THERMOTROPIC FLUID -] ORDERED DISCONTINUOUS PHASE SEPARATION IN MICROSOMAL LIPIDS OF TETRAHYMENA - X-RAY-DIFFRACTION STUDY

The thermotropism of total lipids extracted from microsomal membranes which were isolated from Tetrahymena pyriformis cells grown at either 18 or 28 °C was investigated by small- and wide-angle x-ray diffraction. Furthermore, the 18 °C lipids were chromatographically separated into different lipid fractions, i.e., total neutral lipids, total phospholipids, ethanolamine phosphoglycerides (PE), and PE-deficient phospholipids; these were studied as a function of temperature by small-angle x-ray diffraction. Our data show that total 18 and 28 °C lipids undergo a broad thermotropic fluid to ordered phase separation. At 27 °C, all the 18 °C lipids are in the fluid state, about 60% of which are transformed into the ordered state upon lowering the temperature to 0 °C; fluid and ordered lipids coexist in two separate lamellar phases. Remarkably, this phase separation reveals a discontinuity at ~10 and ~16 °C in the 18 and 28 °C lipids, respectively. Such a discontinuous separation also occurs in the 18 °C phospholipids. However, only one lamellar phase can be detected in the 18 °C PE-deficient phospholipids and the 18 °C PE; the Bragg spacings of the latter show a dramatic change at ~10 °C. Thus, the PE constituting °55% of the total lipids is assumed to play a dominant role in the fluid to ordered “discontinuous” phase separation of total lipids. We suggest that the lipids which are transformed from the fluid into the ordered state above and below the discontinuity point differ in their percent distribution of PE. Finally, our present data support and extend our concept previously suggested for the thermotropic lipid clustering within T. pyriformis membranes [Wunderlich, F., Ronai, A., Speth, V., Seelig, J., and Blume, A. (1975), Biochemistry 14, 3730],. © 1978, American Chemical Society. All rights reserved.