Multiplicity of cytochrome P-450 in microsomal membranes from the housefly, Musca domestica

Biochemical Mechanisms of Resistance to Insecticides

The principal biochemical mechanisms of resistance to insecticides involve either modified, less sensitive cholinesterase, esterase action, glutathione S-transferase action or cytochrome P-450-dependent monooxygenation. Both quantitative and qualitative differences in cytochrome P-450 isozymes are under genetic control and both are related to resistance. Recent characterization studies involving ligand binding and multiplicity of isozymes in Musca domestica (the housefly) are discussed in relation to resistance. The recent demonstration that multiple isozymes of glutathione S-transferase exist in susceptible and resistant insects is of interest, and some re-examination of their role in the mechanism of resistance is required. Esterases are a heterogeneous group of enzymes whose role in resistance has often been suggested but seldom rigorously defined. Purification studies in the green rice leafhopper, Nephotettix cincticeps, have involved an enzyme with carboxylesterase, phosphotriesterase and pyrethroid esterase activities. A similar enzyme, but without pyrethroid esterase activity, is also found in the housefly. In resistance such enzymes may serve either to catalyse hydrolysis or as binding proteins. It has been suggested, from time to time, that regulator genes, enzyme induction and gene magnification all play a part in controlling biochemical mechanisms of resistance, although clearly defined evidence has not always been brought forward. These hypotheses are re-examined.

Cytochromes P450 and insecticide resistance

The cytochrome P450-dependent monooxygenases (monooxygenases) are an extremely important metabolic system involved in the catabolism and anabolism of xenobiotics and endogenous compounds. Monooxygenase-mediated metabolism is a common mechanism by which insects become resistant to insecticides as evidenced by the numerous insect species and insecticides affected. This review begins by presenting background information about P450s, the role of monooxygenases in insects, and the different techniques that have been used to isolate individual insect P450s. Next, insecticide resistance is briefly described, and then historical information about monooxygenase-mediated insecticide resistance is reviewed. For any case of monooxygenase-mediated resistance, identification of the P450(s) involved, out of the dozens that are present in an insect, has proven very challenging. Therefore, the next section of the review focuses on the minimal criteria for establishing that a P450 is involved in resistance. This is followed by a comprehensive examination of the literature concerning the individual P450s that have been isolated from insecticide resistant strains. In each case, the history of the strain and the evidence for monooxygenase-mediated resistance are reviewed. The isolation and characterization of the P450(s) from the strain are then described, and the evidence of whether or not the isolated P450(s) is involved in resistance is summarized. The remainder of the review summarizes our current knowledge of the molecular basis of monooxygenase-mediated resistance and the implications for the future. The importance of these studies for development of effective insecticide resistance management strategies is discussed.

Separation of cytochrome P-450 containing vesicles from the midgut microsomal fraction of Manduca sexta

1. Microsomes prepared from midgut tissue of Manduca sexta by differential centrifugation are a heterogeneous population of vesicles. 2. Upon centrifugation of microsomes in a sucrose gradient of 30%, 15% and 10%, specific activity of cytochrome P-450 catalyzed O-demethylation of p-nitroanisole (a marker enzyme for endoplasmic reticulum) was increased 2.5 to 3.5-fold in the 15% fraction. 3. Specific activities of alkaline phosphatase and leucine aminopeptidase (marker enzymes for brush border membranes) were increased 3.1 to 5.7-fold in the pellet. 4. Differential centrifugation is a useful step in the purification of cytochrome P-450 enzymes.

Cytochrome P-450 monooxygenases in insects

1. The insect monooxygenase system is involved in the oxidative metabolism of both endogenous and exogenous substrates. 2. Monooxygenases appear to be important in insect growth and development, in adaptation to multiple food plants in phytophagous insects and in pesticide resistance. 3. The genetics, purification and properties of cytochrome P-450 isozymes of the house fly (Musca domestica) are discussed in relation to their role in resistance to xenobiotics. 4. Induction of insect cytochrome P-450 isozymes is discussed in comparison to induction in mammals and with reference to polyphagy.