BIOSYNTHESIS OF POLYUNSATURATED LIPIDS IN THE DIATOM, PHAEODACTYLUM-TRICORNUTUM

The marine diatom, Phaeodactylum tricornutum, was pulse-labelled and c hased with [1-C-14]acetate, [1-C-14] 18:1 (n-9), [1-C-14] 18:2 (n-6) o r [1-C-14] 20:4 (n-6). Changes of radioactivities in lipid classes and molecular species were examined during the chase. The results obtaine d showed that phosphatidylcholine (PC) and monogalactosyldiacylglycero l (MGDG) were actively involved in the synthesis of polyunsaturated fa tty acids by P. tricornutum. The changes in labelled PC during the cha se suggest that 18:1 (n-9) synthesized de novo is successively desatur ated to 18:4 (n-3) through the (n-3)-family in PC-linked processes, bu t 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) i n 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, toget her with the desaturation of 20:4 (n-3) to 20:5 (n-3) in sn-2 of PC, s uggests 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 o ccur exclusively in sn-2 of PC and that from 20:4 (n-6) in both sn-1 a nd sn-2 of PC. Changes of radioactivities in molecular species of PC a nd 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 th e 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)/1 6:1-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 eukaryo tic 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).