ACETYLCHOLINESTERASE AT HIGH CATALYTIC EFFICIENCY AND SUBSTRATE-SPECIFICITY IN THE OPTIC LOBE OF ELEDONE MOSCHATA (CEPHALOPODA, OCTOPODA) -BIOCHEMICAL-CHARACTERIZATION AND HISTOCHEMICAL-LOCALIZATION

In the optic lobe of the cephalopod mollusc Eledone moschata, two acet ylcholinesterase forms I and II were detected, both showing a marked a ctive site specificity with differently sized substrates. Catalytic ef ficiency (k(cat)/K-m) of the prevailing form II is similar to that of acetylcholinesterases from vertebrate nervous system. Enzyme forms I a nd II were co-purified from a high-salt-Triton X-100 soluble extract o f optic lobe by consecutive affinity chromatographies on procainamide- and concanavalin A-Sepharose columns and then separately obtained by preparative density gradient centrifugation. According to gel-filtrati on chromatography, sedimentation analysis and SDS-PAGE, the major form II is an amphiphilic globular dimer (135-136 kDa, 6.3-7.4 S) of monom ers (66 kDa) S-S linked between terminal segments. Phosphatidylinosito l anchors give cell membrane insertion, self-aggregation and detergent (Triton X-100, Brij 97) interaction. Form I, characterized only in pa rt owing to its small amount, showed molecular size (129 kDa) and sedi mentation coefficient (7.5 S) similar to those of form II; it is likel y to be attached to the cell membrane by electrostatic interactions. B oth forms behaved similarly with various inhibitors and underwent exce ss-substrate inhibition. The results obtained suggest a common origin of both form I and II from a single gene. The former could be a degrad ation product of the prevailing one (II), which is likely to be functi onal in cholinergic synapses. (C) 1998 Elsevier Science Ltd. All right s reserved.