New proctolin analogues: Synthesis and biological investigation in insects

SummaryWe have extended our work on structure/activity relationship studies of the neuropeptiden proctolin (H-Arg-Tyr-Leu-Pro-Thr-OH) by evaluating the effects of the following proctolin analogues: H-X1-Tyr-Leu-Pro-Thr-OH, where X1=d-Arg(I),N-Me-Arg (II), Can (III), Orn(di-Me) (IV), Orn (iPr) (V), Lys(N, N-di-Me) (VI), Lys(iPr) (VII), Lys(Nic) (VIII) andd-Lys(Nic) (IX). In analogues I–IX, the N-terminal Arg residue was replaced by basic amino acid derivatives with peptides containing amino acid residues with an isosteric system on the back side chain relative to Arg (compounds III, V and VI) orhomo-Arg (compound VII). Analogues I–IX were evaluated for myotropic activity on thein vitro heart preparation ofTenebrio molitor, whereas peptides II, V, and VII–IX were tested for contractile activity on the isolated foregut of locustSchistocerca gregaria. Peptide II and III showed full cardiotropic activity inT. molitor while peptides V and VII showed 40% and 15%, respectively, locust-gut contracting activity of proctolin.