Effects of chlorpyrifos on the catalytic efficiency and expression level of acetylcholinesterases in the bivalve mollusc Scapharca inaequivalvis

(AChE) forms were detected and recovered from foot or gill tissues of the benthonic bivalve mollusc Scapharca inaequivalvis. A study was performed to investigate changes in catalytic and hydrodynamic features of these enzymes as well as in their expression levels after a 4 or a15 day exposure to a sublethal 79 concentration (0.1 ppm) of the pesticide chlorpyrifos (CPF). Both considered organs hold, in either CPF exposed or untreated animals, two non-amphiphilic AChE forms, G2 and G4, which copurified on a procainamide-containing affinity gel, and were separated by density gradient centrifugation. A third AChE form, an amphiphilic membrane-anchored G2, was also purified on the same affinity matrix from both organs. All enzymatic forms are true AChEs and are poorly inhibited by CPF. They show differently sized increases in the maximum velocity (Vmax) and in the Michaelis constant (Km) values after CPF exposure. Consequently, catalytic efficiency of AChEs, as defined by the ratio Vmax / Km, rises in the gills and drops in the foot. This would produce an over-expression of AChE specific mRNAs. The effect was longer lasting in the foot. The combined results suggest that over-expression of enzymes in the presence of organophosphate (OP) may be a consequence of OP-resistance itself. Again, the resistance of the organism to CPF seem to depend mainly on the resulting increase in AChE content.