Properties of acetylcholinesterase in lobster axonal membrane

1. Acetylcholinesterase (AChE) activity of axonal membrane vesicles prepared from lobster (Homarus americanus) walking leg nerve is about 200 mumol ACh/hr/mg protein. 2. The enzyme is a membrane-bound glycoprotein that can be solubilized by lysolecithin and digitonin but is rapidly inactivated by Triton X-100 and other nonionic detergents. 3. The walking leg nerve enzyme contrasts most markedly with postsynaptic AChE in its lack of inhibition by excess substrate and its poor susceptibility to peripheral anionic site ligands. 4. Axonal AChE is competitively blocked by eserine sulfate (1 microM) and by low concentrations of procaine (0.1-0.5 mM). 5. Lidocaine, a tertiary amine, and its quaternary ammonium derivative QX-314 are equipotent non-competitive enzyme inhibitors (Kiapp 2-3 mM), while the primary amine compound GX-HCl is totally inactive. 6. Calcium (10 50 mM), propidium (1 microM) and decamethonium (1 microM), peripheral anionic site ligands in the synaptic enzyme, do not protect the axonal enzyme against inhibition by local anesthetics. 7. Although it is possible that this enzyme is an isozyme of the true AChE of excitable membranes, or that it is an incomplete enzyme fragment, it is also possible that there is a unique membrane-bound AChE present in large quantities in crustacean peripheral nerve.