BIOSYNTHESIS OF POLYUNSATURATED FATTY-ACIDS IN THE MARINE DIATOM, PHAEODACTYLUM-TRICORNUTUM

To determine the pathway for 20:5 (n-3) synthesis in the marine diatom, Phaeodactylum tricornutum, pulse-labelling and chase experiments with labelled fatty acids were carried out. [C-14] 20:5(n-3) was synthesized from [1-C-14] 18:1 (n-9), [1-C-14] 18:2 (n-6), [1-C-14] 18:3 (n-3), [2-C-14] 20:3 (n-6) or [1-C-14] 20:4(n-6). The rates of [C-14] 20:5 (n-3)synthesis from [C-14] fatty acids were [1-C-14] 20:4 (n-6)> [2-C-14] 20:3 (n-6)> [1-C-14] 18:2 (n-6)> [1-C-14] 18:3 (n-3)> [1-C-14] 18:1 (n-9). When pulse-labelled with [1-C-14] 18:1 (n-9) to synthesize 20:5 (n-3), the intermediates such as [C-14] 18:2 (n-6), [C-14] 18:4 (n-3), [C-14] 20:4 (n-6) and [C-14] 20:4 (n-3) were accumulated during the chase. When pulse-labelled with [1-C-14] 18:2 (n-6) instead, both [C-14] 18:3 (n-6) and [C-14] 18:3 (n-3) were accumulated during the chase. Similarly, [C-14] 20:3 (n-3) was accumulated in pulse-chase with [1-C-14] 18:3 (n-3) and [C-14] 20:4 (n-6) in pulse-chase with [2-C-14] 20:3 (n-6). Incorporation of[1-C-14] 18:1 (n-9) into [C-14] 20:5 (n-3) was inhibited in the presence of cold fatty acids such as 18: 1 (n-9), 18:2 (n-6), 18:3 (n-3), 18 :3 (n-6), 20:3 (n-3), 20:3 (n-6), 20:4 (n-6) or 20:5 (n-3), but not inhibited in the presence of 20:1 (n-9) or 20:2 (n-6). From these results, it is concluded that 20:5 (n-3) is able to be synthesized from 18:1 (n-9) in P. tricornutum by a network pathway which contains four routes from 18:2 (n-6) to 20:5 (n-3); two routes pass through (n-3)-fatty acids and one route through (n-6)-fatty acids as intermediates. The other route passes through both (n-3)- and (n-6)-fatty acids as intermediates.