FabI, FabZ and FabG, Three Key Enzymes from the Type II Fatty Acid System of Plasmodium falciparum, as Possible Drug Targets of Polymethoxyflavones of Artemisia annua

The sesquiterpene lactone artemisinin is the antimalarial principle of Artemisia annua L. (Asteraceae). The antimalarial activity of artemisinin and its derivatives has been postulated to result from the inhibition of SERCA, a P. falciparum Ca²⁺-ATPase [1]. The plant contains also some polymethoxyflavones, which were shown to potentiate the antimalarial activity of artemisinin [2]. However, no data concerning the mechanism related to their synergistic effect has been reported. Recent discoveries reveal that Plasmodium is able to synthesize its own fatty acids in the apicoplast [3]. Plasmodium fatty acid synthase (FAS) is a type II multienzyme complex, as found in plants and bacteria, and as such, differs markedly from human type I FAS. FabG, FabI and FabZ represent three key enzymes of the FAS-II system and are ideal targets for malaria drug discovery. After discovering a flavonoid as the first natural product inhibiting the plasmodial FabI enzyme [4], we suspected the A. annua polymethoxyflavones to have similar effects. Thus, three flavonoids, crysophenol D, crysyoplenitin and artemetin isolated from A. annua were tested against purified FabG, FabI and FabZ. Indeed, crysophenol D and artemetin inhibited all three enzymes (IC₅₀s 15–50µg/mL). Crysoplenitin inhibits both FabI and FabZ (IC₅₀s 20 and 40µg/mL), but is inactive against FabG. These findings identify the FAS-II enzymes as possible targets of A. annua flavonoids and provide logical explanations for their synergistic activity when combined with artemisinin. The inhibition of multiple enzymes from the same pathway is very useful, as it increases the efficacy of the drug and reduces the risk of resistance. Consequently, the combinations of polymethoxyflavonoids with artemisinin(s) might be a promising option for treating drug-resistant malaria.

Acknowledgements: The financial support of MIUR (PRIN 2004) and Ente Cassa di Risparmio di Firenze are acknowledged.

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