Received for publication May 7, 2008.
Revised June 20, 2008.
Accepted for publication June 23, 2008.

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

Abstract

There are species-differences between human histamine H₁ receptor (hH₁R) and guinea pig 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 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. Additionally, we constructed new active-state H₁R models based on the crystal structure of the human₂-adrenergic receptor. Compared to 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 Lys-187 (gpH₁R) interacts with large histaprodifens in the binding pocket, but in the h₂AR based model Lys-187 (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.

Key words: GPCR, Histamine H1-Receptor, Histaprodifen, N-Terminus, Phenoprodifen, second extracellular loop