Multiple antimicrobial peptides and peptides related to bradykinin and neuromedin N isolated from skin secretions of the pickerel frog, Rana palustris

Abstract

The skin secretions of the North American pickerel frog Rana palustris are toxic to both microorganisms and predators. A total of 22 peptides with differential growth-inhibitory activity towards bacteria and yeast were isolated from the electrostimulated secretions of R. palustris skin and were characterized structurally. Thirteen of the antimicrobial peptides belong to five of the known families previously identified in the skins of other species of Ranid frogs: brevinin-1 (3 peptides), esculentin-1 (2 peptides), esculentin-2 (1 peptide), ranatuerin-2 (6 peptides), and temporin (1 peptide). Nine peptides show little structural similarity towards other known antimicrobial peptides and so are classified in new families: palustrin-1 (4 peptides) with 27-28 amino acid residues and a cystine-bridged heptapeptide ring; palustrin-2 (3 peptides) with 31 amino acids and a cyclic heptapeptide region and palustrin-3 (2 peptides) with 48 amino acids and a cyclic hexapeptide region. Peptides belonging to the esculentin-1, esculentin-2 and palustrin-3 families are the most potent (minimal inhibitory concentrations approximately 1 microM against Escherichia coli) whereas peptides of the brevinin-1 and esculentin-2 families show the broadest spectrum of activity. As well as bradykinin that is identical to the human peptide, a further 4 peptides structurally related to [Leu(8)]bradykinin and two peptides related to neuromedin-N (the hexapeptide KKPYIL and a larger, cystine-containing form HLRRCGKKPYILMACS) were purified from the skin secretions.