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Infections with parasitic helminths are important causes of morbidity and mortality worldwide. New drugs that are parasite specific and minimally toxic to the host are needed to counter these infections effectively. Here we report the finding of a previously unidentified compound, nafuredin, from Aspergillus niger. Nafuredin inhibits NADH-fumarate reductase (complexes I ؉ II) activity, a unique anaerobic electron transport system in helminth mitochondria, at nM order. It competes for the quinone-binding site in complex I and shows high selective toxicity to the helminth enzyme. Moreover, nafuredin exerts anthelmintic activity against Haemonchus contortus in in vivo trials with sheep. Thus, our study indicates that mitochondrial complex I is a promising target for chemotherapy, and nafuredin is a potential lead compound as an anthelmintic isolated from microorganisms. Helminthiasis is a crucial problem worldwide, because it is not only a major cause of human morbidity in the tropics as well as temperate climates (1), but is responsible also for enormous economic losses in livestock animals (2). In addition, recent results suggest that helminth infections impair the immune response of the host to HIV and tuberculosis and might contribute to the spread of these diseases (3). Furthermore, the emergence of resistance against generally used anthelmintics makes the problem more serious (2, 4). Therefore, the development of novel and potent anthelmintics is urgent. Different classes of broad-spectrum agents such as levamisole, pyrantel, benzimidazoles, and the macrocyclic lactones (e.g., avermectin) have been used widely to treat helminthiasis (5). However, the discovery of potent new classes of anthelmintics has been rare after our discovery of avermectin (6), which was isolated from Streptomyces avermitilis and has been used to treat filariasis worldwide. Regular treatment of livestock causes the emergence of resistance against these classes of drugs. Therefore, there is a steady and urgent need to develop novel anthelmintics and to find new potential targets for such compounds.Because helminths have exploited a variety of energytransducing systems in their adaptation to the peculiar habitats in their hosts, differences in energy metabolisms between the host and helminths are attractive therapeutic targets of helminthiasis. NADH-fumarate reductase is part of a unique respiratory system in parasitic helminths (7-10) and is the terminal step of the phosphoenolpyruvate carboxykinase-succinate pathway, which is found in many anaerobic organisms (11-13). The composition and linear sequential order of the respiratory components of NADH-fumarate reductase have been elucidated with mitochondria from the parasitic nematode, Ascaris suum ( Fig. 1; refs. 14-17). Electrons from NADH are accepted by rhodoquinone through complex I (NADH-rhodoquinone oxidoreductase), and then transferred to fumarate through complex II (rhodoquinol-fumarate reductase). This anaerobic electron transport couples site I phosphorylation in complex I by t...

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