Biosynthesis of polyunsaturated fatty acids in the marine diatom, Phaeodactylum tricornutum

Abstract

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. [ 14C] 20:5 ( n−3) was synthesized from [1- 14C] 18:1 ( n−9), [1- 14C] 18:2 ( n−6), [1- 14C] 18:3 ( n−3), [2- 14C] 20:3 ( n−6) or [1- 14C] 20:4 ( n−6). The rates of [ 14C] 20:5 ( n−3) synthesis from [ 14C] fatty acids were [1- 14C] 20:4 ( n−6)>[2- 14C] 20:3 ( n−6)>[1- 14C] 18:2 ( n−6)>[1- 14C] 18:3 n−3)>[1- 14C] 18:1 ( n−9). When pulse-labelled with [1- 14C] 18:1 ( n−9) to synthesize 20:5 ( n−3), the intermediates such as [ 14C] 18:2 ( n−6), [ 14C] 18:4 ( n−3), [ 14C] 20:4 ( n−6) and [ 14C] 20:4 ( n−3) were accumulated during the chase. When pulse-labelled with [1- 14C] 18:2 ( n−6) instead, both [ 14C] 18:3 ( n−6) and [ 14C] 18:3 ( n−3) were accumulated during the chase. Similarly, [ 14C] 20:3 ( n−3) was accumulated in pulse-chase with [1- 14C] 18:3 ( n−3) and [ 14C] 20:4 ( n−6) in pulse-chase with [2- 14C] 20:3 ( n−6). Incorporation of [1- 14C] 18:1 ( n−9) into [ 14C] 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.