Submitted to: Insect Biochemistry and Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/31/2010
Publication Date: 4/1/2011
Citation: Niu, G., Rupasinghe, S.G., Zangerl, A.R., Siegel, J.P., Schuler, M.A., Berebbaum, M.R. 2011. A substrate-specific cytochrome P450 monooxygenase, CYP6AB11, from the polyphagous navel orangeworm (Amyelois transitella). Insect Biochemistry and Molecular Biology. 41(4):244-253. Interpretive Summary: The navel orangeworm (NOW) Amyelois transitella is a moth that is the principal pest of almonds and pistachios in California. Navel orangeworm also serves as a vector for the aflatoxin-releasing fungi Aspergillus flavus and A. parasiticus, and feeding damage facilitates the entry of fungal spores. In order to meet the industry goal of constantly improving quality, minimizing NOW infestation of almonds and pistachios is a priority. In comparison with most other insects, NOW is remarkably resistant to aflatoxin toxins because it has an extremely active mechanism, known as P450, for detoxifying chemicals. This same system allows the NOW to thrive on a broad range of chemically diverse host plants because it can deactivate their anti-insect toxic chemicals. This active detoxification system may also enhance the ability of NOW to become resistant to synthetic organic insecticides. In this paper, the ability of one enzyme in the P450 detoxification system to degrade both plant defense compounds and a synergist (a chemical used with insecticides to increase their toxicity) commonly used to enhance the activity of pyrethroid insecticides was evaluated. The NOW successfully degraded the plant defense compound Imperatorin, which is unusual because the mechanism was specific to this chemical, suggesting that it may have occurred on the original host of NOW. This moth was also able to slowly degrade the synergist piperonyl butoxide, and this ability may rule out the use of this synergist in the future.
Technical Abstract: The navel orangeworm Amyelois transitella (Walker) (NOW: Lepidoptera: Pyralidae) is a serious pest of many tree crops in California orchards, including almonds, pistachios, walnuts and figs. To understand the molecular mechanisms underlying detoxification of phytochemicals, insecticides and mycotoxins by this species, full-length CYP6AB11 cDNA was isolated from larval midguts using RACE PCR. Phylogenetic analysis of this insect cytochrome P450 monooxygenase established its evolutionary relationship to a P450 that selectively metabolizes imperatorin (a linear furanocoumarin) and myristicin (a natural methylenedioxyphenyl compound) in another lepidopteran species. Metabolic assays conducted with baculovirus-expressed P450 protein, P450 reductase and cytochrome b5 on 16 compounds, including phytochemicals, mycotoxins, and synthetic pesticides, indicated that CYP6AB11 efficiently metabolizes imperatorin (0.88 pmol/min/pmol P450) and slowly metabolizes piperonyl butoxide (0.11 pmol/min/pmol P450). LC-MS analysis indicated that the imperatorin metabolite is an epoxide generated by oxidation of the double bond in its extended isoprenyl side chain. Predictive structures for CYP6AB11 suggested that its catalytic site contains a doughnut-like constriction over the heme that excludes aromatic rings on substrates and allows only their extended side chains to access the catalytic site. That CYP6AB11 can metabolize the principal insecticide synergist piperonyl butoxide, a synthetic methylenedioxyphenyl compound in use to circumvent resistance, raises the possibility that, once acquired, insecticide resistance in this species may be difficult to counter by conventional means.