The reversible inhibition of electric eel acetylcholinesterase
(EC 3.1.1.7) by P2AM (2-(hydroxyimino)methyl-1-methyl-pyridinium
chloride) and Toxogonin (1,1\u27-[oxybis(methylene)] bis(4-(hydroxyimino)
methyl-pyridinium) dichloride) was studied using ac.etylthiocholine
as substrate. Two techniques were applied for
measuring acetylthiocholine hydrolysis, the conventional spectrophotometric
and the stopped-flow (at 25 °c in 100 mM phosphate
buffer pH = 7.4).
The correlation between the degree of inhibition, and acetylthiocholine
and oxime concentrations fits a theoretical model which
postulates that the substrate and the inhibitor bind to two sites on
the enzyme: the catalytic site and an allosteric, substrate-inhibition,
site. The calculated dissociation constants for the two sites
are: 0.13 and 0.76 mM for P2AM, and 0.16 and 2.0 mM for Toxogonin.
The suggested model is an alternative to the hypothesis that
two types of binding occur within the catalytic site.
Horse serum cholinesterase and bovine erythrocyte acetylcholinesterase
are also inhibited by P2AM and Toxogonin to about
the same degree as the electric eel enzyme.
Acetylthiocholine reacts with P2AM and Toxogonin; assuming
that the reaction is bimolecular the corresponding rate constants
are 13.4 and 22.4 M-1 min-
