BIOSYNTHESIS OF POLYUNSATURATED LIPIDS IN THE DIATOM, PHAEODACTYLUM-TRICORNUTUM

The marine diatom, Phaeodactylum tricornutum, was pulse-labelled and chased with [1-C-14]acetate, [1-C-14] 18:1 (n-9), [1-C-14] 18:2 (n-6) or [1-C-14] 20:4 (n-6). Changes of radioactivities in lipid classes and molecular species were examined during the chase. The results obtained showed that phosphatidylcholine (PC) and monogalactosyldiacylglycerol (MGDG) were actively involved in the synthesis of polyunsaturated fatty acids by P. tricornutum. The changes in labelled PC during the chase suggest that 18:1 (n-9) synthesized de novo is successively desaturated to 18:4 (n-3) through the (n-3)-family in PC-linked processes, but C-18 polyunsaturation was unfavourable in MGDG-linked processes. The synthesis of 20:5 (n-3) from 18:1 (n-9) in PC suggests that 18:4(n-3), and probably 18:3 (n-3), are released from PC to be elongated to 20:4 (n-3) and 20:3 (n-3), respectively, then desaturated to 20:5 (n-3) in PC after the esterification. The synthesis of 20:5 (n-3) from 20:4(n-6) in PC and the synthesis of 20:5 (n-3) from 18:1 (n-9) in PC, together with the desaturation of 20:4 (n-3) to 20:5 (n-3) in sn-2 of PC, suggests that the synthesis of 20:5 (n-3) from both 20:4 (n-3) and 20:4 (n-6) occurs in PC, although the synthesis from 20:4 (n-3) seems to occur exclusively in sn-2 of PC and that from 20:4 (n-6) in both sn-1 and sn-2 of PC. Changes of radioactivities in molecular species of PC and MGDG during the chase suggest the desaturation of 20:5 (n-3)/20:4 (n-3)-PC to 20:5 (n-3)/20:5 (n-3)-PC, that of 20:5 (n-3)/18:1 (n-9)-PC to 20:5 (n-3)/18:2 (n-6)-PC and successive desaturation of 20:5 (n-3)/16:1-MGDG to 20: 5 (n-3)/16:1-MGDG. The results also indicated that the labelled molecular species of 20:5 (n-3)/16:1 and 20:5 (n-3)/18:1 (n-9) occurred in both PC and MGDG, leading to the view that 20:5(n-3)/16:1-PC is converted to 20:5 (n-3)/16:1-MGDG and 20:5 (n-3)/18:1 (n-9)-PC is converted to 20:5(n-3)/18:1 (n-9)-MGDG, as occurs in the eukaryotic pathway in higher plants. Based on the results mentioned above, we propose a tentative pathway for the synthesis of 20:5 (n-3) from 18:1 (n-9).