Biofunctional evaluation of a hydrogen bond stabilizing the beta-turn in the acyclic part of oxytocin

Analogs of oxytocin containing a pseudopeptide Leu-Gly bond of cis and trans configuration

Analogs of deamino-oxytocin wherein the Leu-Gly peptide bond has been replaced by a tetrazole moiety or by a double bond of trans configuration were synthesized and their biological activities evaluated. Trans double bond was found to be the most appropriate substitution for the amide bond (uterotonic activity 24% of the deamino-oxytocin). In the case of all three analogs low but prolonged galactogogic activity was found and the ratio of uterotonic in vitro and in vivo activity was surprisingly high (ranging from 4.5 to 20).

Analogs of oxytocin containing a modified peptide bond

Analogs of deamino-oxytocin and deamino-oxypressin containing a CH2-NH group instead of an amide bond between positions 8 and 9 were synthesized. All tested compounds exhibit significantly lowered biological activities.

Role of the carboxamide groups of the asparagine and glycinamide residues of oxytocin. Syntheses and biological properties of [5-beta-cyanoalanine] oxytocin and [9-alpha-aminoacetonitrile] oxytocin

In an attempt to see whether the C=O and the NH2 of CONH2 of asparagine5 and glycinamide9 are both essential for biological activity, [5-beta-cyanoalanine] oxytocin and [9-alpha-aminoacetonitrile] oxytocin have been synthesized. Each of these analogs contains a nitrile group in place of the carboxamide group of Asn5 and GlyNH92 respectively; the nitrile group can simulate the carbonyl portion of the carboxamide, but lacks the hydrogen-bond donating capacity of its NH2 portion. Substitution of a nitrile group produced opposite biological effects in the 5 and the 9 positions; the 5-substituted analog showed very low activities (less than 3% of those of oxytocin) while the 9-substituted analog showed extremely high activities (with an in vivo uterine activity of 906 U/mg almost twice that of oxytocin). The results clearly suggest that the mechanisms of interaction of the carboxamide groups with the receptor sites are different for residues 5 and 9.

Biofunctional evaluation of two hydrogen bonds stabilizing the beta-turn in the acyclic component of oxytocin

The depsipeptide [8-alpha-hydroxyisocaproic acid, 9-glycolic amide]-oxytocin, which has ester linkages replacing the peptide linkages between the 7th and 8th and the 8th and 9th residues of oxytocin, has been synthesized by a (6 + 3) condensation of Boc-tocinoic acid with Pro-O-HyIc-O-Glyc-NH2, followed by deprotection of the resulting product. The analog exhibited the following activities in rats: 258 +/- 11 and 28 +/- 5 U/mg, uterus in vitro in the absence and presence, respectively, of Mg+2; 54 +/- 4 U/mg, uterus in vivo; 19.3 +/- 2.1 U/mg, milk ejection; 0.153 +/- 0.026 U/mg, antidiuretic activity; and no pressor activity. The need for the presence of the peptide linkages mentioned above as sources for internal hydrogen bonds to stabilize the "biologically significant" conformation is discussed.

Biofunctional evaluation of a hydrogen bond stabilizing the conformation in the cyclic part of oxytocin

[5-beta-Malamidic acid]oxytocin was synthesized to study the importance of the hydrogen bond between the C=O of Tyr2 and the peptide N-H of Asn5 for the stabilization of a biologically functional conformation of oxytocin. This analog lacks the peptide N-H at residue 5 required for the formation of a hydrogen bond with the C-O of Tyr2. [5-beta-Malamidic acid] oxytocin exhibited 45.1 +/- 2.5 U/mg and 65.6 +/- 5.9 U/mg of uterotonic activity, in vitro, in the absence and in the presence, respectively, of Mg2+, 147 +/- 14 U/mg of uterotonic activity in vivo, 203 +/- 13 U/mg of milk-ejecting activity, 0.37 +/- 0.03 U/mg of pressor activity and 0.32 +/- 0.29 U/mg of antidiuretic activity. It is concluded that devoid of the hydrogen bond under question, an oxytocin-like peptide can still assume the conformation needed to interact with the oxytocin receptors.