Structure-activity relationships of an inhibitory nematode FMRFamide-related peptide, SDPNFLRFamide (PF1), on Ascaris suum muscle.

FMRFamide-related peptides are widespread among the Nematoda. Among them is a family of extended PNFLRFamide peptides encoded on the flp-1 peptide precursor gene in Caenorhabditis elegans. The most studied peptide from this series is SDPNFLRFamide (PF1). Each residue in this peptide was sequentially substituted with either alanine or the corresponding d-isomer of the native amino acid in order to define structure-function relationships in this peptide using an Ascaris suum muscle tension assay. In general, substitutions in the N-terminal tetrapeptide had only minor consequences for efficacy, while substitutions in the C-terminal tetrapeptide caused more dramatic changes. Such substitutions typically markedly diminished efficacy, but d-isomer substitution at either position 5 (Phe) or 6 (Leu) converted the inhibitory activity of the prototype into excitation. In addition, it has been evident that KPNFLRFamide and SDPNFLRFamide, though encoded on flp-1 and sharing a PNFLRFamide hexapeptide, act through different receptors. KPNFLRFamide directly gates a chloride channel in A. suum muscle cells, while SDPNFLRFamide acts through nitric oxide synthase to open K+ channels in the same tissue. The use of K+ channel blockers and nitric oxide synthase inhibitors in electrophysiological experiments employing A. suum muscle membranes allowed the unambiguous conclusion that the N-terminal lysine is absolutely required for activation of the chloride channel and excludes interaction with the SDPNFLRFamide receptor.