Effect of temperature on the structure of rat liver microsomes studied by electron spin resonance, fluorescence and activity of enzymes involved in fatty acid biosynthesis.

The structural properties of rat liver microsomes were studied by physical and kinetic methods. The microsomes and the lipids extracted from the microsomes were labeled with 16-doxyl-stearic acid- and N-phenyl-1-naphthylamine. The electron spin resonance spectra and the fluorescence intensities were respectively determined at different temperatures from approximately 10 to 40 C. Both methods suggested the absence of a transition temperature indicative of a phase change in the bulk of the lipids of the microsomes in the temperature range studied. The fluidity of the lipid bilayer increased smoothly with the temperature. The Arrhenius plots of the NADH-ferricyanide reductase, NADH-cyt.c reductase, delta 9 desaturase, delta 6 desaturase and palmitic elongation to stearic acid also indicated the absence of a detectable change of phase from crystalline to liquid crystalline in the boundary lipids of these enzymes from 10 C to 40 C. The transference of electrons from the NADH-cyt.b5 reductase to the cyt.b5 is the rate limiting step in the first parts of the electron transport chain. However, the delta 9 desaturase is the rate limiting step of all the series of reactions involved in the delta 9 fatty acid desaturation. Similar conclusions may be extended to the delta 6 desaturation of fatty acids. The physical state of the lipids surrounding the desaturating system would be different from boundary lipids of the cyt.P450 system.