Purification, conformational analysis and cytotoxic activities of host-defense peptides from the Tungara frog Engystomops pustulosus (Leptodactylidae; Leiuperinae)

The amphibian family Leptodactylidae is divided into three sub-families: Leiuperinae, Leptodactylinae, and Paratelmatobiinae. Host-defense peptides (HDPs) present in the skins of frogs belonging to the Leptodactylinae have been studied extensively, but information is limited regarding peptides from Leiuperinae species. Peptidomic analysis of norepinephrine-stimulated skin secretions from the Tungara frog Engystomops pustulosus (Leiuperinae) collected in Trinidad led to the isolation and structural characterization of previously undescribed pustulosin-1 (FWKADVKEIG KKLAAKLAEELAKKLGEQ), [Q28E] pustulosin-1 (pustulosin-2), and pustulosin-3 (DWKETAKELLKKIGAKVAQVISDKLNPAPQ). The primary structures of these peptides do not resemble those of previously described frog skin HDPs. In addition, the secretions contained tigerinin-1EP (GCKTYLIEPPVCT) with structural similarity to the tigerinins previously identified in skin secretions from frogs from the family Dicroglossidae. Pustulosin-1 and -3 adopted extended α-helical conformations in 25% trifluoroethanol–water and in the presence of cell membrane models (sodium dodecylsulfate and dodecylphosphocholine micelles). Pustulosin-1 and -3 displayed cytotoxic activity against a range of human tumor-derived cell lines (A549, MDA-MB-231, and HT29), but their therapeutic potential for development into anti-cancer agents is limited by their comparable cytotoxic activity against non-neoplastic human umbilical vein endothelial cells. The peptides also displayed weak antimicrobial activity against Escherichia coli (MIC = 125 µM) but were inactive against Staphylococcus aureus. Tigerinin-1EP was inactive against both the tumor-derived cells and bacteria.