Motilin is an intestinal peptide hormone that binds to a membrane bound receptor located in the gut tissue. Some reports have demonstrated that the motilins from different species has different amino acid and their function is not the same.
Some reports displayed the difference of motilin among mammalians. species-related structural alterations in motilin molecules generate different bioactive capacities in some animal species, suggests that the middle portion of the motilin molecule is important for its bioactive expression, suggests the presence of motilin receptors on canine duodenal muscle, and suggests that an influx of extracellular calcium is involved in the canine duodenal muscle contraction elicited by canine motilin. The experiment motilins purified from porcine and canine intestine differ in their amino acid composition in positions 7-8-12-13-14. The researcher studied in vitro the contractile response of longitudinal duodenal muscles from various animals (guinea pig, rabbit, dog) to porcine and canine synthetic motilins. Both substances failed to elicit contraction of the guinea pig duodenum but were active and equally potent on rabbit muscle. Another report is between rat and porcine.Rat intestinal immunoreactive motilin did not co-elute with natural porcine motilin following high pressure liquid chromatography on a Waters microBondapak C18 reversed-phase column using a linear gradient of water-acetonitrile (10-45%) over 30 min. Although of similar molecular size, rat motilin is probably structurally dissimilar to other mammalian motilins.
Some another reports shows the difference between viviparous species and mammalian. motilin was isolated from acid extracts of the small intestine of chickens, The sequence is FVPFFTQSDIQKMQEK-ERNKGQ. Although the six residues differing from porcine motilin (4, 7-10, and 12) are mostly in the pharmacophore of porcine motilin, the affinity of chicken motilin and of the (1-14) fragment of chicken motilin for the motilin receptor of rabbit antral smooth muscle is not much reduced .With smooth muscle tissue of the chicken, however, receptors could not be demonstrated with binding studies. In the tissue bath chicken motilin induced a dose-dependent tonic contraction, which was most pronounced with muscle strips prepared from chicken jejunum. This response was blocked by the Ca2+ antagonist verapamil, but atropine, TTX, L-NNA, guanethidine, prazosin, and yohimbine had no effect. The pEC50 for chicken motilin in the chicken jejunum was 7.41. Motilins from other species had lower potencies, and [Phe3, Leu13]porcine motilin, an antagonist in the rabbit, was an agonist in the chicken. The motilin agonists erythromycin A and EM-523 were almost without effect. Tested against rabbit duodenum, chicken motilin had a smaller potency than mammalian motilins. Thus, chicken motilin and the chicken motilin receptor differ from their mammalian counterparts.
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Reference: Poitras P, Lahaie RG, St-Pierre S, et al. Comparative stimulation of motilin duodenal receptor by porcine or canine motilin. Gastroenterology. 1987,92(3):658-662. Vogel LB, Brown JC. Characterization of immunoreactive motilin from the rat small intestine. Can J Physiol Pharmacol. 1990,68(8):1124-1130.  Peeters TL, Bormans V, Matthijs G, et.al. Comparison of the biological activity of canine and porcine motilin in rabbit. Regul Pept. 1986,15(4):333-339.