The relationship between membrane fluidity and permeabilities to water, solutes, ammonia, and protons.

Abstract

Several barrier epithelia such as renal collecting duct, urinary bladder, and gastric mucosa maintain high osmotic pH and solute gradients between body compartments and the blood by means of apical membranes of exceptionally low permeabilities. Although the mechanisms underlying these low permeabilities have been only poorly defined, low fluidity of the apical membrane has been postulated. The solubility diffusion model predicts that lower membrane fluidity will reduce permeability by reducing the ability of permeant molecules to diffuse through the lipid bilayer. However, litde data compare membrane fluidity with permeability properties, and it is unclear whether fluidity determines permeability to all, or only some substances. We therefore studied the permeabilities of a series of artificial large unilamellar vesicles (LUV) of eight different compositions, exhibiting a range of fluidities encountered in biological membranes. Cholesterol and sphingomyelin content and acyl chain saturation were varied to create a range of fluidities. LUV anisotropy was measured as steady state fluorescence polarization of the lipophilic probe DPH. LUV permeabilities were determined by monitoring concentration-dependent or pH-sensitive quenching of entrapped carboxyfluorescein on a stopped-flow fluorimeter. The relation between DPH anisotropy and permeability to water, urea, acetamide, and NH3 was well fit in each instance by single exponential functions (r > 0.96), with lower fluidity corresponding to lower permeability. By contrast, proton permeability correlated only weakly with fluidity. We conclude that membrane fluidity determines permeability to most nonionic substances and that transmembrane proton flux occurs in a manner distinct from flux of other substances.

Aggregation and Dynamics of Oligocholate Transporters in Phospholipid Bilayers Revealed by Solid-State NMR Spectroscopy

“…21 Since hydrophobic interactions are hypothesized to drive the stacking of the oligocholate macrocycles, DLPC was chosen for its lower hydrophobicity. 42 The glucose leakage from POPC/POPG liposomes reached 100% in 30 min with >1 mol % cyclic trimer 1 in the membrane. 21 Thus, CT is expected to be well above its critical aggregation concentration in the membrane at [oligocholate]/[lipids] = 1:7.5 used for the NMR studies.…”

Amt/MEP/Rh proteins conduct ammonia

“…The so-called passive diffusion of small neutral molecules across lipid bilayers is generally characterized by the diffusional membrane permeability P d which permits to express net flux (per area) simply as the product P [7,14,20,22]. For equilibration experiments it can be important to have some estimate of the time needed for equilibration of a concentration gradient across the membrane.…”

“…Monitoring this pH change via a pH sensitive fluorescent dye has been used to follow NH 3 equilibration processes in liposomes [22] or resealed red blood cell ghosts [34]. In oocytes, rapid alkalinization can apparently induce secondary responses like the activation of other transport systems, pumps, or channels [5].…”

Differential mesenteric fat deposition in bovines fed on silage or concentrate is independent of glycerol membrane permeability

“…Nevertheless, literature addressing bovines' fatty acid deposition at the cell membrane level and its outcome on membrane physical properties is unavailable. Given that the membrane bilayer permeability to water and solutes is tightly related to phospholipid composition and membrane fluidity (Lande et al, 1995), it seems reasonable that distinct fat depots with specific metabolic characteristics would affect membrane physical properties. Fatty acid incorporation into cellular membranes is known to affect permeability to water and, most importantly, to glycerol (Soveral et al, 2009;Martins et al, 2010).…”