Active-site studies of neurohypophyseal hormones: synthesis and pharmacological properties of [5-(N4,N4-dimethylasparagine)]oxytocin

Abstract

Synthesis and biological properties of [5-(N4,N4-dimethylasparagine)]oxytocin are reported. In this analogue, the hydrogens of the primary carboxamide moiety in the side chain of the asparagine residue in position 5 of the posterior pituitary hormone oxytocin have been replaced by two methyl groups. The protected nonapeptide intermediate was prepared by a stepwise procedure using solution techniques. The analogue possesses 4.60 +/- 0.03 units/mg (mean +/- SEM) uterotonic activity on the isolated rat uterine horn and 9.14 +/- 0.03 units/mg of avian vasodepressor activity. Moreover, it displays an identical intrinsic activity in the in vitro rat uterotonic assay as oxytocin, when tested in the presence of either 0.5 mM Ca2+ (standard assay conditions) or at reduced levels of Ca2+ (0.3, 0.15, and 0.05 mM). This result is significant in view of the proposed biologically active model of oxytocin, in which the side chain of the 5 position residue was assigned to contain an "active element" responsible for the intrinsic activity of the hormone when bound to the uterine receptor.

Structural studies with weak oxytocin antagonists

“…The synthesis and biological activities of [5-(N4,N 4-dimethylasparagine)] oxytocin [5] indicated the significance of the carbonyl moiety in the y-position of the side chain of asparagine in position 5.…”

Structure ofN-tritylglycine methylamide

“…N-Tritylglycine methylamide was prepared by the method previously described (Matsoukas, Cordopatis & Theodoropoulos, 1977). It is an intermediate in the synthesis of oxytocin analogues which differ dramatically in their biological activities (Walter, Stahl, Caplaneris, Cordopatis & Theodoropoulos, 1979;Ting, Smith, Stahl, Walter, Cordopatis & Theodoropoulos, 1980).…”