Rapid assessment of organophosphorous/carbamate exposure in the bivalve mollusc Mytilus edulis using combined esterase activities as biomarkers.

Carboxylesterases in bivalve molluscs exhibit greater sensitivity to organophosphorous and carbamate pesticides than acetylcholinesterase and are present at higher levels. The aim of the present study was to combine measurement of both acetylcholinesterase and carboxylesterase activities in the marine bivalve Mytilus edulis in order to detect the effects of pesticide exposure. Spectrophotometric assays in microtitreplate format were optimised for use with M. edulis haemolymph and tissue homogenate samples. This permitted the nature and distribution of the enzymes to be determined. One predominant pharmacological form of activity consistent in its patterns of activation and inhibition with acetylcholinesterase was identified in the haemolymph with an apparent K(m) for acetylthiocholineiodide of 1.33 mM. Carboxylesterase activity in the tissues was characterised by its preferential hydrolysis of the substrate analogue phenylthioacetate. Concentration-dependent inhibition of both activities was demonstrated following in vitro incubation with diisopropylfluorophosphate (DFP), paraoxon and eserine in the range 0.1-3.0 mM. When M. edulis (n=10) were exposed for 24 h to concentrations of eserine or paraoxon of 0.05-1.0 mM, the percentage inhibition of acetylcholinesterase was in each case greater than for carboxylesterase and reached statistical significance at lower concentrations. In all exposures, a proportion of carboxylesterase activity was present which remained resistant to inhibition by either organophosphorous or carbamate compounds. The ecotoxicological significance of these findings for the environmental monitoring of pesticide exposure is discussed.