Institute of Specific Prophylaxis and Tropical Medicine, Center for Pathophysiology, Infectiology and Immunology, Medical University of Vienna, Vienna, Austria.
J Antimicrob Chemother. 2011 Aug;66(8):1756-65. doi: 10.1093/jac/dkr192. Epub 2011 May 22.
The mechanism of action of, and resistance to, metronidazole in the anaerobic (or micro-aerotolerant) protozoan parasite Giardia lamblia has long been associated with the reduction of ferredoxin (Fd) by the enzyme pyruvate:ferredoxin oxidoreductase (PFOR) and the subsequent activation of metronidazole by Fd to toxic radical species. Resistance to metronidazole has been associated with down-regulation of PFOR and Fd. The aim of this study was to determine whether the PFOR/Fd couple is the only pathway involved in metronidazole activation in Giardia.
PFOR and Fd activities were measured in extracts of highly metronidazole-resistant (MTR(r)) lines and activities of recombinant G. lamblia thioredoxin reductase (GlTrxR) and NADPH oxidase were assessed for their involvement in metronidazole activation and resistance.
We demonstrated that several lines of highly MTR(r) G. lamblia have fully functional PFOR and Fd indicating that PFOR/Fd-independent mechanisms are involved in metronidazole activation and resistance in these cells. Flavin-dependent GlTrxR, like TrxR of other anaerobic protozoa, reduces 5-nitroimidazole compounds including metronidazole, although expression of TrxR is not decreased in MTR(r) Giardia. However, reduction of flavins is suppressed in highly MTR(r) cells, as evidenced by as much as an 80% decrease in NADPH oxidase flavin mononucleotide reduction activity. This suppression is consistent with generalized impaired flavin metabolism in highly MTR(r) Trichomonas vaginalis.
These data add to the mounting evidence against the dogma that PFOR/Fd is the only couple with a low enough redox potential to reduce metronidazole in anaerobes and point to the multi-factorial nature of metronidazole resistance.
长期以来，甲硝唑在厌氧（或微需氧）原生动物寄生虫贾第鞭毛虫中的作用机制和耐药性一直与酶丙酮酸：铁氧还蛋白氧化还原酶（PFOR）还原铁氧还蛋白（Fd）以及随后 Fd 将甲硝唑激活为有毒自由基物质有关。甲硝唑耐药性与 PFOR 和 Fd 的下调有关。本研究旨在确定 PFOR/Fd 对是否是贾第虫中甲硝唑激活的唯一途径。
测量了高度甲硝唑耐药（MTR(r)）系提取物中的 PFOR 和 Fd 活性，并评估了重组贾第虫硫氧还蛋白还原酶（GlTrxR）和 NADPH 氧化酶的活性，以确定它们是否参与甲硝唑的激活和耐药性。
我们证明了几种高度 MTR(r) 贾第虫系具有完全功能的 PFOR 和 Fd，表明 PFOR/Fd 独立的机制参与了这些细胞中甲硝唑的激活和耐药性。黄素依赖型 GlTrxR，与其他厌氧原生动物的 TrxR 一样，还原包括甲硝唑在内的 5-硝基咪唑化合物，尽管 MTR(r) 贾第虫中的 TrxR 表达没有降低。然而，高度 MTR(r) 细胞中的黄素还原受到抑制，如 NADPH 氧化酶黄素单核苷酸还原活性下降多达 80% 所示。这种抑制与高度 MTR(r) 阴道毛滴虫中普遍受损的黄素代谢一致。
这些数据增加了越来越多的证据，证明 PFOR/Fd 不是唯一具有足够低的氧化还原电位还原厌氧菌中甲硝唑的偶联物，并指出甲硝唑耐药性的多因素性质。
