USE OF ACETYLCHOLINESTERASE ACTIVITY TO DETECT SUBLETHAL TOXICITY IN STREAM INVERTEBRATES EXPOSED TO LOW CONCENTRATIONS OF ORGANOPHOSPHATE INSECTICIDES

Acetylcholinesterase (AChE) activity was measured in selected species of aquatic invertebrates using two methodologies based on the Ellman technique - a modified kit procedure using the spectrophotometer (BMC procedure) and a microplate assay. Levels of AChE ranged from 12.7 mU/mg protein for Daphnia magna to 96.9 mU/mg protein for Hydropsyche spp. The coefficients of variation (CVs) for the microplate technique were much lower than the CVs for the BMC procedure (e.g., 6.9 to 25.5% vs. 34.6 to 55.4%) and allowed measurements within single whole organisms or individual head capsules. Exposure of invertebrates to low concentrations of the organophosphate insecticides, azinphosmethyl and fenitrothion, did not result in a significant depression in AChE levels with one exception, i.e. levels of AChE in Hyalella azteca declined to 55.2% of control levels following 24 h exposure to 2.0 μg azinophosmethyl/l. Exposure for longer periods (48 to 96 h) did not result in reduced levels. Exposure of the stonefly, Claassenia sp., to chlorpyrifos significantly lowered AChE levels by 30.7-45.1% at concentrations approaching lethality (≥40 μg/l). The results indicate that measurement of the activity of AChE in aquatic invertebrates exposed to field concentrations of organophosphate insecticides may be a useful biochemical technique but only for detecting acute toxicity following exposure in the field. In addition, choice of species may be important in the detection of sublethal effects. © 1990.