A considerable body of biological evidence has accumulated that suggests that docosahexaenoic acid (22:6n3) is an essential component in the nervous system. Moreover, it appears from these studies that long chain polyunsaturates of the n-6 family such as arachidonate (20:4n6) and docosapentaenoate (22:5n6) cannot substitute for 22:6n3. This evidence is briefly reviewed and two hypotheses centering upon either biochemical or biophysical aspects of polyunsaturate function are presented and discussed. It is concluded that a bioactive metabolite of 22:6n3 is not responsible for its function in brain and that the best hypothesis asserts that a membrane function of a 22:6n3-containing species of phospholipid, such as phosphatidylserine, is critical for optimal neural function. Moreover, data are presented indicating that the biophysical properties of various highly unsaturated species of phospholipid are distinguishable. It is further contended that these species are not randomly distributed in membranes and thus the differences in physical properties may be amplified. It is concluded that a conceptual framework is needed in which the distinct membrane roles of phospholipid species may be understood as a function of the positions and numbers of double bonds. Only then may the critical role of the highly unsaturated n-3 polyunsaturates in the brain and retina be understood.

Although mammalian tissues contain high levels of polyunsaturated fatty acids, our knowledge of the effects of the degree of unsaturation and double-bond location upon bilayer organization is limited. Therefore, a series of mixed-chain unsaturated phosphatidylcholines (PC) comprised of 18:0 at the sn-1 position and various unsaturates at the sn-2 position (18:1n9, 18:2n6, 18:3n6, 18:3n3, 20:2n6, 20:3n6, 20:4n6, 20:5n3, 22:4n6, 22:5n6, or 22:6n3) was studied with differential scanning calorimetry, and their gel to liquid-crystalline phase transitions yielded measurements of Tm, Tonset, delta H, and delta S. Minimal delta H values were obtained for the diene species, 1.7 and 2.9 kcal/mole for 18:2n6 and 20:2n6, respectively. These results are consistent with the dienes having an acyl chain conformation that results in perturbed chain packing. Increasing the degree of unsaturation to three or more double bonds resulted in higher delta H values, 3.7, 4.3, and 4.6 kcal/mole for 18:3n6, 20:3n6, and 20:4n6, respectively, consistent with the occurrence of a gel-state chain conformation(s), which is more tightly packed than the dienes. The 18:0,22:6n3-PC species yielded the highest delta H (6.1 kcal/mole) and delta S(22.7 cal/mol degree) of all the polyunsaturates studied. The distinctive packing properties of phospholipid bilayers containing 22:6n3 may underlie its essential role in the nervous system.

Time-resolved fluorescence anisotropy (TRFA) and steady-state anisotropy measurements and fluorescence intensification microscopic observations were made on RAW264 macrophages labeled with 1,6-diphenyl-1,3,5-hexatriene (DPH) or 1-[4-(trimethylammonio)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH). Microscopic analysis revealed that the fluorescent probe DPH was found in association with plasma membranes and small vesicles. Macrophages treated with immune complexes could not be distinguished from untreated cells, indicating that the same membrane compartments were labeled. The probe TMA-DPH was exclusively localized to the plasma membrane. Steady-state anisotropy measurements indicated that in vitro culture conditions did not significantly affect membrane fluidity. TRFA measurements were conducted to determine the physical properties of macrophage membranes during immune recognition and endocytosis. Data were analyzed by iterative deconvolution to yield phi, the rotational correlation time, and r infinity, the limiting anisotropy. These parameters may be interpreted as the "fluidity" and order parameter of the membrane environment, respectively. Typical values for untreated macrophages were phi = 7.8 ns and r infinity = 0.12. Binding and endocytosis of immune complexes prepared in 4-fold antigen excess increase these values to phi = 22.1 ns and r infinity = 0.15. However, receptor-independent phagocytosis of latex beads decreases these values to phi = 2.2 ns and r infinity = 0.10. Addition of catalase before, but not after, immune complex incubation with cells diminishes the effect upon membrane structure, suggesting that H2O2 participates in fluidity changes. Pretreatment of macrophages with the membrane-impermeable sulfhydryl blocker p-(chloromercuri)benzenesulfonic acid also diminished these effects.(ABSTRACT TRUNCATED AT 250 WORDS)