Effects of arginine substitutions on the cardioinhibitory activity of the Led-NPF-I neuropeptide

Effects of structural changes on the cardioinhibitory activity of the Led-NPF-I peptide (Ala-Arg-Gly-Pro-Gln-Leu-Arg-Leu-Arg-Phe-amide) were examined by replacing Arg residues in positions 2, 7 and 9. Replacement of L-Arg2 with another basic amino acid, such as Lys, His or D-Arg, did not abolish but rather promoted cardioinhibitory activity in giant mealworm beetle Zophobas atratus Fab. Agonistic peptides were also obtained by substitution of Arg residue in position 7 with Lys or D-Arg, and Arg in position 9 with His or D-Arg, respectively. All these analogues showed stronger cardioinhibitory effects than the native peptide at low concentration (10-9 M), and [Lys7]-, [D-Arg7]- and [D-Arg9]-Led-NPF-I also at the higher concentration (10-6 M). However, substitutions of the Arg residues in position 7 with His or in position 9 with Lys caused a loss of the cardioinhibitory activity. In addition, the replacement of Arg residues in all three positions with Lys or Orn caused a reduction of cardioinhibitory activity, although a single substitution of Arg in positions 2 or 7 with Lys yielded agonistic peptides. We conclude that the Arg2 position in the N-terminal region is more tolerant to structural modification than the other two Arg positions located in the C-terminal region.