Affinity of 1,2-substituted oxytocin analogues to the uterus receptor: Free-Wilson and Hansch analysis

Antagonists as "affinity probes" for peptide hormone receptors: QSAR studies on oxytocin receptor in the rat myometrium

The use of competitive antagonists in receptor investigations has been based mainly on empirically established abilities to suppress biological effects of certain agonists. This information serves primarily for classification of receptors according to ligand specificities. Methods of quantitative structure-activity analysis can extend their use for investigations of preference substituent in the desired position, estimates of ligand-receptor affinities, etc., when a set of antagonists differing solely in the substituents in that position and characterized by their pA2 values, is available. In the case of a multiple substitution, contributions of single substituents on individual positions can be estimated by the Free-Wilson method and then subjected to correlation analysis. The approach is demonstrated on a group of oxytocin analogues acting as uterotonic inhibitors of oxytocin; the "intramolecular force" descriptors are used for correlation analysis.

Amino acid side chain parameters for correlation studies in biology and pharmacology

Fifteen physicochemical descriptors of side chains of the 20 natural and of 26 non-coded amino acids are compiled and simple methods for their evaluation described. The relevance of these parameters to account for hydrophobic, steric, and electric properties of the side chains is assessed and their intercorrelation analyzed. It is shown that three principal components, one steric, one bulk, and one electric (electronic), account for 66% of the total variance in the available set. These parameters may prove to be useful for correlation studies in series of bioactive peptide analogues.

Structural requirements of the oxytocin receptor in rat uterus. Free-Wilson analysis in a series of competitive oxytocin inhibitors

Published and newly calculated pA2-values of 147 neurohypophyseal hormone analogues (7 positions varied) acting as inhibitors of oxytocin on isolated rat uterus in vitro have been subjected to fractionation according to the method by Free and Wilson which was slightly modified for this purpose. The computation was carried out in several steps. After each step, substances with outlying pA2-values were eliminated. The reduced group containing 73-79% of the original substances displayed a high degree of additivity of side chain contributions (SCC). This group seems to follow the "participation" rule as formulated by Free and Wilson. Analysis of the group of eliminated substances and of the resulting SCC-spectrum (level diagram) enabled us to draw some conclusions concerning the structural requirements of receptor binding: i) The intact ring structure is necessary for the peptide-receptor interaction: linear peptides or peptides with an extended ring are always outliers; ii) Carba analogues (substitution with CH2 in the disulfide ring) display better affinities than peptides with an S-S ring; D-Arg8 substitution decreases the binding affinity; iii) Considerably better additivity is achieved when peptides are divided into subgroups with vasopressin-like and oxytocin-like features; populations of receptors more specific for vasopressin and for oxytocin, respectively, can be assumed. Estimates of the "true" receptor-peptide dissociation constants can be obtained by summation of the corresponding SCC's in each investigated position. The value obtained for oxytocin is identical with the medium affinity binding site on myometrial cells, and not with the high affinity site. A nonlinear relationship exists between SCC's computed from pA2-values for magnesium-free and magnesium-containing (0.5 mM) media but no evidence speaks in favor of a Mg-potentiating effect on receptor binding.

Pharmacological approaches to the identification and classification of myometrial oxytocin receptors

Three binding sites have been recently reported on the rat, calf and sheep myometrial cells, with dissociation constants (Kd) roughly 10(-9), 10(-7) and 10(-5) mol/l. Oxytocin receptor for the uterotonic response in vitro was identified pharmacologically: 1) The analysis of dose-response curves has been based on a partial irreversible inhibition of the receptor on isolated rat uterus by the method of Furchgott and Bursztyn, and by the newly suggested plotting of Kd vs. maximal response for an increasing degree of irreversible inhibition. 2) pA2- values (reflecting Kd) of structural analogues of oxytocin acting as competitive inhibitors of the parent hormone have been analysed according to Free and Wilson. Contribution of side chains in individual positions of the nonapeptide chain were computed, tested on additivity and then used for back-computation of a Kd for oxytocin. Results of all experiments reveal a Kd for oxytocin receptor (rat uterus in vitro) of 2 x 10(-7) mol/l. Possible endocrine functions of the high and low affinity sites have not been clarified as yet.

Pharmacological properties of two analogues of angiotensin II containing carboranylalanine (Car)

The pharmacological properties of two angiotensin II analogues containing carboranylalanine (Car) are reported. Sar-Arg-Val-Tyr-Val-His-Pro-Car is a weak partial agonist showing 15% intrinsic activity and a very long lasting action, specific for the angiotensin II receptor of the rabbit aorta. Sar-Arg-Val-Car-Val-His-Pro-Phe is inactive on the angiotensin II receptor. These properties are discussed in view of recent findings on the action of angiotensin II on its smooth muscle receptor.