Contribution of the peptide backbone to the action of oxytocin analogs.

Abstract

This investigation was undertaken to evaluate the functional contribution of the peptide backbone of oxytocin in its interaction with receptor. CoreyPauling-Koltun models of [Gly7]deaminooxytocin, de-aminotocinamide, and their respective retro-D-analogs built in any specific conformation (e.g., the Walter-Urry model for oxytocin) have a quasi-equivalent topochemical arrangement of amino-acid side chains; however, the CO and NH elements of the peptide backbone of the retro-Danalog are reversed. The retro-D-analogs of deaminotocinamide and [Gly7jdeaminooxytocin (prepared using D-alle for L-Ile) and their respective N-formyl derivatives were assayed for uterotonic activity relative to related L-peptides. All retro-D-analogs (tested at concentrations ranging from 10-10 to 1O-5 M) were devoid of agonistic (or antagonistic) activity in the isolated rat uterus, except for the retro-D-[D-alle3, Gly7jdeaminooxytocinamine, which retains a terminal NH2 group on the tail; the latter is a partial agonist with very low affinity. The results obtained with retro-D-analogs indicate that one or more of the elements of the peptide backbone of the tocinamide ring are essential for "occupation" and "activation" of uterine receptors. Oxytocin action may be the resultant of multiple hydrogen-bonding interactions between CO, NH, NH2, and OH groups of the hormone with complementary groups on receptor, made possible by appropriate hydrophobic bonding.The distinctive topological arrangement of amino-acid side chains of a peptide hormone at the receptor surface is believed to be the primary determinant for those interactions that establish "occupancy" and "activation" of receptors (1). The contribution of the peptide backbone has received little attention. The fact that retro-D-analogs (2) * of certain cyclic peptides have similar biological activities as those exhibited by the parent compound (3-5) is consistent with the idea that the peptide backbone serves only as a structural scaffold for projecting amino acid side chain functionalities.t In a retro-Danalog, the "overall shape" and topochemical pattern of side chains are "quasi-equivalent" to that of a normal peptide (provided that Pro is not part of the sequence) (2-4), and differs from the normal peptide in that the CO and NH elements of the peptide bonds, as well as the amino and carboxyl termini, are reversed.If the amide elements of the backbone do not serve an important functional role in hormone action, appropriate To determine whether the CO and NH elements of the pel)-tide backbone participate in hormone interaction with receptor, we made the N-formyl derivatives of retro-D-analogs of two biologically active oxytocin analogs (6-9), deaminotocinamide and [Gly7]deaminooxytocin, which contain neither Pro nor terminal COOH or NH2 groups. The uterotonic activities of these (and related) retro-D-analogs were compared to the parent L-peptides in a rat oxytocic assay. The N-formyl derivatives of these retro-D-analogs (tested over a concentration range from 10-10 to 10-5 M) wer...

Solution conformation of a retro-D analogue of tocinamide

“…In contrast, the peptide H-D-Cys-D-Gln-D-aIle-D-Tyr-NHCH2CH2-SJ called [o-alle3]retro-D-deaminotocinamide and its TV-formyl derivative are only weak competitive inhibitors of oxytocin in the oxytocic assay, at doses 106-107 larger than that of the hormone. Since similarities in the topochemical arrangement of side chains in [D-aIle3]retro-D-deaminotocinamide and deaminotocinamide are possible, it was suggested by Hechter et al (1975) that one reason for the lack of activity of the retro-D compound might be alteration of the sense of the backbone amide units, which might have an essential role in transmitting the hormonal message.…”

Improvement in potency of an oxytocin antagonist after systematic substitutions with L-tryptophan

“…We have previously hypothesized that Tyr2 and Asn5 have a critical role in occupation and activation of receptors. 40 It has also been suggested on conformational grounds that certain amino acids, for example aromatic amino acids of the D-configuration with para substituents, may possess high inhibitory potency. 46 The peptide sequence of the analogue may promote high binding affinity; however, the side chain of the aromatic D-amino acid instead of being located on top of the plane of the peptide ring, in proximity to the asparagine side chain, would be located away from the plane of the 20-membered ring, unable to participate in binding leading to transduction of the hormonal message.…”

“…Replacement of Asn5 with Trp gave analogue 5, [Pmp1j>Trp2,Trp5Arg8]OT, pA2 = 5.21, with the expected low biological activity, since L-Asn seems to be essential for binding to the neurohypophyseal hormone receptors and initiation of the hormon¿ message40 and its substitution usually leads to substantial loss of potency. 40,41,6 Hence, all of the Trp substitutions in the ANTAG ring led to poorer antagonists.…”

Biofunctional evaluation of a hydrogen bond linking the ring and tail beta-turns of oxytocin.

“…The restoration of the characteristic biological activities of oxytocin and deamino-oxytocin on addition of the tripeptide tail, Pro-LeuGly-NH2, to the ring structure might be due to the addition of residues at positions 7 and 8 which, together with residues at positions 3 and 4, provide binding sites for interactions with the receptors. However, the effect of the interaction of the complete natural acyclic tripeptide with the ring moiety has also to be considered (9,(34)(35)(36)(37) (38), the "cooperative model" (9,35) for the biologically active conformation of oxytocin can no longer include an absolute requirement for hydrogen bonding between the carboxamide carbonyl of asparagine and the peptide N-H of leucine because the oxytocin analogs described herein can assume the conformation needed to evoke the characteristic biological activities of oxytocin-albeit with lower potency-even though this hydrogen bond cannot form.…”