Comparisons of steady-state anisotropy of the plasma membrane of living cells with different probes

We have used an extended Perrin equation which was in agreement with literature data for steady-state anisotropy (rSS) for a wide variety of artificial and isolated biological membranes labeled with various probes (Van der Meer et al. (1986) Biochim. Biophys. Acta 854, 38-44 to obtain the static component (r infinity) for the intact plasma membranes of living cells. We show that lipid structural order parameters can be obtained for DPH and TMA-DPH in the plasma membranes of intact cells. We have examined the relationship between 'fractional limiting hindered anisotropy', r infinity/r0, which is related to the lipid structural order parameter, of DPH, TMA-DPH, DPHpPC, and a series of depth-dependent probes (n-(9-anthroyloxy) fatty acids, with n = 2-16), using data from 19 cell types. There was a linear relationship between r infinity/r0 values of DPH and TMA-DPH, but the relationship between either of these probes was non-linear with respect to DPHpPC or the series of fatty acid probes. The relationship between r infinity/r0 values of DPHpPC and the series of fatty acid probes was linear, suggesting that they not only undergo similar motions in the membrane, but also experience similar types of restriction to motion, a type which is different from that experienced by DPH and TMA-DPH. We show that for the plasma membranes of living cells, 'second degree' order parameters can be estimated for DPH and TMA-DPH, and propose that the parameter r infinity/r0, or the 'fractional limiting hindered anisotropy', analogous to a 'first degree' order parameter, can be estimated for DPHpPC and the depth-dependent fatty acid probes to evaluate the density of membrane packing.