Cultured fish cells metabolize octadecapentaenoic acid (all-cis Delta 3,6,9,12,15-18 : 5) to octadecatetraenoic acid (all-cis Delta 6,9,12,15-18 : 4)via its 2-trans intermediate (trans Delta 2, all-cis Delta 6,9,12,15-18 : 5)

Octadecapentaenoic acid (all-cis Delta3,6,9,12,15-18:5; 18:5n-3) is an unus ual fatty acid found in marine dinophytes, haptophytes, and prasinophytes. It is not present at higher trophic levels in the marine food web, but its metabolism by animals ingesting algae is unknown. Here we studied the metab olism of 18:5n-3 in cell lines derived from turbot (Scophthalmus maximus), gilthead sea bream (Sparus aurata), and Atlantic salmon (Salmo salar). Cell s were incubated in the presence of approximately 1 muM [U-C-14]18:5n-3 met hyl ester or [U-C-14]18:4n-3 (octadecatetraenoic acid; all-cis Delta6,9,12, 15-18:4) methyl ester, both derived from the alga Isochrysis galbana grown in (HCO3-)-C-14, and also with 25 muM unlabeled 18:5 n-3 or 18:4n-3. Cells were also incubated with 25 muM trans Delta2, all-cis Delta6,9,12,15-18:5 ( 2-trans 18:5n-3) produced by alkaline isomerization of 18:5n-3 chemically s ynthesized from docosahexaenoic acid (all-cis Delta4, 7,10,13,16,19-22 :6). Radioisotope and mass analyses of total fatty acids extracted from cells i ncubated with 18:5n-3 were consistent with this fatty acid being rapidly me tabolized to 18:4n-3 which was then elongated and further desaturated to ei cosatetraenoic acid (all-cis Delta8,11,14,17,19-20:4) and eicosapentaenoic acid (all-cis Delta5,8,11,14,17-20:5). Similar mass increases of 18:4n-3 an d its elongation and further desaturation products occurred in cells incuba ted with 18:5n-3 or 2-trans 18:5n-3. We conclude that 18:5n-3 is readily co nverted biochemically to 18:4n-3 via a 2-trans 18:5n-3 intermediate generat ed by a Delta (3),Delta (2)-enoyl-CoA-isomerase acting on 18:5n-3. Thus, 2- trans 18:5n-3 is implicated as a common intermediate in the beta -oxidation of both 18:5n-3 and 18:4n-3.