A comparison of α-linolenic acid (18:30ω3) and γ-linolenic acid (18:3ω6) in phosphatidylcholine bilayers

Abstract

α-Linolenic acid (18:3 Δ9,12,15), an omega-3, and γ-linolenic acid (18:3 Δ6,9,12), on omega-6, are two 18 carbon fatty acids which vary only by the position of their unsaturations. Here we compare these two isomeric fatty acids to understand better what effect location of the double bond has on fluidity, fusion and permeability in phosphatidylcholine (PC) bilayers. Any difference measured between the two fatty acids may be attributed primarily to the position of the unsaturation. The fatty acids were investigated as both free acids added to PC bilayers and mixed chain 18:0,α-18:3 and 18:0,γ-18:3 phosphatidylcholines. Fluorescence polarization of a series of anthracene stearic acid probes and 1,6-diphenyl-1,3,5-hexatriene indicates that 20 mol% incorporation of the fatty acids in dipalmitoylphosphatidylcholine bilayers broadened and depressed the onset temperature of the gel to liquid crystalline phase transition. In the liquid crystalline state, polarization ( P) values for membranes containing the two fatty acids were essentially the same. Similar results were obtained in the liquid crystalline state of the mixed chain PCs using the same set of fluorescent probes. In contrast to the fluidity measurements where little difference was detected between the fatty acids, α-linolenic acid produced greater enhancement of fusion and permeability properties, relative to γ-linoleic acid. This was demonstrated by resonance energy transfer (fusion) and osmotic swelling in isotonic erythritol (permeability). It is concluded that these two fatty acids affect the fusogenic and permeability properties of PC bilayers differently, while exhibiting little difference in their influence on bilayer fluidity. From these experiments a distinction can be made between the effects of omega-3 and omega-6 fatty acids on membranes.