Differential Regulation of Thermodynamic Binding Forces of Levocetirizine and ()-Cetirizine by Lys191 in Human Histamine H₁ Receptors.

Cetirizine is a zwitterionic second-generation antihistamine containing - and -enantiomers, levocetirizine, and ()-cetirizine. Levocetirizine is known to have a higher affinity for the histamine H₁ receptors than ()-cetirizine; ligand-receptor docking simulations have suggested the importance of the formation of a salt bridge (electrostatic interaction) between the carboxylic group of levocetirizine and the Lys191 residue at the fifth transmembrane domain of human histamine H₁ receptors. In this study, we evaluated the roles of Lys191 in the regulation of the thermodynamic binding forces of levocetirizine in comparison with ()-cetirizine. The binding enthalpy and entropy of these compounds were estimated from the van 't Hoff equation, by using the dissociation constants obtained from their displacement curves against the binding of [³H]mepyramine to the membrane preparations of Chinese hamster ovary cells expressing wild-type human H₁ receptors and their Lys191 mutants to alanine at various temperatures. We found that the higher binding affinity of wild-type H₁ receptors for levocetirizine than ()-cetirizine was achieved by stronger forces of entropy-dependent hydrophobic binding of levocetirizine. The mutation of Lys191 to alanine reduced the affinities for levocetirizine and ()-cetirizine, through a reduction in the entropy-dependent hydrophobic binding forces of levocetirizine and the enthalpy-dependent electrostatic binding forces of ()-cetirizine. These results suggested that Lys191 differentially regulates the binding enthalpy and entropy of these enantiomers, and that Lys191 negatively regulates the enthalpy-dependent electrostatic binding forces of levocetirizine, contrary to the predictions derived from the ligand-receptor docking simulations.