Spin label studies of microsomal membranes from Acanthamoeba castellanii in different states of differentiation

Phase transitions of rat stratum corneum lipids by an electron paramagnetic resonance study and relationship of phase states to drug penetration

In order to relate barrier function to stratum corneum structure and the thermal transitions of corneum lipids, samples from hairless rat skin were investigated by using ESR and drug penetration techniques. The phase transition of stratum corneum lipids was estimated using a deeper probe (16-doxyl-stearic acid) inserted in the lipid bilayers and measuring the rotational correlation time, tau(c). Results of ESR study showed that stratum corneum lipids underwent thermal transitions at 39.3 +/- 1.6 degrees C and 63.6 +/- 2.6 degrees C roughly similar to the data obtained by differential scanning calorimetry measurements. Cholesterol oxidase treatment decreased the fluidity of the lipids at lower temperatures. The treatment of stratum corneum with laurocapram (1%) and isopropyl myristate (IPM, 2%) little changed both phase transition temperatures, although the treatment highly increased the molecular motion of the lipids. The flux (J(s)) of lipophilic drugs (beta-estradiol, indomethacin and betahistine) through the skin was enhanced with increasing temperatures, with an increase in the diffusion constant within skin and a decrease in the lag time. There was a good relationship between log J(s) or log permeability coefficient (K(p)) and 1/tau(c) in the temperature range of 45 to 64 degrees C. The calculated activation energy (delta E) for diffusion of these drugs across skin was 17-40 kcal/mol. Judging from our data, stratum corneum lipids of rat probably exist as the gel, crystalline state below 39 degrees C, the mesomorphic state between 39 and 64 degrees C and the fluid, liquid-crystalline state at temperatures of 64 degrees C or above. These results are in line with the permeability of these lipophilic drugs through the intercellular lipids disordered is highly increased.

Exogenous polyamines alter membrane fluidity in bean leaves - a basis for potential misinterpretation of their true physiological role

Changes in the rotational motion of paramagnetic and fluorescent lipid-soluble probes were used to assess the effects of putrescine, spermidine and spermine on the fluidity of microsomal membranes from primary leaves of bean (Phaseolus vulgaris L.). Surface probes were more strongly immobilized by physiological concentrations of the polyamines than probes that partitioned deep into the bilayer interior. Spermidine and spermine were more effective than putrescine at reducing membrane fluidity, and at equimolar concentrations, the polyamines and calcium had similar effects on the mobility of the membrane probes. Spermine had essentially equivalent effects on the fluidity of native membranes, heat-denatured membranes and liposomes prepared from the total lipid extract of the membranes, indicating that polyamines associate with membrane lipid. These results raise the possibility that some of the physiological effects previously attributed to exogenously added polyamines could reflect membrane rigidification rather than a true physiological response.

Spin probe clustering in human erythrocyte ghosts

A model has been developed for 5-nitroxide stearate, I(12,3), distribution in human erythrocyte ghosts which accurately predicts ESR spectral alterations observed with increased probe/total lipid (P/L) at 37 degrees C. This spin probe occupies a class of high-affinity, noninteracting sites at low loading. Saturation occurs with increasing probe concentration, and, at higher loading, the probe inserts itself at initially dilute sites to form membrane-bound clusters of variable size. No 'low' probe remains at high P/L where all I(12,3) clusters in a 'concentrated' phase. This model allows determination of the dilute/clustered probe ratio, and shows that I(12,3) segregates in erythrocytes at what might otherwise be considered low P/L (e.g., 1/359). These findings validate the earlier use of empirical parameters to estimate probe sequestration in biological membranes.