Cholinesterase and Phosphatase Activities in Adults and Infective-stage Larvae of Levamisole-resistant and Levamisole-susceptible Isolates of Haemonchus contortus

Cholinesterase (ChE) and acid phosphatase (AP) activities, but not alkaline phosphatase activities, were detected in cytosolic and membrane-bound fractions of adult and infective-stage larvae of levamisole-resistant and levamisole-susceptible Haemonchus contortus. In contrast to other gastrointestinal nematodes, the ChE activity was higher in L₃ than in adults and, in both cases, was mainly associated with membranes. ChE activity was inhibited by Triton X-100 and was only detected in membrane-bound fractions when the detergent was removed. Differences between resistant and susceptible L₃ were observed in the response to inhibitors (cytosolic fraction) and in the enzymatic content (membrane-bound fraction). Phosphatase activity was detected at acidic pH in all fractions, being higher in the adult than in the L₃ stage. In the former, most of the enzyme was localized in the membrane-bound fractions, whereas in the latter it was mainly in cytosolic fractions. This difference could be correlated with the activity in the gut. In inhibition assays, a difference between cytosolic fractions from resistant and susceptible adults was observed in their response to 1 mmol/L tartaric acid.