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In a continued effort to relate the three-dimensional structure of a peptide hormone to its biological activity, the dose-response relationships of [3-phenylalanine] oxytocin (oxypressin), with an aromatic amino acid residue in position 3, and [3-beta-cyclohexylalanine]oxytocin, with an aliphatic amino acid residue to position 3, were determined in the rat uterine assay in vitro and compared to that of oxytocin. Oxypressin has not only a lower affinity for the smooth muscle receptor than the natural hormone, but also a decreased maximal response (efficacy). [3-beta-Cyclohexylalanine]oxytocin exhibits an even lower affinity than oxypressin, but retains the same maximal response as oxytocin. A reorientation of the tyrosine sidechain, caused by the presence of a neighboring aromatic sidechain in position 3, away from the surface of the 20-membered ring is though to remove the phenolic hydroxyl group from its optimal position in the "active center" of oxytocin and give rise to the reduced efficacy of oxypressin.

proceedings of the national academy of sciences of the united states of america

Importance of the third amino acid residue of oxytocin for its action on isolated rat uterus: study of relationship between hormone conformation and activity.