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Instituto de Investigaciones Bioquímicas de Bahía Blanca, Universidad Nacional del Sur-Consejo Nacional de Investigaciones Científicas y Técnicas, Bahía Blanca, Argentina
1 To whom correspondence should be addressed. e-mail: silviant{at}criba.edu.ar
About one-fourth the phosphatidylcholines (PC) from retina photoreceptor rod outer segment (ROS) membranes contain docosahexaenoic acid (22:6n-3) at sn-2 and a very long chain polyunsaturated fatty acid (VLCPUFA) (C24 to C36) at the sn-1 position of the glycerol backbone. In order to study the thermotropic behavior of these PCs, subfractions and molecular species of PC (16:0/22:6, 18:0/22:6, 22:6/22:6, 32:5/22:6, 32:6/22:6, 34:5/22:6), were isolated from bovine ROS, and liposomes containing different proportions of these PCs and dimyristoyl-PC (DMPC) or dipalmitoyl PC (DPPC) were compared using the fluorescence probes Laurdan and 1,6-diphenyl-1,3,5-hexatriene (DPH). With both probes, the 22:6n-3 containing PCs from ROS, in all proportions tested, decreased the transition temperature (Tt) of both DMPC and DPPC. Below the transition temperature, coexistence of phases was evidenced in all cases. Liposomes formed with 100% of any of these PCs did not show phase transitions in the temperature range studied (8°C to 50°C). At physiological temperatures, as it is likely to be the case in ROS membranes, all of these PC species were in the liquid-crystalline state. With Laurdan, all dipolyunsaturated PCs seemed to behave similarly: despite the large number of double bonds per molecule, all of them decreased the Tt of DPPC less than did the hexaenoic PCs. With DPH, an ample difference was detected between the dipolyunsaturates, 22:6/22:6-PC and VLCPUFA/22:6-PCs, and between the latter and hexaenoic PCs throughout the temperature range studied.
This difference is consistent with the interpretation that the largest "disorder" produced by PCs containing a VLCPUFA like 32:6n-3 at the sn-1 position occurs toward the center of the membrane.
Abbreviations: DMPC, dimyristoylphosphatidylcholine; DPH, 1,6-diphenyl-1,3,5-hexatriene; DPPC, dipalmitoylphosphatidylcholine; GC, gas chromatography; GP, generalized polarization; HPLC, high pressure liquid chromatography; PC, phosphatidylcholine; ROS, rod outer segments; VLCPUFA, PUFA with 24 to 36 carbon atoms. The fatty acids are designed by the convention, number of carbon atoms: number of double bonds. The notation n-3 or n-6 refer to the position of the first double bond counting from the methyl end
Supplementary key words dipolyunsaturated phosphatidylcholine • polarization • fluorescence • Laurdan • transition temperature
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