship (SAR) data. In this review, alpha-helical antimicrobial peptides are considered from the point of view of six interrelated structural and physicochemical parameters that modulate their activity and specificity: sequence, size, structuring, charge, amphipathicity, and hydrophobicity. It begins by providing an overview of how these vary in peptides from different natural sources. It then analyzes how they relate to the currently accepted model for the mode of action of alpha-helical peptides, and discusses what the numerous SAR studies that have been carried out on these compounds and their analogues can tell us. A comparative analysis of the many alpha-helical, antimicrobial peptide sequences that are now available then provides further information on how these parameters are distributed and interrelated. Finally, the systematic variation of parameters in short model peptides is used to throw light on their role in antimicrobial potency and specificity. The review concludes with some considerations on the potentials and limitations for the development of alpha-helical, antimicrobial peptides as antiinfective agents.

The antibiotic and anticancer active aurein peptides from the Australian Bell Frogs Litoria aurea and Litoria raniformis the solution structure of aurein 1.2

Seventeen aurein peptides are present in the secretion from the granular dorsal glands of the Green and Golden Bell Frog Litoria aurea, and 16 from the corresponding secretion of the related Southern Bell Frog L. raniformis. Ten of these peptides are common to both species. Thirteen of the aurein peptides show wide-spectrum antibiotic and anticancer activity. These peptides are named in three groups (aureins 1-3) according to their sequences. Amongst the more active peptides are aurein 1.2 (GLFDIIKKIAESF-NH2), aurein 2.2 (GLFDIVKKVVGALGSL-NH2) and aurein 3.1 (GLFDIVKKIAGHIAGSI-NH2). Both L. aurea and L. raniformis have endoproteases that deactivate the major membrane-active aurein peptides by removing residues from both the N- and C-termini of the peptides. The most abundant degradation products have two residues missing from the N-terminal end of the peptide. The solution structure of the basic peptide, aurein 1.2, has been determined by NMR spectroscopy to be an amphipathic alpha-helix with well-defined hydrophilic and hydrophobic regions. Certain of the aurein peptides (e.g. aureins 1.2 and 3.1) show anticancer activity in the NCI test regime, with LC50 values in the 10-5-10-4 M range. The aurein 1 peptides have only 13 amino-acid residues: these are the smallest antibiotic and anticancer active peptides yet reported from an anuran. The longer aurein 4 and 5 peptides, e.g. aurein 4.1 (GLIQTIKEKLKELAGGLVTGIQS-OH) and aurein 5. 1 (GLLDIVTGLLGNLIVDVLKPKTPAS-OH) show neither antibacterial nor anticancer activity.

Maculatin 1.1, an anti-microbial peptide from the Australian tree frog, Litoria genimaculata solution structure and biological activity

The dorsal glands of Australian tree frogs from the Litoria species contain a diversity of antibiotic peptides that forms part of the defence system of the animal. Here, the antibiotic activity and structure of maculatin 1.1, a 21 amino acid peptide from Litoria genimaculata, are compared. The activity data on maculatin 1.1 and a series of its analogues imply that the mechanism of action of maculatin 1.1 involves binding to, and subsequent lysis of, the bacterial cell membrane. The structure of maculatin 1.1 was determined using NMR spectroscopy in a trifluoroethanol/water mixture and when incorporated into dodecylphosphocholine micelles. Under both conditions, the peptide adopts a very similar conformation, i.e. a helical structure with a central kink in the vicinity of Pro15. The kink allows the peptide to adopt a well-defined amphipathic conformation along its entire length. The similar structures determined under both solvent conditions imply that structures of membrane-interacting peptides in trifluoroethanol/water mixtures are representative of those adopted in a membrane environment, e.g. when incorporated into micelles. The synthetic Ala15 analogue of maculatin 1.1 has markedly reduced activity and its NMR-derived structure is a well-defined helix, which lacks the central kink and flexibility of the parent molecule. It is concluded that the kink is important for full biological activity of the peptide, probably because it allows maximum amphipathicity of the peptide to facilitate interaction with the membrane. The structure of maculatin 1.1 is compared with a related peptide, caerin 1.1 [Wong, H., Bowie, J.H. and Carver, J.A. (1997) Eur. J. Biochem. 247, 545-557], which has an additional central proline residue and enhanced central flexibility compared with maculatin 1.1. The role of central flexibility within antibiotic peptides in their interaction with bacterial membranes is discussed.

The antibiotic and anticancer active aurein peptides from the Australian Bell Frogs Litoria aurea and Litoria raniformis. Part 2. Sequence determination using electrospray mass spectrometry

Sixteen aurein peptides are present in the host defence secretion from the granular dorsal glands of the Green and Golden Bell Frog Litoria aureus and seventeen from those of the related Southern Bell Frog Litoria raniformis. All peptides have been sequenced using a combination of electrospray mass spectrometry and Lys-C digestion, with each sequence confirmed by automated Edman sequencing. The peptides are named in five groups, viz. aureins 1-5. Ten of these peptides are common to both species of frog. Thirteen of the aurein peptides show wide-spectrum antibiotic and anticancer activity. Amongst the more active peptides are aurein 1.2 (GLFDIIKKIAESF-NH2), the smallest peptide from an anuran reported to have both antibiotic and anticancer activity; aurein 2.2 (GLLDIVKKVIGAFGSL-NH2) and aurein 3.1 (GLFDIVKKIAGHIAGSI-NH2). The aurein 4 and 5 peptides, e.g. aurein 4.1 (GLIQTIKEKLKELAGGLVTGIQS-OH) and aurein 5.1 (GLLDIVTGLLGNLIVDVLKPKTPAS-OH), show neither antibacterial nor anticancer activity.