Active Site Studies of Neurohypophyseal Hormones. Comparison of Oxytocin and Arginine Vasopressin Analogues Containing 2-D-Tyrosine

On the basis of differences between the proposed “biologically active” model of oxytocin at the uterine smooth muscle receptor and the proposed “biologically active” conformation of vasopressin when bound to its mammalian antidiuretic receptor, it may be anticipated that [D-Tyr²]oxytocin would exhibit markedly reduced uterotonic activity when compared to oxytocin, while [D-Tyr²]vasopressin would retain most of the antidiuretic activity of its natural congener. To test this hypothesis [D-Tyr²]oxytocin and [D-Tyr²]arginine vasopressin were prepared by the solid-phase method of peptide synthesis and evaluated pharmacologically. DL-Tyrosine was introduced into the growing peptide chain and the diastereoisomers of each of the hormones were separated by partition chromatography. The biological results of the D-Tyr-2 analogues were in agreement with the prediction derived from the “biologically active” models. [D-Tyr²]oxytocin possesses only 8.4 ± 0.3 units/mg in vitro rat uterotonic potency (cf. 546 ± 18 for oxytocin) and exhibited a reduced intrinsic activity. [D-Tyr²]arginine vasopressin retained nearly 50% (207 ± 10 units/mg) of the antidiuretic potency of arginine vasopressin (cf. 503 ± 53 units/mg) and exhibited both the same affinity and intrinsic activity in the renal medullary adenylate cyclase assay of rat as the mammalian antidiuretic hormone.