Fatty acid and molecular species compositions of phospholipids and diacylglycerols from rat retinal membranes

Abstract

The relative proportions and compositions of the diacyl molecular species of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidic acid (PA), and diacylglycerol (DG) from rat retinal membranes were determined. Two membrane fractions were derived by discontinuous sucrose gradient floatation: rod outer segment (ROS) and ‘rest of the retina’ (ROR). ROR is defined as those membranes pelleted at 100 000 g following removal of the ROS. Diacylglycerols were prepared from PC, PE and PS by phospholipase C treatment and were converted into the corresponding 1,2-diacylglycerobenzoates (DGBZ), PI, PA, and DG were converted into 1,2-diacylglyceroacetates (DGAC) by acetolysis. The molecular species of the DGBZ and DGAC were resolved by reverse-phase HPLC and detected by UV absorption at 230 and 210 nm, respectively. Fatty acid methyl esters of PC, PE, PS, PI and DG from ROS and ROR were prepared and analysed by GLC. The fatty acid and molecular species patterns of PC, PE and PS were similar in both membrane fractions, although the levels of docosahexaenoic acid (22:6ω3) and 22:6-containing molecular species were lower in ROR than in ROS. PE and PS were enriched in 22:6ω3 and 18:0, as evidenced by the high levels of 18:0–22:6 and 22:6–22:6 molecular species. PC contained relatively more saturated and monoene species, such as 16:0–16:0, 16:0–18:0, 16:0–18:0, 16:0–18:1 and 18:0–18:1. The fatty acids and molecular species patterns of DG, PI and PA in ROS and ROR differed from those of PC, PE and PS. The major fatty acids of DG and PI were 16:0, 18:0 and 20:4ω6, and the predominant molecular species were 18:0–20:4 and 16:0–20:4. Thus, whereas the most abundant glycerolipids (PC, PS and PC) of rat retina contain 22:6ω3 as the major polyunsaturated fatty acid (PUFA), the minor lipids (DG, PI, and PA) have 20:4ω6 as the major PUFA.