[Speaker] Mojca Krzan:1
[Co-author] Jan Keuschler:1, Nika Jurisevic:1, Robert Vianello:2, Janez Mavri:3
1:Department of Pharmacology and Experimental Toxicology, Faculty of Medicine, University of Ljubljana, Slovenia, 2:Rudjer Boskovic Institute, Zagreb, Croatia, 3:National Chemical Institute, Slovenia

In this presentation we report a combined experimental and computational study concerning the effects of deuteration on the binding of histamine and other histaminergic ligands to 3H-tiotidine-labeled histamine H2 receptor in neonatal rat astrocytes. Binding affinities were measured by displacing radiolabeled tiotidine from H2 receptor binding sites present on cultured neonatal rat astrocytes. Quantum-chemical calculations were performed by employing the empirical quantization of nuclear motion within a cluster model of the receptor binding site extracted from the homology model of the entire H2 receptor. After incubation of the receptor in deuterium oxide (D2O), the hydrogen atoms, bound to electronegative elements, were replaced with deuterium atoms, which lead to a change in inter- and intramolecular bond lengths which lead to a change in the affinity of a agonists to histamine H2 receptor. Experiments clearly demonstrate that deuteration affects the binding by increasing the affinity for histamine and 4-methylhistamine and reducing the affinity for 2-methylhistamine, while basically leaving it unchanged for antagonists cimetidine and famotidine. Ab initio quantum-chemical calculations on the cluster system extracted from the homology H2 model along with the implicit quantization of the acidic N-H and O-H bonds demonstrate that these changes in the binding can be rationalized by the altered strength of the hydrogen bonding upon deuteration known as the Ubbelohde effect.
Our computational analysis also reveals a new mechanism of histamine binding, which underlines an important role of Tyr250 residue. The ligand H/D substitution is relevant for therapy in the context of perdeuterated and thus more stable drugs that are expected to enter therapeutic practice in the near future. Moreover, presented approach may contribute towards understanding receptor activation, while a distant goal remains in silico discrimination between agonists and antagonists based on the receptor structure.

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