Histamine H₃‐receptor agonists and imidazole‐based H₃‐receptor antagonists can be thermodynamically discriminated

Abstract

Background and purpose: Studies suggest that measurement of thermodynamic parameters can allow discrimination of agonists and antagonists. Here we investigate whether agonists and antagonists can be thermodynamically discriminated at histamine H 3 receptors. 0 values for agonists remained unchanged or decreased with decreasing temperature, while antagonist pK I values increased with decreasing temperature; agonist binding was entropy-driven while antagonist binding was enthalpy and entropy-driven. In buffer A Ca , temperature had no effect on antagonist and agonist pK I values; both agonist and antagonist binding were enthalpy and entropy-driven. Conclusions and implications:The binding of H 3 -receptor agonists and antagonists can be thermodynamically discriminated under conditions where agonist pK I 0 values are over-estimated (pK I 0 apK app ). However, under conditions when agonist pK I BpK app , the thermodynamics underlying the binding of agonists are not different to those of antagonists.

Contribution of Binding Enthalpy and Entropy to Affinity of Antagonist and Agonist Binding at Human and Guinea Pig Histamine H1-Receptor

“…Weiland et al (1979) were the first to find that antagonists and agonists can be thermodynamically discriminated at the ␤-adrenergic receptor. Subsequently, several radioligand binding studies at the cholecystokinin CCK 2 receptor (Harper et al, 2007b), ␤-adrenergic receptor (Weiland et al, 1979;Contreras et al, 1986a,b), serotonin 5-HT 1A -receptor , histamine H 3 receptor (Harper et al, 2007a), dopamine D 2 receptor (Kilpatrick et al, 1986;Duarte et al, 1988) and adenosine A 1 and A 2A receptors were performed at different temperatures to determine thermodynamic parameters such as binding enthalpy (⌬H°) and binding entropy (⌬S°) of the ligand to the receptor. These studies showed that a thermodynamic discrimination between agonism and antagonism is only possible at some of the receptors (e.g., ␤-adrenergic receptor, 5-HT 3 , A 1 , and A 2A receptors).…”

Binding of Ligands to GPCRs ? How Valid is a Thermodynamic Discrimination of Antagonists and Agonists?

“…Moreover, a temperature dependent measurement of rate constants followed by the calculation of the Gibbs energy of activation, leads to the enthalpy and entropy of activation by the same machinery as in pure thermodynamics. For example, at the guinea-pig histamine H 3 receptor, the rate constants for association and dissociation of [ 3 H]clobenpropit was determined in dependence of temperature [14]. Based on these rate constants, the Gibbs energy, enthalpy and entropy of activation for the association process can be calculated ( Figure 3, Table 2).…”

“…Based on these rate constants, the Gibbs energy, enthalpy and entropy of activation for the association process can be calculated ( Figure 3, Table 2). Taking into account the experimental errors based on the data in literature [14], the entropy term of activation is zero. Thus, the reaction rates of association and dissociation are only determined by enthalpy (Table 2).…”

“…The thermodynamic analysis of ligand binding to the guinea-pig histamine H 3 receptor was performed at two different assay conditions [14]. Within the first series HEPES-NaOH buffer (buffer A) was used, whereas in a second series HEPES-NaOH buffer, containing 300 mM CaCl 2 (buffer A Ca ) was used.…”

“…Weiland et al (1979) were the first, who observed a thermodynamic discrimination of antagonists and agonists at the β-adrenergic receptor [6]. Subsequently, a large number of studies at different GPCRs were performed in order to determine thermodynamics of ligand binding: For example, addressing the β 2 adrenergic receptor [6][7][8][9], the serotonin 5-HT 1A -receptor [10], the dopamine D 2 receptor [11,12], the histamine H 1 receptor [13] and the histamine H 3 receptor [14]. These receptors belong to the aminergic GPCRs of the GPCR family A [3].…”