BIOSYNTHESIS OF PHOSPHOLIPID MOLECULAR-SPECIES IN ISOLATED LIVER-CELLS STUDIED BY COMBINING FATTY-ACID SUBSTRATES ESTERIFIED IN THE SN-1 AND SN-2 POSITIONS

The simultaneous incorporation of a saturated fatty acid in the sn-1 p osition and an unsaturated fatty acid in the sn-2 position in phosphat idylcholine (PC) and ethanolamine (PE) was studied in isolated liver c ells. We combined a saturated fatty acid, 16:0 or 18:0 and an unsatura ted fatty acid substrate, 18:2,n-6 or 20:4,n-6. In this situation the saturated fatty acids were preferentially oxidized and the unsaturated fatty acids were preferentially esterified in PL and TG. Addition of unlabelled 16:0 increased the incorporation of [C-14]18:2 in 16:0-18:2 in PC and PE, reduced the incorporation in 18:2-18:2 but did not redu ce the incorporation in 18:0-18:2. 18:0 increased the esterification o f [C-14]18:2 in 18:0-18:2, reduced the incorporation in 18:2-18:2 but did not reduce the incorporation in 16:0-18:2. The latter is the domin ating C-14-labelled species formed from [C-14]18:2 also in the presenc e of unlabelled 18:0. Addition of 20:4 stimulated the incorporation of [C-14]16:0 in 16:0-20:4 and markedly reduced the formation of 16:0-18 :2, 16:0-18:1 and 16:0-22:6. Addition of 18:2 increased the incorporat ion of [C-14]16:0 in 16:0-18:2 and reduced the formation of 16:0-20:4 and 16:0-18:1. It is concluded that the unsaturated fatty acids 18:2 o r 20:4 have a stronger impact on the synthesis of phospholipid molecul ar species than the saturated fatty acids 16:0 or 18:0 have. Thus 20:4 ,n-6 and 18:2,n-6 are able to direct available [C-14]16:0 or [C-14]18: 0 to the sn-1 position. 16:0 and 18:0 are not in the same way able to direct [C-14]18:2,n-6 to the synthesis of 16:0-18:2 or 18:0-18:2 at th e expense of other C-14-labelled molecular species.