Alternate pathways in the desaturation and chain elongation of linolenic acid, 18:3(n-3), in cultured glioma cells

Cultured C6 glioma cells rapidly incorporate and metabolize the essential fatty acids, 18:2(n-6) and 18:3(n-3), to 20- and 22-carbon polyunsaturated fatty acids. Using several deuterated fatty acid substrates we have obtained data that suggest alternate pathways, one possibly involving Δ⁸-desaturation, may exist in glioma cells for formation of 20:5(n-3) and 22:6(n-3) from 18:3(n-3). With 18:3(n-3)-6,6,7,7-d₄ practically no 18:4(n-3)-6,7-d₂ or 20:4(n-3)-8,9-d₂ was detected whereas 20:3(n-3)-8,8,9,9-d₄ accounted for 3.4% and Δ^5,11,14,17-20:4-8,8,9,9-d₄ for 21.1% of the total deuterated fatty acids recovered in phospholipids after a 16 h incubation; 20:5(n-3)-8,9-d₂, 22:5(n-3)-10,11-d₂, and 22:6(n-3)-10,11-d₂ accounted for 42.4%, 13.2%, and 2.8% of deuterated acyl chains, respectively. When added exogneously, 20:3-8,8,9,9,-d₄ was extensively converted to Δ^5,11,14,17-20:4(n-3)-8,8,9,9-d₄ (45%) and 20:5(n-3)-8,9-d₂ (24%); a small amount (4%) of 18:3(n-3)-d₄ also was detected. Both 20:4(n-3)-8,9-d₂ and 18:4(n-3)-12,13,15,16-d₄ were also converted to 20:5(n-3) and 22:6(n-3) with 8 and 0% of the respective original deuterated substrate remaining after 16 h. A possible pathway for 18:3(n-3) metabolism in glioma cells is described whereby an initial chain elongation step is followed by successive Δ⁵ and Δ⁸ desaturation reactions resulting in 20:5(n-3) formation and accounting for the ordered removal of deuterium atoms. Alternatively, extremely effective retroconversion may occur to chain shorten 20:3(n-3)-d₄ to 18:3(n-3)-d₄ followed by rapid conversion through the classical desaturation and chain elongation sequence. The relative contribution of these possibilities cannot be quantitated with labeled isomers currently available. Whether these alternate routes of polyunsaturated fatty acid metabolism have physiological significance in other cell types or tissues, or may relate to the nutritional state, such as essential fatty acid deficiency, remains unresolved but such possibilities for 20:5(n-3) formation should be considered particularly when the classical pathway may be impaired.