Biochemical Factors Responsible for an Extremely High Level of Diazinon Resistance in a Housefly Strain

Abstract

A housefly colony collected at a sanitary land-fill was selected with diazinon for several generations in the laboratory. The resultant strain was 1400-fold resistant to diazinon (LD₅₀: 50μg per fly) when compared to the susceptible CSMA strain. It had increased activity in each of the three enzyme systems involved in detoxification. Cytochrome P-450-dependent monooxygenases catalyzed diazinon degradation, producing diethyl phosphorothioic acid and diethyl phosphoric acid. Glutathione transferase mediating dealkylation and dearylation reactions degraded both diazinon and diazoxon. Phosphorotriester hydrolase activity for diazoxon was detected only in the resistant strain and was present in all the subcellular fractions examined. The activity was highest in the soluble fraction. The resistant strain also showed a decreased sensitivity to inhibition at the site of action, acetylcholinesterase (AChE). Interstrain comparison in AChE sensitivity using diazoxon as the inhibitor revealed ca. 6-fold difference in the bimolecular rate constant k_i. An extremely high resistance of this housefly strain to diazinon may be attributable to a synergistic effect of multiple factors.