PubMed · 5886109
The source did not provide an abstract. Follow the original record for more information.
Explore connections, maps & timelines
W D Seufert. 1965-07-10. Induced permeability changes in reconstituted cell membrane structure.. https://doi.org/10.1038/207174a0
Cite the original work for its findings. Save a collection to share your selection of sources.
KEEP EXPLORING
Water rapidly crosses most membranes, but only slowly crosses apical membranes of barrier epithelia such as bladder and kidney collecting duct, a feature essential to barrier function. How apical membrane structure reduces permeabilities remains unclear. Cell plasma membranes contain two leaflets of distinct lipid composition; the role of this bilayer asymmetry in membrane permeability is unclear. To determine how asymmetry of leaflet composition affects membrane permeability, effects on bilayer permeation of reducing single leaflet permeability were determined using two approaches: formation of asymmetric bilayers in an Ussing chamber, with only one of two leaflets containing cholesterol sulfate, and stabilization of the external leaflet of unilamellar vesicles with praeseodymium (Pr3+). In both systems, permeability measurements showed that each leaflet acts as an independent resistor of water permeation. These results show that a single bilayer leaflet can act as the barrier to permeation and provide direct evidence that segregation of lipids to create a low permeability of barrier epithelial apical membranes.
It is well known that continuous-wave EPR spectra of nitroxide probes (labels) introduced into phospholipid bilayers are sensitive to molecular oxygen. However, accurate determination of oxygen broadening from these experiments is complicated by the complex shapes of EPR spectra, which are strongly influenced by anisotropic restricted motion of the probe molecules. An accurate method is presented to extract the oxygen broadening from the spectra measured with and without oxygen and at the same temperature. The method is based on a fast convolution algorithm with Levenberg-Marquardt optimization. This method was previously applied to EPR oximetry with nitroxides exhibiting rotational motion in the fast limit. It is shown that for several membrane spin probes, the oxygen broadening can be described as homogeneous; thus, a one-linewidth-parameter fitting model is appropriate. The method is applied to measure permeability profiles of model membranes composed from 1,2-dimyristoyl-sn-glycero-3-phosphocholine above and below the main phase transition. For both membrane phases, the broadening of doxyl- and sterol-type labels is found to be homogeneous, a finding consistent with the model of Heisenberg exchange between molecular oxygen and spin probes. As an example, the method is applied to study the ethanol effect on local oxygen permeability of a phospholipid bilayer. It is shown that ethanol concentrations as low as 1% (v/v) increase oxygen permeability of the bilayer. The effect is larger at the surface of the membrane than at its center, indicating that ethanol molecules interact primarily within the polar head region of the bilayer.
Nitrogen is available to the plant in the form of NH+4 in the soil solution. Here it is shown that a voltage-independent K+ channel in the plasma membrane of rye (Secale cereale L.) roots is permeable to NH+4. The channel was studied following its incorporation into planar 1-palmitoyl-2-oleoyl phosphatidyl ethanolamine bilayers. The unitary conductance of the channel was greater when assayed in the presence of 100 mM NH4Cl than 100 mM KCl. However, the probability of finding the channel open (Po) was lower in the presence of 100 mm NH4Cl (Po = 0.63) than in 100 mM KCl (Po = 0.8), suggesting that Po can be regulated by the (permeant) ions present in solution. When assayed in equimolar concentrations of NH4Cl (cis) and KCl (trans), the zero-current (reversal) potential for the channel (Erev) exhibited a complex concentration dependence. At low cation concentrations, the apparent permeability of NH+4 relative to K+ (PNH4/PK) was greater than 1.0. However, as the cation concentration was increased, PNH4/PK initially decreased to a minimum of 0.95 at 3 mM before increasing again to a maximum of 1.89 at 300 mM. At cation concentrations above 300 mM, PNH4/PK decreased slightly. This implies that the pore of the channel can be occupied by more than one cation simultaneously. Ammonium permeation through the pore was simulated using a model which is composed of three energy barriers and two energy wells (the ion-binding sites). The model (3B2S) allowed for single-file permeation, double cation occupancy, ion-ion repulsion within the pore and surface potential effects. Results indicated that energy peaks and energy wells were situated asymmetrically within the electrical distance of the pore, that cations repel each other within the pore and that the vestibules to the pore contain negligible surface charge. The energy profile obtained for NH+4 is compared with ones obtained for K+ and Na+. This information allows the fluxes through the K+ channel of the three major monovalent cations present in the soil solution to be predicted.