A single form of cholinesterase was detected in the parasitic nematode
Parascaris equorum and purified from a low-salt
Triton X-100 extract of whole animals by affinity chromatography on an
edrophonium–Sepharose matrix. Based on gel-filtration chromatography,
sedimentation analysis and SDS–PAGE, such a cholinesterase is an
amphiphilic globular (G2)
dimer (125–129 kDa, 6·1 S). It includes some hydrophobic
domain other than phosphatidylinositol, which gives auto-aggregation,
detergent interaction and also anchors the molecule to the cell membrane.
enzyme, probably functional
in cholinergic neurotransmission, is an acetylcholinesterase showing a
fairly low substrate specificity with thiocholine
esters. Electrostatic interactions seem to play a major role in the catalytic
activity. Studies with inhibitors gave complete
inhibition with 1 mM eserine, low sensitivity for procainamide
for tetra(monoisopropyl)pyrophosphortetramide as
well as higher inhibition with edrophonium chloride and
1,5-bis(4allyldimethylammoniumphenyl)-pentan-3-one dibromide. The enzyme
also showed excess-substrate inhibition with acetylthiocholine. No
between the gene(s) encoding acetylcholinesterase in P. equorum
and ace-1 from the free-living nematode Caenorhabditis
elegans. The expression of a single cholinesterase form in P.
equorum, unusual in free-living nematodes, could be due to
parasitic life adaptation with resulting reduction of locomotor activity.