A key detoxifying protein in Anopheles mosquitoes that metabolizes DDT may be important in mosquitoes developing resistance to the insectcide, researchers at the University of Illinois have found.
The findings show that a protein produced at elevated levels in DDT-resistant Anopheles gambiae mosquitoes actually metabolizes the insecticide.
Anopheles gambiae as a species includes many closely related mosquito strains that transmit the malarial parasite to humans and other animals.
The protein that metabolized DDT, CYP6Z1, belongs to a class of cytochrome P450 monooxygenases (P450s) that are known to be important detoxifying agents in many species.
Previous studies in a variety of insect species have shown that P450s play key roles in insect defences against plant toxins.
With the help of molecular modelling techniques based on the three-dimensional structure of a similar protein found in humans, principal investigator Mary A. Schuler and postdoctoral researchers Ting-Lan Chiu and Sanjeewa Rupasinghe were able to visualize the likely orientation of the molecules that allowed CYP6Z1 to bind to, and inactivate, DDT.
The model predicted that the active site of CYP6Z1 could accommodate a single molecule of DDT and inactivate it by adding oxygen to a chlorinated side group on the DDT molecule.
Their model of a similar protein, CYP6Z2, which is also produced at elevated levels in some DDT-resistant Anopheles mosquito strains, predicted that it was structurally incapable of binding and hence inactivating DDT.
Biochemical studies, conducted by postdoctoral researcher Zhimou Wen, confirmed that CYP6Z1 did in fact inactivate DDT while CYP6Z2 did not.
"To understand the relationship of different P450s, you really need to look at three-dimensional active site predictions in order to see what are critical variations between evolutionarily related P450s," Schuler said.
"The configuration of the CYP6Z1 active site is open enough so that DDT can come in close enough to the reactive center to be oxygenated and, therefore, disabled," she added.
Schuler chose the CYP6Z1 protein for further study from a list of P450 genes that were transcriptionally elevated in resistant mosquitoes because its gene structure closely resembled other P450s that she and entomology department head May Berenbaum had studied in pest insects in the United States.
By comparing models developed for the CYP6Z1 proteins in 'sensitive' and 'resistant' strains of A. gambiae mosquitoes, the researchers found that, from a three-dimensional perspective, the CYP6Z1 proteins were not appreciably different from one another.
Variations did occur, but often these were on the surface of the protein in regions not important for DDT metabolism.
"With biochemical analysis showing that the CYP6Z1 protein can metabolize DDT quite efficiently, you have to ask: What's the difference between the sensitive strain and the resistant strain?" Schuler said.
"It has to be that these transcripts and their proteins are over-expressed in the resistant strains and, as a consequence, are allowing them to exhibit this resistance," she added.
Schuler said that it is probable that exposure to potent, naturally occurring plant toxins or to synthetic insecticides causes the insects to step up production of certain P450 proteins, such as CYP6Z1, that subsequently aid in the detoxification of these compounds.
The study is published in the Proceedings of the National Academy of Sciences.