[Importance of membrane fluidity determination]

Abstract

Translational motions of lipids and proteins and molecular motions within the lipid hydrocarbon chains of biological membranes can be approached by spectroscopic methods. The electron spin resonance (ESR) spectra of spin labels bound on proteins or grafted on fatty acids and incorporated into biological membranes undergo characteristic changes which may be resolved in terms of modifications of membrane environment. However, the method is still scarcely used in routine clinical studies. In contrast, fluorescence methods such as intermolecular excimers or fluorescence polarization might join the ranks of routine analyses. The labeling of lipid compartments with fluorescent probes and the study of their spectroscopic properties give informations on the cohesion of their immediate environment. Polyenic molecules (DPH and TMA-DPH) which are located in different lipid compartments characterize deep and superficial areas of cell membrane, respectively. A set of probes in which a 9-anthroyloxy group is attached to different positions of a long chain fatty acid provides a means of measuring a fluidity gradient into membrane bilayer. Thus, these fluorescence methods which are simple and rapid represent a semi-quantitative approach of the so called "membrane fluidity". Modifications in membrane fluidity can control the expression of proteins, receptors exposed on cell surface and alter functional properties of cells. Moreover, pathological processes can also be related to fluidity modifications. In particular, in hypertension and vascular diseases, a decrease in membrane fluidity has been shown in platelets and red cells. Nevertheless, further investigations combining biophysical, biochemical and immunological methods are needed to determine the exact relations between membrane fluidity, classical rheological properties and cell functions.