Oxytocin receptor binding affinities

Oxytocin and vasopressin are closely related peptides sharing seven out of nine amino acids. Vasopressin is able to bind to all vasopressin and oxytocin receptors with nanomolar affinity, whereas oxytocin has greater affinity for the oxytocin receptor than the vasopressin receptors. The affinity of the peptides for the receptors is shown in Table 7.1 (data from Mouillac et al. [20]). 

The first non-peptide oxytocin antagonists, based on a spiropiperidine template, were described by Merck in 1992 [68-70]. The binding affinity data for key compounds from this series are summarised in Table 7.2. The initial screening hit, L-342,643, (23), had modest (4/iM) affinity for rat uterine oxytocin receptors and very little vasopressin selectivity [71]. A structure activity relationship (SAR) study was carried out around this template, focussing on the toluenesulphonamide group. This work led to the identification of bulky lipophilic substitution as key to improved oxytocin potency, while the introduction of a carboxylic acid group led to improved... 

In CHO cells transfected with human V], and V2 vasopressin receptors, 1 inhibits [3H]-AVP binding with K/s of 4.3 and 1.9 nM, respectively.15 Compound 1 demonstrates similar activity on rat Vla and V2 receptors, with Kj s of 0.48 nM and 3.0 nM (Table 1). As a result of significant structural homology between the vasopressin and the oxytocin receptors, 1 and AVP also demonstrate significant oxytocin receptor affinities (rat receptor Kt s of 44.4 nM and 3.4 nM, respectively).16 As seen in Table 1, the balanced binding affinities of 1 toward rat Via and V2 receptors closely parallel those of AVP in contrast, vasopressin receptor antagonists mozavaptan hydrochloride (2) and tolvaptan (3) demonstrate moderate to significant V2 receptor selectivity. 

Similarly, a three-dimensional model of the vasopressin/oxytocin receptor has been defined and the binding mode of the neuropeptides has been analysed (Figure 3 and 5). Residues likely to contribute to the affinity, efficacy and selectivity have been proposed from the model. These qualitative predictions have subsequently been confirmed by site directed mutagenesis [20-22]. A number of general comments can be made ... 

In competition binding, the affinity of a compound is measured by its ability to compete with labelled oxytocin (or an analogue). Human receptors are... 

The structures of the two peptides are similar, so it might seem surprising that their functions are so different. However, closer inspection shows that there is one small difference and one major difference in their amino acid composition. Both have a neutral, nonpolar side chain at residue 3, but residue 8 in oxytocin is the nonpolar amino acid leucine, whose side chain is a sec-butyl group, but residue 8 in vasopressin is arginine, whose side chain has a positive charge. As a result, the for oxytocin has a very low affinity for vasopressin and the receptor for vasopressin has a very low affinity for oxytocin. Since they bind different receptors, they have different functions. 

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