ISOLATION, SEQUENCE, AND BIOACTIVITY OF CHICKEN MOTILIN

Motilin was isolated from acid extracts of the small intestine of chic kens by a combination of gel filtration chromatography, ion-exchange, and reverse-phase HPLC. The purification was monitored using a radiore ceptor assay. The sequence of chicken motilin is FVPFFTQSDIQKMQEK-ERNK GQ. Although the six residues differing from porcine motilin (4, 7-10, and 12) are mostly in the pharmacophore of porcine motilin, the affin ity of chicken motilin and of the (1-14) fragment of chicken motilin f or the motilin receptor of rabbit antral smooth muscle is not much red uced (pK(d)s of 8.90 and 8.45), compared with the affinity of [Nle(13) ]porcine motilin (pK(d) 9.12). With smooth muscle tissue of the chicke n, however, receptors could not be demonstrated with binding studies. In the tissue bath chicken motilin induced a dose-dependent tonic cont raction, which was most pronounced with muscle strips prepared from ch icken jejunum. This response was blocked by the Ca2+ antagonist verapa mil, but atropine, TTX, L-NNA, guanethidine, prazosin, and yohimbine h ad no effect. The pEC(50) for chicken motilin in the chicken jejunum w as 7.41. Motilins from other species had lower potencies, and [Phe(3), Leu(13)]porcine motilin, an antagonist in the rabbit, was an agonist i n the chicken. The motilin agonists erythromycin A and EM-523 were alm ost without effect. Tested against rabbit duodenum, chicken motilin ha d a smaller potency than mammalian motilins. Thus, chicken motilin and the chicken motilin receptor differ from their mammalian counterparts .