Differential Regulation of Thermodynamic Binding Forces of Levocetirizine and (<i>S</i>)-Cetirizine by Lys191 in Human Histamine H₁ Receptors

• Main Authors: Shigeru Hishinuma, Yuri Tamura, Chihiro Kobayashi, Chizuru Akatsu, Masaru Shoji
• Format: Article
• Language: English
• Published: MDPI AG 2018-12-01
• Series: International Journal of Molecular Sciences
• Subjects: affinity; enthalpy; entropy; histamine H₁ receptor; levocetirizine; (<i>S</i>)-cetirizine
• Online Access: https://www.mdpi.com/1422-0067/19/12/4067

Cetirizine is a zwitterionic second-generation antihistamine containing <i>R</i>- and <i>S</i>-enantiomers, levocetirizine, and (<i>S</i>)-cetirizine. Levocetirizine is known to have a higher affinity for the histamine H₁ receptors than (<i>S</i>)-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 (<i>S</i>)-cetirizine. The binding enthalpy and entropy of these compounds were estimated from the van &#8216;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 (<i>S</i>)-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 (<i>S</i>)-cetirizine, through a reduction in the entropy-dependent hydrophobic binding forces of levocetirizine and the enthalpy-dependent electrostatic binding forces of (<i>S</i>)-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.