Oleic Acid Biosynthesis in <em>Plasmodium falciparum</em>: Characterization of the Stearoyl-CoA Desaturase and Investigation as a Potential Therapeutic Target

BackgroundPlasmodium falciparum parasitization of erythrocytes causes a substantial increase in the levels of intracellular fatty acids, notably oleic acid. How parasites acquire this monounsaturated fatty acid has remained enigmatic. Here, we report on the biochemical and enzymatic characterization of stearoyl-CoA desaturase (SCD) in P. falciparum. Methodology/Principal FindingsMetabolic labeling experiments allowed us to demonstrate the production of oleic acid from stearic acid both in lysates of parasites incubated with [14C]-stearoyl-CoA and in parasite-infected erythrocytes labeled with [14C]-stearic acid. Optimal SCD activity was detected in schizonts, the stage of maximal membrane synthesis. This activity correlated with a late trophozoite stage-specific induction of PFE0555w transcripts. PFE0555w harbors a typical SCD signature. Similar to mammalian SCDs, this protein was found to be associated with the endoplasmic reticulum, as determined with PFE0555w-GFP tagged transgenic P. falciparum. Importantly, these parasites exhibited increased rates of stearic to oleic acid conversion, providing additional evidence that PFE0555w encodes the plasmodial SCD (PfSCD). These findings prompted us to assess the activity of sterculic acid analogues, known to be specific Δ9-desaturase inhibitors. Methyl sterculate inhibited the synthesis of oleic acid both with parasite lysates and infected erythrocytes, most likely by targeting PfSCD. This compound exhibited significant, rapid and irreversible antimalarial activity against asexual blood stages. This parasiticidal effect was antagonized by oleic acid. Conclusion/SignificanceOur study provides evidence that parasite-mediated fatty acid modification is important for blood-stage survival and provides a new strategy to develop a novel antimalarial therapeutic based on the inhibition of PfSCD.背景 恶性疟原虫（Plasmodium falciparum）寄生红细胞会显著升高细胞内脂肪酸水平，尤以油酸最为突出。然而疟原虫获取该单不饱和脂肪酸的具体机制至今仍不明确。本研究针对恶性疟原虫中的硬脂酰辅酶A去饱和酶（stearoyl-CoA desaturase, SCD）开展生化与酶学表征。 方法与主要结果 代谢标记实验证实，在添加[¹⁴C]-硬脂酰辅酶A的寄生虫裂解液中，以及用[¹⁴C]-硬脂酸标记的感染红细胞内，均可检测到硬脂酸向油酸的转化过程。在膜合成最为旺盛的裂殖体阶段，可检测到最优的SCD活性。该活性与晚期滋养体阶段特异性上调的PFE0555w转录本水平密切相关。PFE0555w编码的蛋白具有典型的SCD特征序列。与哺乳动物SCD类似，通过对PFE0555w与绿色荧光蛋白（GFP）融合的转基因恶性疟原虫进行亚细胞定位分析，发现该蛋白定位于内质网。重要的是，这类转基因寄生虫的硬脂酸向油酸转化速率显著提升，进一步验证了PFE0555w编码疟原虫来源的SCD（PfSCD）。基于上述发现，我们评估了已知可特异性抑制Δ9去饱和酶的苹婆酸（sterculic acid）类似物的活性。实验结果显示，苹婆酸甲酯可通过靶向PfSCD，在寄生虫裂解液与感染红细胞中均抑制油酸的合成。该化合物对疟原虫无性血液阶段表现出显著、快速且不可逆的抗疟活性，且此种杀寄生虫效应可被油酸所拮抗。 结论与意义 本研究证实寄生虫介导的脂肪酸修饰对疟原虫血液阶段的存活至关重要，并为基于抑制PfSCD开发新型抗疟疗法提供了全新的策略。