Strategies for transformation of naturally-occurring amphibian antimicrobial peptides into therapeutically valuable anti-infective agents.

The emergence of strains of pathogenic microorganisms with resistance to commonly used antibiotics has necessitated a search for novel types of antimicrobial agents. Many frog species produce amphipathic alpha-helical peptides with broad spectrum antimicrobial activity in the skin but their therapeutic potential is limited by varying degrees of cytolytic activity towards eukaryotic cells. Methods for development of such peptides into anti-infective drugs are illustrated by the example of temporin-1DRa (HFLGTLVNLAK KIL.NH(2)). Studies with model alpha-helical peptides have shown that increase in cationicity promotes antimicrobial activity whereas increases in hydrophobicity, helicity and amphipathicity promote hemolytic activity and loss of selectivity for microorganisms. Analogs of temporin-1DRa in which each amino acid is replaced by L-lysine and D-lysine were synthesized and their cytolytic activities tested against a range of microorganisms and human erythrocytes. Small changes in structure produced marked changes in conformation, as determined by retention time on reversed-phase HPLC, and in biological activity. However, peptides containing the substitutions (Val(7) -->L-Lys), (Thr(5)-->D-Lys) and (Asn(8)-->D-Lys) retained the high solubility and potent, broad spectrum antimicrobial activity of the naturally occurring peptide but were appreciably (up to 10-fold) less hemolytic. In contrast, analogs in which Leu(9) and Ile(13) were replaced by the more hydrophobic cyclohexylglycine residue showed slightly increased antimicrobial potencies (up to 2-fold) but a 4-fold increase in hemolytic activity. The data suggest a strategy of selective increases in cationicity concomitant with decreases in helicity and hydrophobicity in the transformation of naturally-occurring antimicrobial peptides into non-toxic therapeutic agents.

Brevinin-1BYa: a naturally occurring peptide from frog skin with broad-spectrum antibacterial and antifungal properties.

Brevinin-1BYa (FLPILASLAAKFGPKLFCLVTKKC) is a cationic alpha-helical peptide containing an intramolecular disulphide bridge that is present in skin secretions of the foothill yellow-legged frog Rana boylii. A synthetic replicate of the peptide showed growth inhibitory activity against a range of reference strains of Gram-positive and Gram-negative bacteria, against clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) (minimum inhibitory concentration (MIC)=2.5 microM), and against reference strains and clinical isolates of the opportunistic yeast pathogens Candida albicans, Candida tropicalis, Candida krusei and Candida parapsilosis (MIC

Purification and properties of laticeptin, an antimicrobial peptide from skin secretions of the South American frog Leptodactylus laticeps.

Norepinephrine-stimulated skin secretions from the Sante Fe frog Leptodactylus laticeps contained high concentrations of a peptide, termed laticeptin, with the primary structure Gly-Val-Val-Asp-Ile-Leu-Lys-Gly-Ala-Ala-Lys-Asp-Leu-Ala-Gly-His-Leu-Ala-Thr-Lys-Val-Met-Asn-Lys-Leu.NH(2). Laticeptin inhibited the growth of selected Gram-negative bacteria but the lack of activity against Gram-positive bacteria and the very low hemolytic activity is probably a consequence of the weak amphipathicity of the peptide in its alpha-helical conformation.

Antimicrobial peptides from diverse families isolated from the skin of the Asian frog, Rana grahami.

Seven peptides with antimicrobial activity were isolated in pure form from an extract of the skin of the Yunnanfu Kunming frog Rana grahami Boulenger, 1917. The peptides were identified as belonging to the nigrocin-2 (three peptides), brevinin-1 (one peptide), brevinin-2 (three peptides), and esculentin-1 (one peptide) families. Nigrocin-2GRb (GLFGKILGVGKKVLCGLSGMC) containing three lysine residues, represented the peptide with highest potency against microorganisms (MIC = 3 microM against Escherichia coli, 12.5 microM against Staphylococcus aureus and 50 microM against Candida albicans) and the greatest hemolytic activity against human erythrocytes (LD50 = 40 microM). In contrast, nigrocin-2GRa (GLLSGILGAGKHIVCGLSGLC) and nigrocin-2GRc (GLLSGILGAGKNIVCGLSGLC), with only a single lysine residue, showed weak antimicrobial and hemolytic activity. Phylogenetic relationships among Eurasian ranid frogs are less well understood than those of North American ranids but the primary structures of the R. grahami antimicrobial peptides suggest a close relationship of this species with the Japanese pond frogs R. nigromaculata and R. porosa brevipoda.

Antimicrobial peptides from the skin of the Tsushima brown frog Rana tsushimensis.

The Tsushima brown frog Rana tsushimensis Stejneger, 1907 exists in reproductive isolation on the island of Tsushima, Japan. Six peptides with antimicrobial activity were isolated in pure form from an extract of the skin of this species and their amino acid sequences identified them as members of the brevinin-1 (one peptide), brevinin-2 (one peptide) and temporin (four peptides) families. The C-terminally alpha-amidated brevinin-1 peptide (FLGSIVGALASALPSLISKIRN.NH2) lacks the cyclic heptapeptide domain Cys18-(Xaa)4-Lys-Cys24 at the COOH-terminus of the molecule that characterizes other members of that family. A structurally similar brevinin-1 peptide, also lacking the cyclic domain, was previously isolated from the skin of the Ryukyu brown frog Rana okinavana, indicative of a close phylogenetic relationship between the species. Brevinin-2TSa (GIMSLFKGVLKTAGKHVAGSLVDQLKCKITGGC) showed broad-spectrum growth inhibitory activity against a range of Gram-negative and Gram-positive bacteria (including methicillin-resistant Staphylococcus aureus) (minimum inhibitory concentrations< or =25 microM) and relatively low hemolytic activity against human erythrocytes (LD50=100 microM). The peptide therefore represents a candidate for drug development.

Pentadactylin: an antimicrobial peptide from the skin secretions of the South American bullfrog Leptodactylus pentadactylus.

Norepinephrine-stimulated skin secretions were obtained from male specimens of the South American bullfrog, Leptodactylus pentadactylus and shown to contain two peptides that inhibited the growth of microorganisms. The primary structure of a previously undescribed peptide, termed pentadactylin, was established as Gly-Leu-Leu-Asp-Thr-Leu-Lys-Gly-Ala-Ala-Lys-Asn-Val-Val-Gly-Ser-Leu-Ala-Ser-Lys-Val-Met-Glu-Lys-Leu.NH2. The second peptide, which differs from pentadactylin by eight amino acid residues, is identical to fallaxin previously isolated from skin secretions of the Caribbean mountain chicken frog L. fallax. Pentadactylin inhibited the growth of reference strains of both Gram-negative bacteria (Escherichia coli, Enterobacter cloacae, Klebsiella pneumoniae, Pseudomonas aeruginosa) and Gram-positive bacteria (Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus faecalis, Streptococcus group B) but potencies were relatively low (MIC values in the range 25-200 microM). The peptide showed very low hemolytic activity against human erythrocytes (LD50>400 microM).

Purification and characterization of antimicrobial peptides from the skin secretions of the carpenter frog Rana virgatipes (Ranidae, Aquarana).

The members of the Aquarana (or Rana catesbeiana species group) form a well-supported monophyletic clade but phylogenetic relationships between species within the group are incompletely understood. Peptides that differentially inhibited the growth of bacteria were purified from electrically stimulated skin secretions of the carpenter frog Rana virgatipes. Structural characterization identified members of the ranatuerin-2 (3 peptides) and temporin (3-peptides) families, previously found in the skins of R. catesbeiana, R. clamitans, R. grylio and R. septentrionalis. Ranalexin, a peptide previously found only in the Aquarana, was isolated together with a variant (FFGLHNLVPSMLCVVRKKC) that lacks the propensity to adopt an alpha-helical conformation and so was devoid of antimicrobial activity. Two C-terminally alpha-amidated peptides belonging to the brevinin-2 family were isolated from the skin secretions that, like an ortholog from R. septentrionalis, lacked the C-terminal cyclic heptapeptide domain associated with members of this family. Ranatuerin-1, previously isolated from R. catesbeiana, R. clamitans and R. grylio but absent from R. septentrionalis, was also not identified in R. virgatipes. Synthetic replicates of temporin-1Va (FLSSIGKILGNLL.NH2), temporin-IVb (FLSIIAKVLGSLF.NH2) and temporin-1Vc (FLPLVTMLLGKLF.NH2) potently inhibited growth of Gram-positive bacteria (including methicillin-resistant Staphylococcus aureus). Temporin-1Va was also active against Gram-negative bacteria and the opportunistic yeast pathogen Candida albicans and had relatively weak hemolytic activity (LD50=120 microM) and may therefore represent a candidate for drug development. Our data support the placement of R. virgatipes in the Aquarana and indicate a closer phylogenetic relationship of R. virgatipes with R. septentrionalis than with R. catesbeiana, R. clamitans and R. grylio.

Antimicrobial and cytolytic properties of the frog skin peptide, kassinatuerin-1 and its L- and D-lysine-substituted derivatives.

Kassinatuerin-1, a 21-amino-acid C-terminally alpha-amidated peptide first isolated from the skin of the African frog Kassina senegalensis, adopts an amphipathic alpha-helical conformation in a membrane-mimetic solvent (50% trifluoroethanol) and shows broad-spectrum antimicrobial activity. However, its therapeutic potential is limited by its relatively high cytolytic activity against mammalian cells. The antimicrobial and cytolytic properties of a peptide are determined by an interaction between cationicity, hydrophobicity, alpha-helicity and amphipathicity. Replacement of the C-terminal alpha-amide group in kassinatuerin-1 by carboxylic acid decreased both cationicity and alpha-helicity, resulting in an analog with decreased potency against Escherichia coli (4-fold) and Staphylococcus aureus (16-fold). Low cytolytic activities against human erythrocytes (LD50>400 microM) and L929 fibroblasts (LD50=105 microM) were also observed. Increasing cationicity, while maintaining amphipathic alpha-helical character, by progressively substituting Gly7, Ser18, and Asp19 on the hydrophilic face of the alpha-helix with L-lysine, increased antimicrobial potency against S. aureus and Candida albicans (up to 4-fold) but also increased hemolytic and cytolytic activities. In contrast, analogs with d-lysine at positions 7, 18 and 19 retained activity against Gram-negative bacteria but displayed reduced hemolytic and cytolytic activities. For example, the carboxylic acid derivative of [D-Lys7, D-Lys18, D-Lys19]kassinatuerin-1 was active (minimum inhibitory concentration (MIC)=6-12.5 microM) against a range of strongly antibiotic-resistant strains of E. coli but showed no detectable hemolytic activity at 400 microM and was 4-fold less cytolyic than kassinatuerin-1. However, the reduction in alpha-helicity produced by the D-amino acid substitutions resulted in analogs with reduced potencies against Gram-positive bacteria and against C. albicans.