Catalyzes the last of the four reactions of the long-chain fatty acids elongation cycle. This endoplasmic reticulum-bound enzymatic process, allows the addition of 2 carbons to the chain of long- and very long-chain fatty acids/VLCFAs per cycle. This enzyme reduces the trans-2,3-enoyl-CoA fatty acid intermediate to an acyl-CoA that can be further elongated by entering a new cycle of elongation. Thereby, it participates in the production of VLCFAs of different chain lengths that are involved in multiple biological processes as precursors of membrane lipids and lipid mediators. VLCFAs serve for instance as precursors for ceramide and sphingolipids. Required for normal biogenesis of piecemeal microautophagy of the nucleus (PMN) bleps and vesicles during nutrient stress. a very-long-chain 2,3-saturated fatty acyl-CoA + NADP(+) = a very-long-chain (2E)-enoyl-CoA + H(+) + NADPH NADP(+) + octadecanoyl-CoA = (2E)-octadecenoyl-CoA + H(+) + NADPH (2E)-eicosenoyl-CoA + H(+) + NADPH = eicosanoyl-CoA + NADP(+) (2E)-docosenoyl-CoA + H(+) + NADPH = docosanoyl-CoA + NADP(+) (2E)-tetracosenoyl-CoA + H(+) + NADPH = NADP(+) + tetracosanoyl-CoA (2E)-hexacosenoyl-CoA + H(+) + NADPH = hexacosanoyl-CoA + NADP(+) Lipid metabolism; fatty acid biosynthesis. Interacts with the fatty acid elongation system components ELO2 and ELO3. Interacts with NVJ1. Accumulates at nucleus-vacuole (NV) junctions. Sequestred to NV junctions by NVJ1. Accumulates in nuclear PMN bleps and vesicles during stationary phase and nitrogen starvation. Present with 23600 molecules/cell in log phase SD medium. Belongs to the steroid 5-alpha reductase family.