Kinetics of inhibition of acetylcholinesterase by 9-hydrazino-1,2,3,4-tetrahydroacridine and 9-amino-10-methyl-1,2,3,4-tetrahydroacridinium in vitro

The kinetics was studied of the inhibition of solubilized rat brain acetylcholinesterase by 9-hydrazino-1,2,3,4-terahydroacridine (THH) and 9-amino-10-methyl-1,2,3,4-tetrahydroacridinium (QTHA); the inhibitory effect was compared with the effect of tacrine (9-amino-1,2,3,4-tetrahydroacridine, THA). It was observed that THH is a reversible, noncompetitive inhibitor of rat brain acetylcholinesterase (Ki = 0.16 μM) and that it binds, similarly to THA, to the hydrophobic domain of the active center of acetylcholinesterase thus simultaneously inhibiting the formation of complex ES2 with acetylcholine as substrate. This eliminates the inhibition of acetylcholinesterase by an excess of substrate. QTHA is a mixed, competitive-non-competitive inhibitor characterized by KI comp = 5.3 μM and Ki noncomp = 0.08 μM. QTHA binds to an entirely different site of the active surface of acetylcholinesterase than THA and THH. This binding site is most likely the so-called β-anionic or also peripheric anionic site to which, e.g. atropine is also bound. Both inhibitors studied form with acetylcholinesterase a reversible, enzymatically inactive complex in which one inhibitor molecule is bonded to each active center of the enzyme.