Influence of methylation or substitution of the histidine of HVFLRFamide on biological activity and binding of locust oviduct.

The FMRFamide-related peptide PDVDHVFLRFamide (SchistoFLRFamide) is involved in the neural control of locust oviducts, where it acts as a potent inhibitor of spontaneous and induced contractions. Previous studies have shown that the His residue in the truncated analogue HVFLRFamide is critical for the retention of inhibitory biological activity, whereas VFLRFamide, in which inhibitory biological activity is lost, is the minimum sequence for binding of comparable affinity to the parent compound. In the present study we have used binding and bioassay to further investigate the properties of the His residue in determining the inhibitory biological activity of HVFLRFamide. Substitution of His by the D-isomer or Phe produced analogues with stimulatory rather than inhibitory activity, confirming the importance of the His moiety. In addition, inhibitory activity was retained when the His moiety was methylated at the N-3 position of the imidazole ring, but methylation of N-1 yielded a peptide that stimulated contractions. Inhibitory activity was further retained when Nn-methyl-L-His and D,L-1',2',4'-triazole-3-Ala were substituted for His. These results, along with binding studies, suggest that transfer of an imidazole proton is not responsible for inhibitory activity and further suggest that hydrogen bonding to the N-1 of the imidazole ring of histidine may be required to evoke the inhibitory response.