EXPRESSION OF A SINGLE DIMERIC MEMBRANE-BOUND ACETYLCHOLINESTERASE INPARASCARIS-EQUORUM

A single form of cholinesterase was detected in the parasitic nematode Parascavis equorum and purified from a low-salt Triton X-100 extract of whole animals by affinity chromatography on an edrophonium-Sepharos e matrix. Based on gelfiltration chromatography, sedimentation analysi s and SDS-PAGE, such a cholinesterase is an amphiphilic globular (G(2) ) dimer (125-129 kDa, 6.1 S). It includes some hydrophobic domain othe r than phosphatidylinositol, which gives autoaggregation, detergent in teraction and also anchors the molecule to the cell membrane. The enzy me, probably functional in cholinergic neurotransmission, is an acetyl cholinesterase showing a fairly low substrate specificity with thiocho line esters. Electrostatic interactions seem to play a major role in t he catalytic activity. Studies with inhibitors gave complete inhibitio n with 1 mM eserine, low sensitivity for procainamide and for tetra(mo noisopropyl)pyrophosphortetramide as well as higher inhibition with ed rophonium chloride and 1,5-bis(4allyldimethylammoniumphenyl)-pentan-3- one dibromide. The enzyme also showed excess-substrate inhibition with acetylthiocholine. No cross-hybridization occurred between the gene(s ) encoding acetylcholinesterase in P. equorum and ace-1 from the free- living nematode Caenorhabditis elegans. The expression of a single cho linesterase form in P. equorum, unusual in free-living nematodes, coul d be due to parasitic life adaptation with resulting reduction of loco motor activity.