Concentration-dependent reversible activation-inhibition of human butyrylcholinesterase by tetraethylammonium ion
Résumé
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 °C 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.