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 mollu sc Helix pomatia: a fully soluble (FS) ChE in the hemolymph, represent ing about 90% of total activity, and a detergent-soluble (DS) membrane -bound enzyme, The FS enzyme seems to be a stable complex forming a la rge particle with a sedimentation coefficient of 32 S. The DS ChE (est imated M(r): 129,000) is likely to be an amphiphilic protein with hydr ophobic domains interacting with non-ionic detergent (Triton X-100) an d 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 hi ndrance and hydrophobic forces; DS ChE shows a lower substrate specifi city level. The study with inhibitors shows a far higher sensitivity o f 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.