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 A 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.
