Kinetic and structural studies on the interaction of cholinesterases with the anti-Alzheimer drug rivastigmine

Rivastigmine, a carbamate inhibitor of acetylcholinesterase, is already in use for treatment of Alzheimer's disease under the trade name of Exelon. Rivastigmine carbamylates Torpedo californica acetylcholinesterase very slowly (k(i) = 2.0 M-1 min(-1)), whereas the bimolecular rate constant for inhibition of human acetylcholinesterase is > 1600-fold higher (k(i) = 3300 M-1 min(-1)). For human butyrylcholinesterase and for Drosophila melanogaster acetylcholinesterase, carbamylation is even more rapid (k(i) = 9 x 10(4) and 5 x 10(5) M-1 min(-1), respectively). Spontaneous reactivation of all four conjugates is very slow, with < 10% reactivation being observed for the Torpedo enzyme after 48 h. The crystal structure of the conjugate of rivastigmine with Torpedo acetylcholinesterase was determined to 2.2 angstrom resolution. It revealed that the carbamyl moiety is covalently linked to the active-site serine, with the leaving group, (-)-S-3-[1 -(dimethylamino)ethyl]phenol, being retained in the "anionic" site. A significant movement of the active-site histidine (H440) away from its normal hydrogen-bonded partner, E327, was observed, resulting in disruption of the catalytic triad. This movement may provide an explanation for the unusually slow kinetics of reactivation.