Concentration-dependent reversible activation-inhibition of human butyrylcholinesterase by tetraethylammonium ion.
Tetraalkylammonium (TAA) salts are well known reversible inhibitors of cholinesterases. However, at concentrations around 10 mm, they have been found to activate the hydrolysis of positively charged substrates, catalyzed by wild-type human butyrylcholinesterase (EC 3.1.1.8) [Erdoes, E.G., Foldes, F.F., Zsigmond, E.K., Baart, N. & Zwartz, J.A. (1958) Science 128, 92]. The present study was undertaken to determine whether the peripheral anionic site (PAS) of human BuChE (Y332, D70) and/or the catalytic substrate binding site (CS) (W82, A328) are involved in this phenomenon. For this purpose, the kinetics of butyrylthiocholine (BTC) hydrolysis by wild-type human BuChE, by selected mutants and by horse BuChE was carried out at 25  degreeC and pH 7.0 in the presence of tetraethylammonium (TEA). It appears that human enzymes with more intact structure of the PAS show more prominent activation phenomenon. The following explanation has been put forward: TEA competes with the substrate at the peripheral site thus inhibiting the substrate hydrolysis at the CS. As the inhibition by TEA is less effective than the substrate inhibition itself, it mimics activation. At the concentrations around 40 mm, well within the range of TEA competition at both substrate binding sites, it lowers the activity of all tested enzymes.
Stojan
Jure
J
Institute of Biochemistry, Medical Faculty, University of Ljubljana, Slovenia. stojan@ibmi.mf.uni.lj.si
Golicnik
Marko
M
Froment
Marie-Thérèse
MT
Estour
Francois
F
Masson
Patrick
P
D000818:Animals; D002091:Butyrylcholinesterase / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D002092:Butyrylthiocholine / Q000378:metabolism; D020134:Catalytic Domain / Q000235:genetics; D002800:Cholinesterase Inhibitors / Q000494:pharmacology*; D003198:Computer Simulation; D004789:Enzyme Activation; D006736:Horses; D006801:Humans; D008956:Models, Chemical; D008958:Models, Molecular; D009154:Mutation; D011994:Recombinant Proteins / Q000378:metabolism; D019789:Tetraethylammonium / Q000494:pharmacology*; D014101:Torpedo
D002800:Cholinesterase Inhibitors; D011994:Recombinant Proteins; D002092:Butyrylthiocholine; D019789:Tetraethylammonium; D002091:Butyrylcholinesterase