Some kinetic and toxicological characteristics of thoracic ganglia cholinesterase of Chasmagnathus granulata (Decapoda, Grapsidae)

In this study, some kinetic and toxicological parameters of the thoracic ganglia cholinesterase from the estuarine crab Chasmagnathus granulata were determined. Effects of the type and concentration of substrate, pH (6.80-8.50), incubation temperature (5-35 degrees C) and eserine on the enzyme activity were studied. Enzymatic activity was higher at pH 7.40 and 8.00 and significantly reduced at lower temperatures (5-10 degrees C). Employing acetylthiocholine iodide (ATCh) as substrate, the K(m) and Vmax were estimated as 0.28 mM and 1.75 mumol.mg protein-1.min-1, respectively. A 14.11 and 24.51% decrease in enzyme activity were registered at 4.62 and 9.24 mM of ATCh, respectively. Using propionylthiocholine iodide as substrate, the K(m) and Vmax were estimated as 0.16 mM and 0.91 mumol.mg protein-1.min-1, respectively. The IC50 for eserine was estimated as 5.3 x 10(-4) mM. The Ki estimated for eserine (8.10 mM-1.min-1) indicates that the thoracic ganglia cholinesterase from C. granulata showed a low ability to generate an irreversible enzyme-inhibitor complex. The higher enzymatic activity registered with ATCh and the enzyme inhibition observed at high concentration of this substrate, suggest that thoracic ganglia cholinesterase from C. granulata is an acetylcholinesterase.