Characterization of acetylcholinesterase from the gut of sea cucumber Stichopus japonicus

An acetylcholinesterase was purified from the gut of sea cucumber Stichopus japonicus by anion exchange chromatography followed by gel filtration chromatography. The enzyme was purified 35.49-fold with a total yield of 7.73 %. The molecular mass of purified acetylcholinesterase was 68 kDa as revealed on sodium dodecyl sulfate polyacrylamide gel electrophoresis. The enzyme displayed maximum activity at pH 7.5 and 35 °C with acetylthiocholine iodide as substrate. The enzyme activity appeared to be stable over pH 6.0–8.0 and up to 40 °C. It displayed an apparent Michaelis–Menten behavior in the concentration range from 0.1 to 0.8 mM with K _m values of 0.62 mM for acetylthiocholine iodide and 2.53 mM for butyrylthiocholine iodide. More than 95 % of acetylcholinesterase activity was inhibited by 1 mM eserine or 1,5-bis(4-allyldimethylammonium phenyl)-pentan-3-one dibromide (BW284C51), but only 19.1 % of the activity was inhibited by tetraisopropylpyrophosphoramide (iso-OMPA) at the same concentration. On the basis of the substrate and inhibitor specificities, the purified enzyme appeared to be a true acetylcholinesterase. Nevertheless, the purified acetylcholinesterase exhibited insensitivity to substrate inhibition phenomenon. Its biochemical properties were compared with those reported for different species.