Polysubstrate Monooxygenases and Other Xenobiotic-Metabolizing Enzymes in Susceptible and Resistant Colorado Potato Beetle

The activities of key defensive enzymes against xenobiotics were studied in two azinphosmethyl-resistant and one susceptible strain of Colorado potato beetle ( Lepinotarsa decemlineata Say) with emphasis on the microsomal polysubstrate monooxygenase system. Monooxygenase activity comparable to that in other coleopterans was observed in the oxidation of NADPH, O-demethylation of p-nitroanisole, N-demethylation,of aminopyrine, and epoxidation of aldrin. Aniline hydroxylase activity was not detected. Monooxygenase activity against aminopyrine was largely confined to the microsomal fraction of homogenates and was more active in the larval gut than in the fat body. Microsomal NADPH oxidation was inhibited strongly in vitro by SKF 525-A and in vivo by prior treatment of insects with piperonyl butoxide. The activity of the monooxygenase system was approximately twice as high in the resistant strains as that in the susceptible strain. Glutathione transferase activity was also elevated in the resistant strains, but no elevation in carboxyesterase activity was seen. Piperonyl butoxide pretreatment increased the toxicity of azinphosmethyl several-fold more strongly in the resistant than susceptible strain, indicating a significant role for monooxygenase activity in resistance. However, in both strains, resistance to azinphosmethyl is multifactorial and enhanced monooxygenase activity is only one contributing factor.