Boophilus microplus: strain differences of the cholinesterase system.

Cholinesterase (EC 31.1.7 and EC 31.1.8) activity was determined for homogenates of two Argentinian strains of larval ticks, Boophilus microplus. One strain (strain A) was sensitive to organophosphate acaricides. The other strain (strain G) was insensitive to coumaphos and to a lesser degree to other acaricides. For both strains maximal activity was found at 8 × 10^?3M substrate, showing inhibition by excess substrate. Maximal activity was recorded at pH 6.7 and 7.8, the latter peak being more evident for strain G. Values of Q₁₀ of 1.15 and 1.25 were obtained for strains A and G, respectively. Centrifugation at 27,000g resulted in the separation of the total cholinesterase activity into two fractions, one soluble and one particulate; the soluble fraction in strain G showed marked resistance to inhibition by an organophosphate ester and to heat inactivation.