Design and synthesis of N-terminal cyclic motilin partial peptides: a novel pure motilin antagonist.

Motilin antagonist was designed and synthesized on the basis of the structure-activity relationship analysis of porcine motilin that we reported recently. The drug design was performed on a specific concept to reduce a flexibility of peptide conformation of porcine motilin partial peptide by its cyclization. The cyclic peptide was synthesized using Boc (tert-butyloxycarbonyl) solid phase methodology, followed by cyclization using the azide procedure, and tested for the binding activity to motilin receptor and smooth muscle contractile activity. The cyclic peptides 3, 4, and 5 showed antagonistic property on contraction assay (pA2 [the negative logarithm of molar concentration of antagonist causing a 2-hold shift to the right of the concentration-response curve for motilin]: 4.5, 4.34, and 4.04, respectively, in rabbit duodenum) and no contractile activity even at high concentration.

Structure-activity study of intact porcine motilin.

Biologically important sites on intact porcine motilin (pMTL) were explored using its partial peptides. The partial peptides were synthesized using Fmoc (9-fluorenylmethyloxycarbonyl) solid phase methodology, and tested for the binding activity to motilin receptor and the smooth muscle contractile activity. The results were as follows: important residues for the contractile activity were found to be Phe1, Ile4, and Tyr7, and an open space existed beyond the N-terminus between motilin and its receptor. On the model of interaction between motilin and motilin receptor evolved from these results, the three points of interaction, due to Phe1, Ile4, and Tyr7, and the presence of an open space were expected. The motilin agonist and antagonist, designed on this model, will help the inquiry into motilin associated diseases.