PHARMACOLOGICAL AND FUNCTIONAL-CHARACTERIZATION OF THE WILD-TYPE AND SITE-DIRECTED MUTANTS OF THE HUMAN H-1 HISTAMINE-RECEPTOR STABLY EXPRESSED IN CHO CELLS

A cDNA clone for the human histamine H-1 receptor was isolated from a lung cDNA library and stably expressed in CHO cells. The recombinant r eceptor protein present in the cell membranes, displayed the functiona l and binding characteristics of histamine H-1 receptors. Mutation of Ser155 to Ala in the fourth transmembrane domain did not significantly change the affinity of the receptor for histamine and H-1 antagonists . However, mutation of the fifth transmembrane Asn198 to Ala resulted in a dramatic decrease of the affinity for histamine binding, and for the histamine induced polyphosphoinositides breakdown, whereas the aff inity towards antagonists was not significantly modified. In addition, mutation of another fifth transmembrane amino acid, Thr194 to Ala als o diminished, but to a lesser extent, the affinity for histamine. Thes e data led us to propose a molecular model for histamine interaction w ith the human H-1 receptor. In this model, the amide moiety of Asn198 and the hydroxyl group of Thr194 are involved in hydrogen bonding with the nitrogen atoms of the imidazole ring of histamine. Moreover, muta tion of Thr194 to Ala demonstrated that this residue is responsible fo r the discrimination between enantiomers of cetirizine.