The identification of the Hi receptor protein was initiated with the use of [3H]azidobenzamide by Yamashita et al. (1991b) to irreversibly label high amounts of receptor peptides (53-58 kDa) in bovine adrenal medulla membranes. Following these findings, this group cloned the gene encoding the bovine H1 receptor (Yamashita et al. 1991a) by an elegant approach of expression cloning. Injection of bovine adrenal medulla mRNA into Xenopus oocytes resulted in the induction of HA-induced chloride current. By fractionation, a single cDNA was finally isolated. This H1 -receptor cDNA encodes for a 491 amino acid receptor protein (apparent molecular weight 56 kDa) with all of the structural features of a GPCR (see Fig. 17.1) (Yamashita et al. 1991a). Since biochemical studies indicate that the H1-receptor protein is glycosylated (Garbarg et al. 1985), the predicted molecular weight of 56 kDa is certainly underestimated. Currently, genes for the rat (Fujimoto et al. 1993), guinea pig (Traiffort et al. 1994), mouse (Inoue et al. 1996) and human (De Backer et al. 1993; Fukui et al. 1994; Moguilevsky et al. 1994) H1 receptor have been cloned. The H1 receptor protein is encoded by a single exon and contains 486 (rat), 488 (guinea pig, mouse), 491 (bovine), or 487 (human) amino acids. De Backer et al. (1998) recently identified a previously unknown intron of approximately 5.8 kB in the 5' untranslated region immediately upstream of the start codon. The importance of this intron has not been clarified. The homology between the H1-receptor proteins across species is very high in the transmembrane domains (ca. 90 per cent), but is significantly lower in the intracellular and extracellular parts. The human H1 receptor is located on chromosome 3 (Table 17.1).
Site-directed mutagenesis studies indicate that binding of agonists and antagonists mainly occurs within the TM domain. The protonated amine function of HA interacts with the Asp107 residue in TM3 (Ohta et al. 1994), whereas the binding of the imidazole ring is suggested to occur via hydrogen bonding contacts with Asp198 and Lys191 residues in TM5 (Leurs et al. 1994a, 1995a). Like HA, the antagonists also use the conserved aspartate residue in TM3 as a counter ion for their protonated amine function (Ohta et al. 1994). Mutagenesis studies revealed that Phe433 and Phe435 are likely to be involved in the binding of the aromatic rings of the H1 antagonists. Moreover, an additional interaction point (Lys191 in TM5) was suggested for the acidic side-chain in the non-sedative, zwitterionic H1 antagonists acrivastine and cetirizine (Gillard et al. 2002; Wieland et al. 1999), whereas Thr194 is suggested to regulate the known H1 antagonist stereospecificity (Gillard etal. 2002).
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