Department of Food and Bioproduct Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5A8, Canada.
Department of Food and Bioproduct Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5A8, Canada.
Biochimie. 2018 Jan;144:108-114. doi: 10.1016/j.biochi.2017.10.024. Epub 2017 Oct 31.
Thraustochytrium is a marine protist producing a specific profile of nutritionally important fatty acids, including very long chain polyunsaturated fatty acids (VLCPUFAs) docosahexaenoic acid (DHA, 22:6n-3), even chain saturated fatty acids (SFAs) palmitic acid (16:0), and odd chain SFAs pentadecanoic acid (15:0). To study how these fatty acids are synthesized, a series of radiolabeled precursors were used to trace the biosynthetic process in vivo and in vitro. When Thraustochytrium was fed with long chain fatty acid intermediates such as [1-C]-oleic acid, [1-C]-linoleic acid and [1-C]-α-linolenic acid, no VLCPUFAs were produced, indicating that the aerobic pathway for the biosynthesis of VLCPUFAs was not functional in Thraustochytrium. When fed with [1-C]-acetic acid, both SFAs and VLCPUFAs were labeled, and when fed with [1-C]-propionic acid, mainly SFAs were labeled. However, when fed with [1-C]-acetic acid in the presence of cerulenin, a type I FAS inhibitor, only VLCPUFAs were labeled, and when fed with [1-C]-propionic acid in the presence of cerulenin, neither SFAs nor VLCPUFAs were labeled. This result clearly indicates that the type I fatty acid synthase (FAS) in Thraustochytrium could use acetic acid and propionic acid as the primers to synthesize even chain and odd chain SFAs, respectively, and VLCPUFAs were synthesized by the PUFA synthase using acetic acid as the primer. In addition, radioactive acetic acid could label both phospholipids (PL) and triacylglycerols (TAG), and VLCPUFAs appeared first and were largely accumulated in PL, whereas TAG accumulated much more SFAs than VLCPUFAs. The in vitro assay with [1-C]-malonyl-CoA in presence of cerulenin showed that the crude protein of Thraustochytrium produced only VLCPUFAs, not SFAs, further confirming the role of the PUFA synthase in the biosynthesis of VLCPUFAs. Collectively, these results have elucidated the biochemical mechanisms for the biosynthesis of all fatty acids in Thraustochytrium.
厚壳虫是一种海洋原生生物，能够产生具有重要营养价值的脂肪酸，包括超长链多不饱和脂肪酸（VLCPUFA）二十二碳六烯酸（DHA，22:6n-3）、偶数链饱和脂肪酸（SFA）棕榈酸（16:0）和奇数链 SFA 十五烷酸（15:0）。为了研究这些脂肪酸的合成途径，使用了一系列放射性标记前体来追踪体内和体外的生物合成过程。当厚壳虫被喂食长链脂肪酸中间体，如 [1-C]-油酸、[1-C]-亚油酸和 [1-C]-α-亚麻酸时，没有产生 VLCPUFAs，表明厚壳虫中用于 VLCPUFA 生物合成的需氧途径不起作用。当喂食 [1-C]-乙酸时，SFA 和 VLCPUFA 都被标记，而当喂食 [1-C]-丙酸时，主要标记 SFA。然而，当在 cerulenin（一种 I 型 FAS 抑制剂）存在下喂食 [1-C]-乙酸时，只有 VLCPUFAs 被标记，而当在 cerulenin 存在下喂食 [1-C]-丙酸时，SFA 和 VLCPUFA 都没有被标记。这一结果清楚地表明，厚壳虫中的 I 型脂肪酸合酶（FAS）可以分别使用乙酸和丙酸作为引物合成偶数链和奇数链 SFA，而 VLCPUFAs 是由多不饱和脂肪酸合酶使用乙酸作为引物合成的。此外，放射性乙酸可以标记磷脂（PL）和三酰基甘油（TAG），VLCPUFAs 首先出现并大量积累在 PL 中，而 TAG 积累的 SFA 比 VLCPUFA 多得多。在 cerulenin 存在下用 [1-C]-丙二酰辅酶 A 进行的体外测定表明，厚壳虫的粗蛋白仅产生 VLCPUFA，而不产生 SFA，这进一步证实了多不饱和脂肪酸合酶在 VLCPUFA 生物合成中的作用。总之，这些结果阐明了厚壳虫中所有脂肪酸生物合成的生化机制。
