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United States Department of Agriculture

Agricultural Research Service

Title: COMPARATIVE INHIBITION KINETICS FOR ACETYLCHOLINESTERASES EXTRACTED FROM ORGANOPHOSPHATE RESISTANT AND SUSCEPTIBLE BOOPHILUS MICROPLUS (ACARI:IXODIDAE)

Author
item Pruett, John

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 24, 2002
Publication Date: October 2, 2002
Citation: PRUETT JR, J.H. COMPARATIVE INHIBITION KINETICS FOR ACETYLCHOLINESTERASES EXTRACTED FROM ORGANOPHOSPHATE RESISTANT AND SUSCEPTIBLE BOOPHILUS MICROPLUS (ACARI:IXODIDAE). JOURNAL OF ECONOMIC ENTOMOLOGY. v. 95. p. 1239-1244.

Interpretive Summary: Acetylcholinesterase (AChE) is associated with nerve transmission in the central nervous system of ticks. Inhibition of AChE activity by organophosphate (OP) acaricides is lethal. Selective pressure through the use of OP acaricides has led to the development of AChEs that are more resistant to inhibition by OP than the wild type enzyme. Resistant ticks are a threat to the cattle fever tick eradication program. Therefore, study of the molecular basis of OP resistance is important to the continued use of these important acaricides that prevent reentry of this important disease vector into the U. S. In this study AChEs were extracted from two Mexican Boophilus microplus strains that demonstrated resistance to the OP acaricide, coumaphos in bioassays. The rate of inhibition of the extracted AChEs by the diethyl-OP paraoxon was determined for two resistant strains and two susceptible strains of B. microplus. The time to inhibition of 50% of AChE activity was approximately two-fold greater for the resistant strains. Kinetic analysis of the interaction of the resistant AChEs with paraoxon revealed reduced bimolecular reaction constants. Apparent conformational changes in the AChEs of the resistant strains were reflected in reduced Km and Vmax values. The bimolecular reaction constants of the resistant strains were most affected by a slower rate of enzyme phosphorylation.

Technical Abstract: Acetylcholinesterase (AChE) is associated with cholinergic synapses in the central nervous system of arthropods. Inhibition of AChE activity by organophosphate (OP) acaricides can be lethal. Selective pressure through the use of OP acaricides has led to the development of AChEs that are more resistant to inhibition by OP than is the wild type enzyme. In this study AChEs were extracted from two Mexican Boophilus microplus strains that demonstrated resistance to the OP acaricide, coumaphos in bioassay. The rate of inhibition of the extracted AChEs by the diethyl-OP paraoxon was determined for the two resistant strains and two susceptible strains of B. microplus. The time to inhibition of 50% AChE activity was approximately two-fold greater for the resistant strains. Kinetic analysis of the interaction of the resistant AChE's with paraoxon revealed reduced bimolecular reaction constants (ki). Apparent conformational changes in the AChE of the resistant strains were reflected in reduced Km and Vmax values. The bimolecular reaction constants (ki) of the resistant strains were most affected by a slower rate of enzyme phosphorylation (k2).

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