Dick Ryan A, Kanne David B, Casida John E
Environmental Chemistry and Toxicology Laboratory, Department of Environmental Science, Policy and Management, University of California, Berkeley, California 94720-3112, USA.
Chem Res Toxicol. 2005 Feb;18(2):317-23. doi: 10.1021/tx049737i.
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.
吡虫啉（IMI）是典型的新烟碱类杀虫剂，在全球范围内用于作物保护和宠物跳蚤防治。它既通过细胞色素P450酶进行氧化代谢，又通过一种此前未明确的胞质“新烟碱类硝基还原酶”在硝基亚氨基部分被还原，这正是本研究的主题。将IMI与兔肝胞质溶胶一起温育时，检测到两种主要代谢产物：亚硝基胍（IMI-NO）和氨基胍（IMI-NH2）。有三条证据表明钼黄素酶醛氧化酶（AOX，EC 1.2.3.1）是新烟碱类硝基还原酶。首先，经典的AOX电子供体底物（苯甲醛、2-羟基嘧啶和N-甲基烟酰胺）显著提高了IMI代谢产物的形成速率。别嘌呤醇和百草枯也是有效的电子供体，而NADPH和黄嘌呤则不是。其次，当使用N-甲基烟酰胺作为电子供体时，AOX抑制剂（氰化钾、甲萘醌和异丙嗪）会抑制代谢产物的形成。不添加电子供体时，兔肝胞质溶胶仅以缓慢的速率将IMI还原为IMI-NO。这种还原也受到氰化钾、甲萘醌和异丙嗪以及其他AOX抑制剂西咪替丁和氯丙嗪的抑制。最后，AOX介导的IMI硝基还原对有氧环境敏感，但程度远低于细胞色素P450 2D6。在肝胞质溶胶的IMI硝基还原活性方面观察到很大的物种差异。虽然兔和猴（食蟹猴）的总代谢产物形成水平最高，但人、小鼠、牛和大鼠也能快速代谢IMI。相比之下，狗、猫和鸡的肝胞质溶胶不会以可观的速率还原IMI。AOX作为新烟碱类硝基还原酶，可能会限制IMI以及可能其他新烟碱类在哺乳动物体内的持久性。
