Cholinesterase in Helix pomatia (Gastropoda: Stylommatophora): presence of a soluble (hemolymph) and a membrane-bound form

Two forms of cholinesterase (ChE) were detected in the gastropod mollusc Helix pomatia : a fully soluble (FS) ChE in the hemolymph, representing about 90% of total activity, and a detergent-soluble (DS) membrane-bound enzyme. The FS enzyme seems to be a stable complex forming a large particle with a sedimentation coefficient of 32 S. The DS ChE (estimated M r : 129,000) is likely to be an amphiphilic protein with hydrophobic domains interacting with non-ionic detergent (Triton X-100) and giving self-aggregation. Based on V max / K m values, the enzymes are an acetylcholinesterase (FS) and a butyrylcholinesterase (DS), even if they hydrolyze propionylthiocholine at the highest rate. FS ChE seems to discriminate among the substrates with an involvement of steric hindrance and hydrophobic forces; DS ChE shows a lower substrate specificity level. The study with inhibitors shows a far higher sensitivity of DS ChE to inhibition by edrophonium. Both FS and DS ChE are totally inhibited by 10 −5 M and 10 −4 M eserine, respectively. Some kinetic and molecular features of FS and DS ChE from H. pomatia are compared with those of other invertebrate enzymes.