Flometoquin, a novel insecticide, acts on mitochondrial complex III as a Q<sub>i</sub> inhibitor

Flometoquin (FLO) is a novel quinoline-type insecticide that elicits a quick knock-down effect against target pests; however, its mode of action (MoA) remains unknown. In this study, we investigated its MoA systematically, using varying biochemical techniques. Since FLO-treated insects exhibited symptoms similar to those induced by respiratory inhibitors, we examined the effect of FLO on respiratory enzyme complexes using mitochondria isolated from different insects (housefly, diamondback moth, and western flower thrips). We found that FLO itself is not active; however, its deacylated metabolite, FloMet, specifically inhibits the activity of ubiquinol-cytochrome <i>c</i> oxidoreductase (complex III) in mitochondria at the nM level. Ligand binding assays and monitoring the reduction kinetics of cytochrome hemes <i>b</i> and <i>c</i> ₁ clearly revealed that FloMet inhibits complex III by binding to the Q_i site.