Orientation and motion of spin-labels in rabbit small intestinal brush border vesicle membranes

The temperature dependence of the packing (order) and fluidity (microviscosity) of rabbit small, intestinal brush border vesicle membranes and of liposomes made from their extracted lipids has been investigated by using a variety of lipid spin probes. The lipids in the brush border membrane are present essentially as a bilayer. Compared to other mammalian membranes, the brush border membrane appears to be characterized by a relatively high packing order as well as microviscosity. At body temperature, the lipid molecules undergo rapid, anisotropic motion, which is essentially a fast rotation about an axis approximately perpendicular to the bilayer normal. Both the order (motional anisotropy) and the microviscosity increase with decreasing temperature and with increasing distance from the center of the bilayer. Qualitatively similar motional or fluidity gradients have been reported for other mammalian and bacterial membranes. The liposomes made from the extracted lipids have a somewhat lower packing order and a slightly higher fluidity than brush border vesicle membranes. The differences are, however, small indicating that the packing and the fluidity (microviscosity) of the membrane are primarily determined by the lipid composition. Membrane-associated proteins and cytoskeleton cannot play a dominant role in determining the order and fluidity of the lipid bilayer. Discontinuities are observed in the temperature dependence of various spectral parameters, the order parameter S, the rotational correlation time tau, and 2,2,6,6-tetramethylpiperidinyloxy partitioning. They are assigned to phase transitions and/or phase separations of the membrane lipids. These discontinuities occur at about 30, 20, and 13 degrees C for 5-doxyl-, 12-doxyl-, and 16-doxylstearic acid, respectively. The apparent transition temperature depends on the location of the spin probe along the bilayer normal, being higher the closer the probe is to the membrane surface. This indicates the possibility that chain melting is progressive and spreads with increasing temperature from the center of the membrane outward.