The mammalian #6-desaturase coded by fatty acid desaturase 2 (FADS2; HSA11q12-q13.1) catalyzes the first and rate-limiting step for the biosynthesis of long-chain polyunsaturated fatty acids. FADS2 is known to act on at least five substrates, and we hypothesized that the FADS2 gene product would have #8-desaturase activity. Saccharomyces cerevisiae transformed with a FADS2 construct from baboon neonate liver cDNA gained the function to desaturate 11,14-eicosadienoic acid (20:2n-6) and 11,14,17-eicosatrienoic acid (20:3n-3) to yield 20:3n-6 and 20:4n-3, respectively. Competition experiments indicate that #8-desaturation favors activity toward 20:3n-3 over 20:2n-6 by 3-fold. Similar experiments show that #6-desaturase activity is favored over #8-desaturase activity by 7-fold and 23-fold for n-6 (18:2n-6 vs 20:2n-6) and n-3 (18:3n-3 vs 20:3n-3), respectively. In mammals, 20:3n-6 is the immediate precursor of prostaglandin E1 and thromboxane B1. 20:3n-6 and 20:4n-3 are also immediate precursors of long-chain polyunsaturated fatty acids arachidonic acid and eicosapentaenoic acid, respectively. These findings provide unequivocal molecular evidence for a novel alternative biosynthetic route to long-chain polyunsaturated fatty acids in mammals from substrates previously considered to be dead-end products.-Park, W. J., K. S. D. Kothapalli, P. Lawrence, C. Tyburczy, and J. T.Brenna. An alternate pathway to long-chain polyunsaturates: the FADS2 gene product #8-desaturates 20:2n-6 and 20:3n-3. J. Lipid Res. 2009Res. . 50: 1195Res. -1202 Supplementary key words polyunsaturated fatty acid biosynthesis • dihomo-g-linolenic acid • eicosanoid precursor biosynthesis Long-chain polyunsaturated fatty acids (LCPUFAs) are ubiquitous in mammalian tissue, achieving highest concentrations in the membranes of neural and other excitable tissue (1). LCPUFA of the n-3 and n-6 families, especially eicosapentaenoic acid (EPA; 20:5n-3), docosahexaenoic acid (22:6n-3), and arachidonic acid (20:4n-6), are bioactive components of membrane phospholipids and serve as substrates for signaling molecules (2). The degree of unsaturation of the membranes is determined by the action of enzymes involved in fatty acid biosynthesis and metabolism (3). Most organisms synthesize unsaturated fatty acids, but the pathways are specific to cell types and species.Fatty acid desaturases are enzymes that catalyze the introduction of cis double bonds at specific positions in a fatty acid chain (4). Desaturases in plants and lower animal species can introduce double bonds near the methyl end. Eukaryotic cells of higher animals, fungi, and dinoflagellates express membrane-bound acyl-CoA front-end desaturases (5, 6) catalyzing double bond introduction into the D6, D5, D8, and D4 positions. Mammalian front-end desaturases operate on diet-derived PUFA to synthesize LCPUFA, which can also be derived from the diet but possibly not in sufficient quantities to optimize health (7).The front-end desaturases are remarkable for their structural similarity and functiona...