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

Peptides quantitative structure-function relationships: an automated mutation strategy to design peptides and pseudopeptides from substitution matrices

The process by which analogs in peptide chemistry are currently designed does not include any quantitative basis for amino acid substitutions from pharmacological leads. Here, we show that substitution matrices such as PAM 250 can provide quantitative constraints compatible with biological activity. This article describes its use in a strategy of rational amino acid substitution in peptides and proteins: we have computed a chemically derived matrix equivalent to the well-known PAM 250 matrix, reflecting the natural mutability rates of amino acids in protein evolutions but that can be extended to all the noncoded amino acids. Some of these noncoded amino acids are widely used to mimic secondary structure, to constrain backbone conformation, or to evade protease degradation. An automated sequence mutation (ASM) strategy has been defined to generate mutations within constraints. Application of such a substitution matrix to quantitative structure-function relationship studies will be of use in the design of proteins and peptides destined to become pharmaceutical drugs. In particular, issues such as which functionally conserved substitutions are able to satisfy conformational restrictions, oral bioavailability, or formulation demands can be quantitatively addressed.

Oxytocin antagonists in preterm labour and delivery

Evidence has been gained that an oxytocin receptor antagonist given by intravenous infusion effectively stops uterine contractions in threatened as well as in actual preterm labour. The findings suggest that the increase of oxytocin receptors is aetiologically important in uncomplicated preterm labour. Oxytocin antagonists could therefore be an attractive alternative to currently used drugs by virtue of their high specificity and lack of serious side-effects. Their use in prophylactic and maintenance therapy may be greater when modified analogues have been developed that allow non-parenteral therapy.

Quantitative structure-activity relationship (QSAR) study of elastase substrates and inhibitors

One hundred Suc-X-Y-Ala-pNA peptides: (Suc: succinyl, pNA: p-nitroanilide, X, Y: Gly, Ala, Val, Leu, Ile, Phe, Pro, alpha-aminobutyric acid, norvaline, norleucine) were synthesized and their reaction constants with porcine pancreatic elastase (Km, kcat and kcat/Km) were determined. These reaction constants were quantitatively analyzed using the Free-Wilson/Fujita-Ban method. The contribution of amino acid side chains to the reaction constants Km, kcat and kcat/Km, expressed logarithmically, was found to be additive. On the other hand, 19 elastase inhibitors of the general formula CF3CO-X-Y-Ala-pNA (X,Y: ten amino acids) were synthesized, and their inhibition constants were compared with the Michaelis constant for the corresponding substrates and analyzed using free-energy-related substituent constants. In the analysis of amino acid side chains in the Y position, the Ki value of the inhibitor was generally correlated to the Km value of the substrate, which corresponded to the inhibitor, thus confirming the validity of the equation. log(1/Ki) = 1.271 log(1/Km) + 4.831 This study may serve as a prototypical approach to unraveling structure-activity relationships of peptide substrates and inhibitors of medicinal or agricultural importance.

The nature of receptor binding of oxytocin in myometrial tissue

The dissociation course of the hormone receptor complexes of rat myometrial oxytocin receptors was studied. The dissociation course was biphasic with a major component displaying an almost irreversible binding character (time constant 0.003 min-1). This irreversibility was independent of hormonal status of the myometrium as well as the type of divalent cation present in the medium, and was not caused by endocytosis of the receptor. However, as addition of EDTA caused an immediate dissociation of all receptor binding, the irreversibility must be of a non-covalent character. It is suggested that new analysis methods for receptor kinetics including the possibility of partly irreversible binding mechanisms should be considered.

Pharmacological versus binding analysis of receptor systems: how do they interplay? Myometrial cell receptors for oxytocin as a paradigm

Binding studies in various biological systems frequently indicate the presence of several binding sites for a biologically active ligand. They differ in their affinity for the ligand in question, binding capacity, and Hill coefficient, which suggests differences in the mechanisms of the binding site-ligand interactions. Identification of the 'true' receptors (sites initiating a cellular response) appears to be difficult. Three clusters of binding sites for oxytocin were found on rat myometrial cells. The oxytocin receptor seems to be linked to the medium-affinity site; the cooperation between the high- and medium-affinity sites in eliciting the uterotonic response seems likely, but lacks experimental proof. Dose-response analysis in partially irreversibly inhibited uterus preparations, the method of equipotent doses (Furchgott-Bursztyn method), and structure-activity analysis of oxytocin-like peptides acting as competitive inhibitors of oxytocin, turned out to be suitable for pharmacological analysis of this receptor system.

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.