Apparent relative retention of the phosphatidylethanolamine molecular species 18:0–20:5( n−3), 16:0–22:6( n−3) and the sum 16:0–20:4( n−6) plus 16:0–20:3( n−9) in the liver microsomes of pig on an essential fatty acid deficient diet

Abstract

Attempts at a better understanding of the cell membrane organization and functioning need to assess the physical properties which partly depend (i) on the positional distribution of the fatty acids in the membrane phospholipids (PLs) and (ii) on the way by which the PL molecular species are affected by exogenous fatty acids. To do that, the effects of essential (polyunsaturated) fatty acid (EFA) deficiency and enrichment were studied in the liver microsomes of piglets feeding on either an EFA-deficient diet or an EFA-enriched diet containing hydrogenated coconut oil or a mixture of soya + corn oils, respectively. After derivatization, the diacylated forms of choline and ethanolamine PLs were analyzed using a combination of Chromatographic techniques and fast-atom bombardmentmass spectrometry. The dinitrobenzoyl-diacylglycerol derivatives corresponding to the molecular species of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were identified. It appears that three factors brought about a marked apparent relative retention: the nature of (i) the base of the polar head, (ii) fatty acids at the sn−1 position and (iii) fatty acids at the sn−2 position. The highest apparent relative retentions were displayed by the 18:0–20:5( n−3)-PE and 16:0–22:6( n−3)-PE. It is noteworthy that the behavior of 20:3 n−9 — which is synthesized during the EFA-deficient diet by the same bioconversion system as 20:4 n−6 — was very similar to that of 20:4 n−6 during the formation of PC and PE molecular species and that the molecular species of PE containing 20:4( n−6) and 20:3( n−9), gathered together as metabolical homologues, were also apparently retained, particularly in association with 16:0. Present observations are consistent with some others showing retention or preferential distribution of EFA in PE and suggest that specific acyltransferase(s), ethanolamine phosphotransferase and methyltransferase would be mainly involved for PE and PC formation in liver endoplasmic reticulum. Fast-atom bombardment-mass spectrometry of intact phospholipids enables us to show that there is no very long chain dipolyunsaturated phospholipid in liver endoplasmic reticulum.