Identification of aldehyde oxidase as the neonicotinoid nitroreductase.

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Dick RA, Kanne DB, Casida JE

Identification of aldehyde oxidase as the neonicotinoid nitroreductase.

Chem Res Toxicol. 2005 Feb;18(2):317-23.

PubMed ID

15720138 [ View in PubMed

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Abstract

Imidacloprid (IMI), the prototypical neonicotinoid insecticide, is used worldwide for crop protection and flea control on pets. It is both oxidatively metabolized by cytochrome P450 enzymes and reduced at the nitroguanidine moiety by a previously unidentified cytosolic "neonicotinoid nitroreductase", the subject of this investigation. Two major metabolites are detected on incubation of IMI with rabbit liver cytosol: the nitrosoguanidine (IMI-NO) and the aminoguanidine (IMI-NH2). Three lines of evidence identify the molybdo-flavoenzyme aldehyde oxidase (AOX, EC 1.2.3.1) as the neonicotinoid nitroreductase. First, classical AOX electron donor substrates (benzaldehyde, 2-hydroxypyrimidine, and N-methylnicotinamide) dramatically increase the rate of formation of IMI metabolites. Allopurinol and diquat are also effective electron donors, while NADPH and xanthine are not. Second, AOX inhibitors (potassium cyanide, menadione, and promethazine) inhibit metabolite formation when N-methylnicotinamide is utilized as an electron donor. Without the addition of an electron donor, rabbit liver cytosol reduces IMI only to IMI-NO at a slow rate. This reduction is also inhibited by potassium cyanide, menadione, and promethazine, as well as by additional AOX inhibitors, cimetidine and chlorpromazine. Finally, IMI nitroreduction by AOX is sensitive to an aerobic atmosphere, but to a much lesser extent than cytochrome P450 2D6. Large species differences are observed in the IMI nitroreductive activity of liver cytosol. While rabbit and monkey (Cynomolgus) give the highest levels of total metabolite formation, human, mouse, cow, and rat also metabolize IMI rapidly. In contrast, dog, cat, and chicken liver cytosols do not reduce IMI at appreciable rates. AOX, as a neonicotinoid nitroreductase, may limit the persistence of IMI, and possibly other neonicotinoids, in mammals.

DrugBank Data that Cites this Article

Drug Enzymes

Drug	Enzyme	Kind	Organism	Pharmacological Action	Actions
N-methylnicotinamide	Aldehyde oxidase	Protein	Humans	Unknown	Substrate	Details