Plasmodium berghei enoyl ACP reductase: Investigations into its suitability as a drug target and its biological role in malarial parasite development
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The type II fatty acid pathway (FASII) has been thought to be a valid target for antimicrobial drug discovery. Enoyl ACP reductase (ENR, also known as FabI) is a key FASII enzyme, and ENR is a proven target of several important antimicrobials including triclosan. However, triclosan is rapidly glucoronidated in the human body leading to rapid readsorption into the biliary system and excretion from the host. This led us to synthesize and investigate triclosan analogs with the goal of identifying ones that have good pharmacokinetic properties and improved oral bioavailability.;However, unexpected results during our studies led us to question whether ENR is an antimalarial drug target. Using a double crossover strategy, we generated ENR knock out parasites in the rodent malaria parasite Plasmodium berghei. ENR knock out parasites and parental lines were equally sensitive to triclosan, and HPLC analysis revealed similar fatty acid profiles in these two lines. This indicates that ENR is not essential in P. berghei asexual blood stages, and ENR is not the target of triclosan and its analogs.;We next examined whether the FASII pathway might be active at other stages of parasite development, particularly the liver stage when replication is prolific and there might be a greater need for high levels of fatty acid production. The analysis of the phenotypes of the ENR knock out parasite line in both the sexual stages in the mosquito host and in the liver stage showed that ENR knockout sporozoites had a marked attenuation in their virulence in the vertebrate host, resulting in delayed blood stage infection. This appeared to be due to a defect in the degradation of the parasitophorous vacuolar membrane in mature liver stages of the ENR knockout parasites, leading to reduced egress of liver stage merozoites. Together, these studies have revealed that ENR is not essential for blood stage parasites but does play an important role in the maturation of the thousands of liver stage parasites produced per infected hepatocyte. These data also suggest a re-evaluation of the mode of action of triclosan, and suggest that antimalarial agents that target fatty acid biosynthesis might be effective as prophylactic agents against liver stage parasites.