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CELLULAR AND MOLECULAR

Ligand-Specific Contribution of the N Terminus and E2-Loop to Pharmacological Properties of the Histamine H₁-Receptor

Departments of Pharmaceutical/Medicinal Chemistry I (A.S.) and Pharmacology and Toxicology (R.S.), School of Pharmacy, University of Regensburg, Regensburg, Germany; and Faculty of Chemistry and Pharmacy, University of Regensburg, Regensburg, Germany (H.-J.W.)

There are species differences between human histamine H₁ receptor (hH₁R) and guinea pig (gp) histamine H₁ receptor (gpH₁R) for phenylhistamines and histaprodifens. Several studies showed participation of the second extracellular loop (E2-loop) in ligand binding for some G protein-coupled receptors (GPCRs). Because there are large species differences in the amino acid sequence between hH₁R and gpH₁R for the N terminus and E2-loop, we generated chimeric hH₁Rs with gp E2-loop (h_gpE2H₁R) and gp N terminus and gp E2-loop (h_gpNgpE2H₁R). hH₁R, gpH₁R, and chimeras were expressed in Sf9 insect cells. [³H]Mepyramine binding assays and steady-state GTPase assays were performed. In the series hH₁R > h_gpE2H₁R > h_gpNgpE2H₁R, we observed a significant decrease in potency of histamine 1 in the GTPase assay. For phenoprodifen 5 and the chiral phenoprodifens 6R and 6S, a significant decrease in affinity and potency was found in the series hH₁R > h_gpE2H₁R > h_gpNgpE2H₁R. In addition, we constructed new active-state H₁R models based on the crystal structure of the human β₂-adrenergic receptor (hβ₂AR). Compared with the H₁R active-state models based on the crystal structure of bovine rhodopsin, the E2-loop differs in its contact to the ligand bound in the binding pocket. In the bovine rhodopsin-based model, the backbone carbonyl of Lys187 (gpH₁R) interacts with large histaprodifens in the binding pocket, but in the hβ₂AR-based model, Lys187 (gpH₁R) is located distantly from the binding pocket. In conclusion, the differences in N terminus and E2-loop between hH₁R and gpH₁R exert an influence on affinity and/or potency for histamine and phenoprodifens 5, 6R, and 6S.

Address correspondence to: Dr. Andrea Straßer, Department of Pharmaceutical and Medicinal Chemistry I, University of Regensburg, Universitätsstraße 31, D-93053 Regensburg, Germany. E-mail: andrea.strasser{at}chemie.uni-regensburg.de