Recovery of cholinesterase activity in the earthworm Eisenia fetida savigny following exposure to chlorpyrifos

Organophosphorus (OP) insecticides inhibit cholinesterase activity, an essential process in the nervous system of most animals. Re‐establishment of active enzymes is slow and depends on elimination of the insecticide from the body followed by two lengthy processes: Reactivation and/or biosynthesis of new enzymes. Earthworms (Eisenia fetida) were exposed to either clean or chlorpyrifos‐containing (240 mg/kg) soil for 48 h. After transfer to clean soil, we monitored two cholinesterases (E1 and E2) and chlorpyrifos content of the earthworms for 12 weeks. After 14 to 21 d of recovery, the exposed and control worms were indistinguishable in terms of appearance and behavior. Chemical analysis showed a rapid elimination of chlorpyrifos from the earthworms, with only minor levels detected after one week. The activities of E1 and E2 were measured spectrophotometrically in whole specimen homogenates using acetylthiocholine as the substrate. Carbaryl, which selectively inhibits E1, was used to discriminate the enzyme activities. Mean ± standard error of mean of E1 and E2 activity in the controls immediately after exposure were 1.57 ± 0.18 nanokatal (nkat)/mg protein (n = 3) and 0.95 ± 0.07 nkat/mg protein, respectively, and 0.48 ± 0.07 nkat/mg and 0.45 ± 0.06 nkat/mg, respectively, in exposed worms. After three weeks, E1 had regained an activity comparable to the controls, whereas E2 remained depressed throughout the 12‐week monitoring period. The non‐ or late recovery of E2 makes this enzyme a potential biomarker candidate for previous OP insecticide exposure in Eisenia fetida, provided the protocol for measurements is improved and standarized.