title: (-)-7'-Isothiocyanato-11-hydroxy-1',1'-dimethylheptylhexahydrocannabinol (AM841), a high-affinity electrophilic ligand, interacts covalently with a cysteine in helix six and activates the CB1 cannabinoid receptor.
abstract: The CB1 cannabinoid receptor has been shown to play important physiological roles in the central nervous system, as well as peripherally, and is a target for development of therapeutic medications. To gain insight on the ligand binding site(s) and structural features of activation, we designed and synthesized (-)-7'-isothiocyanato-11-hydroxy-1',1'-dimethylheptylhexahydrocannabinol (AM841), a classical cannabinoid affinity label that incorporates an isothiocyanate substituent as an electrophilic reactive group capable of interacting irreversibly with a suitably located and properly oriented nucleophilic amino acid residue at or near the binding site. To obtain evidence for the site of covalent attachment of AM841, C6.47, identified in part by interactive ligand docking, was mutated to serine, alanine, and leucine to reduce or eliminate the nucleophilic character. Wild-type (WT) and mutant CB1 receptors were evaluated for their abilities to recognize a series of cannabinergic ligands. Each bound comparably to WT, excluding C6.47L, which displayed a reduced affinity for 3H-labeled (1R,3R,4R)-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-4-(3-hydroxypropyl)cyclohexan-1-ol (CP55940), AM841, 11-hydroxy-1',1'-dimethylheptylhexahydrocannabinol (AM4056), and (-)-7'-bromo-11-hydroxy-1',1'-dimethylheptylhexahydrocannabinol (AM4043) and an improvement in affinity for (-)-trans-delta9-tetrahydrocannabinol (delta9-THC). The affinity of 3H-labeled [2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrrolo-[1,2,3-de]-1,4-benzoxazin-6-yl](naphthyl)methanone (WIN55212-2) was unchanged across all mutants. It is noteworthy that AM841 was shown to bind irreversibly to WT CB1 but exhibited no covalent attachment with the mutants and behaved as an agonist suggesting irreversible attachment to C6.47 maintains CB1 in its active state. The evidence presented identifies C6.47 as the site of covalent bond formation with AM841 and combined with the binding data fully supports the molecular modeling. These studies present the first report of tandem applications of affinity labeling, site-directed mutagenesis, and interactive ligand docking for CB1.
authors.affiliation: Center for Drug Discovery, Department of Pharmaceutical Sciences, University of Connecticut, Storrs, USA.
authors.forename: Robert P
authors.initials: RP
authors.lastname: Picone
authors.forename: Atmaram D
authors.initials: AD
authors.lastname: Khanolkar
authors.forename: Wei
authors.initials: W
authors.lastname: Xu
authors.forename: Lionel A
authors.initials: LA
authors.lastname: Ayotte
authors.forename: Ganesh A
authors.initials: GA
authors.lastname: Thakur
authors.forename: Dow P
authors.initials: DP
authors.lastname: Hurst
authors.forename: Mary E
authors.initials: ME
authors.lastname: Abood
authors.forename: Patricia H
authors.initials: PH
authors.lastname: Reggio
authors.forename: Donna J
authors.initials: DJ
authors.lastname: Fournier
authors.forename: Alexandros
authors.initials: A
authors.lastname: Makriyannis
mesh_terms: D000345:Affinity Labels; D001665:Binding Sites; D002187:Cannabinol / Q000031:analogs & derivatives* / Q000737:chemistry; D003545:Cysteine; D013759:Dronabinol / Q000031:analogs & derivatives* / Q000737:chemistry; D006801:Humans; D008024:Ligands; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D043884:Receptor, Cannabinoid, CB1 / Q000235:genetics / Q000378:metabolism*; D055672:Static Electricity
chemical_list: C507046:7'-Isothiocyanato-11-hydroxy-1',1'-dimethylheptylhexahydrocannabinol; D000345:Affinity Labels; D008024:Ligands; D043884:Receptor, Cannabinoid, CB1; D013759:Dronabinol; D002187:Cannabinol; D003545:Cysteine