Phospholipid surface density determines the partitioning and permeability of acetic acid in DMPC:cholesterol bilayers

Abstract

Relationships between the permeability coefficient (PHA) and partition coefficient (K m/w) of acetic acid and the surface density of DMPC:cholesterol bilayers have been investigated. Permeability coefficients were measured in large unilamellar vesicles by NMR line broadening. Bilayer surface density, sigma, was varied over a range of 0.5-0.9 by changing cholesterol concentration and temperature. The temperature dependence of PHA for acetic acid exhibits Arrhenius behavior with an average apparent activation energy (Ea) of 22 +/- 3 kcal/mole over a cholesterol mole fraction range of 0.00-0.40. This value is much greater than the enthalpy change for acetic acid partitioning between bulk decane and water (delta H degree = 4.8 +/- 0.8 kcal/mole) and the calculated Ea (= 8.0 kcal/mole) assuming a "bulk phase" permeability model which includes the enthalpy of transfer from water to decane and the temperature dependence of acetic acid's diffusion coefficient in decane. These results suggest that dehydration, previously considered to be a dominant component, is a minor factor in determining Ea. Values of 1n PHA decrease linearly with the normalized phospholipid surface density with a slope of kappa = -12.4 +/- 1.1 (r = 0.90). Correction of PHA for those temperature effects considered to be independent of lipid chain order (i.e., enthalpy of transfer from water to decane and activation energy for diffusion in bulk hydrocarbon) yielded an improved correlation (kappa = -11.7 +/- 0.5 (r = 0.96)). The temperature dependence of Km/w is substantially smaller than that for PHA and dependent on cholesterol composition. Values of 1n K m/w decrease linearly with the surface density with a slope of kappa = -4.6 +/- 0.3 (r = 0.95), which is 2.7-fold smaller than the slope of the plot of 1n PHA vs. sigma. Thus, chain ordering is a major determinant for molecular partitioning into and transport across lipid bilayers, regardless of whether it is varied by lipid composition or temperature.

Fatty acid flip-flop and proton transport determined by short-circuit current in planar bilayers

“…The reduction of the leak conductance of asolectin membranes induced by cholesterol is consistent with findings that cholesterol greatly reduces the permeability of lipid membranes to different substances (19)(20)(21)(22), which is generally attributed to the ability of cholesterol to promote order in lipid chains. However, we observed that cholesterol incorporation into asolectin membranes essentially eliminated the PA-induced increase in proton selectivity observed in pure asolectin membranes.…”

Permeability of Small Molecules through a Lipid Bilayer: A Multiscale Simulation Study

“…Experimental data on the free energy difference between the headgroup region and the water phase have been obtained for acetic acid; from the partition coefficient in DMPC vesicles, a value ∆G ≈ -0.45 kJ/mol was calculated. 53 This negative value is in qualitative agreement with our AL-CG results, whereas it is inconsistent with the AL results, which gave a positive free energy difference. 34 However, there is a discrepancy of a factor of ∼5 between that experimental value 53 and our AL-CG result.…”

Quantitative visualization of passive transport across bilayer lipid membranes

“…The difference in behavior that we observe compared with others, both in trend and magnitude, is clear and we briefly consider the reasons below. We note, however, that the trend is not entirely without precedent; the gel-phase dipalmitolyphosphatidylcholine (DPPC) studies of Xiang and Anderson (14,16,17) also found a decreasing permeation coefficient from acetic to propionic acid, but for larger acids P increased again (17). DPPC is a fully saturated lipid, thus P values are orders of magnitude smaller in the gel phase (17) and so are easier to measure accurately.…”

“…On the other hand, some weak acid protonophores may also exert detrimental effects on cellular processes such as oxidative phosphorylation, apoptosis (7), and photosynthesis (8). The transport of weak acids across model cell membranes and the determination of permeation rates have consequently been the subject of a considerable number of studies (9)(10)(11)(12)(13)(14)(15)(16)(17)). …”

“…Investigations of membrane permeation have essentially used two configurations based on either planar bilayer lipid membranes (BLMs) (9)(10)(11)(12)(13)20) or liposomes (10,14,16,17). The former configuration has received the most attention, with flux measurements made between adjacent stirred chambers of different composition separated by the BLM.…”