Soluble and membrane-bound acetylcholinesterases in Mytilus galloprovincialis (Pelecypoda: Filibranchia) from the northern Adriatic sea

Three forms of acetylcholinesterase (AChE) were detected in samples of the bivalve mollusc Mytilus galloprovincialis collected in sites of the Adriatic sea. Apart from the origin of the mussels, two spontaneously soluble (SS) AChE occur in the hemolymph and represent about 80% of total activity, perhaps hydrolyzing metabolism-borne choline esters. These hydrophilic enzymes (forms A and B) copurified by affinity chromatography (procainamide-Sepharose gel) and were separated by sucrose gradient centrifugation. They are, respectively, a globular tetramer (11.0-12.0 S) and a dimer (6.0-7.0 S) of catalytic subunits. The third form, also purified from tissue extracts by the same affinity matrix, proved to be an amphiphilic globular dimer (7.0 S) with a phosphatidylinositol tail giving cell membrane insertion, detergent (Triton X-100, Brij 96) interaction and self-aggregation. Such an AChE is likely functional in cholinergic synapses. All three AChE forms show a good substrate specificity and are inactive on butyrylthiocholine. Studies with inhibitors showed low inhibition by eserine and paraoxon, especially on SS forms, high sensitivity to 1,5-bis(4-allyldimethylammoniumphenyl)-pentan-3-one dibromide (BW284c51) and no inhibition with propoxur and diisopropylfluorophosphate (DFP). The ChE forms in M. galloprovincialis are possibly encoded by different genes. Some kinetic features of these enzymes suggest a genetic polymorphism.