Fluorescence polarization measurements on normal and tumour cells and their corresponding plasma membranes

Using 1,6-diphenyl-1,3,5-hexatriene as a probe, the degree of fluorescence polarization (P) at 25 degrees C of intact and disrupted cells and isolated plasma membranes were compared for a variety of systems. 1. Human erythrocytes, mouse thymocyte and leukemia cells, rat liver and hepatoma cells, and human and mouse milk fat globules displayed P values ranging from 0.300 to 0.120. 2. P values or probe labelling rates of intact and disrupted cells were similar. 3. As compared with whole or disrupted cells, the higher to much higher P values of plasma membranes isolated from the corresponding cells showed only a limited mutual variation. 4. delta P values, being the difference in P values between plasma membranes and whole cells were attributed to the extent to which endomembranes and non-membrane lipids contributed. Among these, triglycerides had the greatest relative effect. 5. Though a particular isolation procedure for plasma membranes may select for more rigid fragments, this effect is by far not sufficient to account for the observed delta P values. It is concluded that the fluorescence polarization technique with a lipophilic probe applied to whole cells represents a measure of the average fluidity of all lipids being present in a cell and thus does not exclusively monitor the cell surface membrane.