A comparison of eel electroplax and snake venom acetylcholinesterase

The effects of some cholinergic ligands, harmala alkaloids and local anesthetics on the activity of eel electroplax and Naja naja siamensis venom acetylcholinesterase have been studied. In most cases, eel electroplax was found to be more susceptible towards inhibition than the venom acetylcholinesterase. No major difference was observed with respect to the type of inhibition in both enzymes. The activation of the two enzyme preparations by inorganic cations (Ca2+, Mg2+ and Na+) showed a similar pattern. In both preparations, the onset of activation was detectable at much lower concentration with the divalent metal ions than with the monovalent Na+. Antagonism between Ca2+ and decamethonium, tubocurarine and tetracaine in both enzymes approached competitive kinetics. The onset of substrate inhibition is delayed by Ca2+ (30 mM) in both enzymes. It is suggested that the Ca2+ binding site overlaps with the substrate inhibitory site. It is concluded that cobra venom acetylcholinesterase has similar allosteric binding sites to those of eel electroplax.