Alcohol's effects on lipid bilayer properties

Abstract

Alcohols are known modulators of lipid bilayer properties. Their biological effects have long been attributed to their bilayer-modifying effects, but alcohols can also alter protein function through direct protein interactions. This raises the question: Do alcohol's biological actions result predominantly from direct protein-alcohol interactions or from general changes in the membrane properties? The efficacy of alcohols of various chain lengths tends to exhibit a so-called cutoff effect (i.e., increasing potency with increased chain length, which that eventually levels off). The cutoff varies depending on the assay, and numerous mechanisms have been proposed such as: limited size of the alcohol-protein interaction site, limited alcohol solubility, and a chain-length-dependent lipid bilayer-alcohol interaction. To address these issues, we determined the bilayer-modifying potency of 27 aliphatic alcohols using a gramicidin-based fluorescence assay. All of the alcohols tested (with chain lengths of 1-16 carbons) alter the bilayer properties, as sensed by a bilayer-spanning channel. The bilayer-modifying potency of the short-chain alcohols scales linearly with their bilayer partitioning; the potency tapers off at higher chain lengths, and eventually changes sign for the longest-chain alcohols, demonstrating an alcohol cutoff effect in a system that has no alcohol-binding pocket.

Figure 1

3-Pentanol's bilayer-perturbing effect, as determined using a gA-based fluorescence assay. Normalized fluorescence time courses for fluorophore-loaded LUVs incubated with varying [3-pentanol], with and without 260 nM gA and with and without external quencher. (A) Average traces (lines) and results from all repeats (>7 per condition; dots) over 1 s. (B) One repeat for each experimental condition in A (dots), as well as stretched exponential fits (2–100 ms) to those repeats (solid lines). The stippled line denotes the 2 ms mark. (C) The normalized quenching rate relative to rates of vesicles with the same amount of gA and no added 3-pentanol. Results are for three different days of experiments (differently shaped symbols); the solid line indicates a f([alc]) = 1 + [alc]/D fit to the results.

Figure 2

Relative changes in quenching rate as a function of alcohol concentration. Results are shown for (A) straight-chain alcohols (methanol through 1-decanol) and (B) 1-pentanol, 2-pentanol, and 3-pentanol. Each alcohol was tested on at least three different days (differently shaped symbols); the solid lines are fits of f([alc]) = 1 + [alc]/D to the results. The dashed lines mark 1 (no change) and the dotted lines mark 2 (the doubling rates, D).

Figure 3

Bilayer-perturbing potency of alcohols. The concentration at which the alcohols double the quenching rate D, as determined by fitting f([alc]) = 1 + [alc]/D to the alcohols' fluorescence quenching dose-response curve; error bars are the fits' standard errors for D. (A) D as a function of alcohol chain length. (B) D as a function of alcohol bilayer partitioning. The alcohol's bilayer partition coefficient is estimated as the calculated octanol/water partition coefficient (cLogP), using the ACD/Labs LogP algorithm (54). A linear fit (solid line) of all alcohols with six or fewer carbons (solid symbols) gives a slope = 0.95, R = 0.97. The dotted line is an extrapolation of the linear fit and meant to guide the eye. (C) The alcohol membrane mole fraction (m_alc) at D. The stippled line indicates m_alc = 0.1.

Figure 4

Bilayer-perturbing effects of long-chain alcohols. Fluorophore-loaded LUVs were made with 20 mol % of long alcohols added to the vesicle-making lipid solution. (A) Average traces without gA for the no-alcohol control, 1-dodecanol, and 1-hexadecanol (top three traces). The remaining six traces have 780 nM gA for 1-hexadecanol, 1-dodecanol, and control (middle three traces), and 816 mM ethanol (added to the aqueous LUV solution) for 1-dodecanol, control, and 1-hexadecanol (bottom three traces). (B) Fluorescence quenching rates (average ± SD, n = 4–6) for vesicles with 780 nM gA; 20 mol % of the following alcohols was added to the vesicle-making solution: nothing/control, 1-octanol, 1-decanol, 1-dodecanol, 1-tetradecanol, 1-hexadecanol, and additional DC_22:1PC lipid. The asterisk indicates a significance of p < 0.01 compared with control.

Figure 5

Alcohol bilayer-perturbing effects in thinner (DC_20:1PC) bilayers. Fluorophore-loaded LUVs were made using DC_20:1PC, and 80 nM of a sequence-shortened gA analog (gA⁻(13)) were added to the gA-containing vesicles. The effects of ethanol, 1-octanol, 1-nonanol, and 1-decanol were determined at twice their doubling rate concentration, D, or 294, 0.82, 0.38, and 1.26 mM respectively. (A) Average traces without gA for control, 1-octanol, and 1-decanol (top three traces), and with added gA⁻(13) for control, 1-decanol, and 1-octanol (bottom three traces). (B) Average and SD of the relative change in quenching rates as a function of added alcohols.

Figure 6

Potency of straight-chain alcohols in different systems. An alcohol's potency is defined differently depending on the system, as follows: D for the gA-based fluorescence assay in DC_22:1PC lipid vesicles (boxes); IC₅₀ inhibition of Na_v1.2 sodium current in oocytes (65) (circles); cholesterol activation measured as the concentration that promotes hemolysis of red blood cells in a cholesterol oxidase assay (66) (up triangles); ED₃ is the injected dose required (in mmole/kg, not mM) to produce ataxia 2 behavior (as described by Majchrowicz (67)) for intoxication in rats (56) (down triangles); IC_67% reduction in unitary conductance of KvAP channels reconstituted in PE:GP/n-decane planar bilayers (23) (diamonds); anesthesia ED₅₀ as determined by the loss of righting reflex of tadpoles (68) (left triangles); 50% suppression of peak inward current in voltage-clamped intact giant squid axons (69) (right triangles); EC₅₀ inhibition of NMDA-induced current in mice hippocampus neurons (70) (hexagons); ED₅₀ 50% reduction of luciferase fluorescence in buffer (27) (pentagons); and EC₅₀ potentiation of GABA-induced current in mice hippocampus neurons (71) (crosses). The dashed lines have slopes of 0.59 log units representing 800 cal/mole per –CH₂ group.