Partitioning of long-chain alcohols into lipid bilayers: implications for mechanisms of general anesthesia.

Abstract

Alcohols act as anesthetics only up to a certain chain length, beyond which their biological activity disappears.Although the molecular nature of general anesthetic target sites remains unknown, presently available data support the hypothesis that this "cutoff" in anesthetic activity could be due to a corresponding cutoff in the absorption of long-chain alcohols into lipid-bilayer portions of nerve membranes. To test this hypothesis, we have developed an extremely sensitive biological assay, based on inhibition of the light-emitting firefly luciferase reaction, which is capable of measuring lipidbilayer/buffer partition coefficients K for very lipid soluble compounds. Contrary to the hypothesis and reported data, we find a strictly linear increase in log(K) as the chain length increases [A(AG")CH2 = -3.63 kJ/mol] for the primary alcohols from decanol to pentadecanol, with no hint of a cutoff. The fact that alcohols continue to partition into lipid bilayers long after their biological activity has ceased is consistent with the view that the primary target sites in general anesthesia are proteins rather than the lipid-bilayer portions of nerve membranes.For nearly a century it has been known that alcohols can act as general anesthetics (1,2). As the chain length of an alcohol increases, so does its potency (-1/ED50 concentration, where ED50 is the dose for 50% effect) as an anesthetic. Eventually, however, anesthetic potency begins to level off, and a point is soon reached beyond which anesthetic activity disappears (the so-called "cutoff effect"). For the primary alcohols, general anesthetic potency levels off after about undecanol (C11) and completely disappears after tridecanol (C13) (3,4). Thus, while 1-dodecanol (C12) is among the most potent of all the alcohols, 1-tetradecanol (C14) is completely inactive. Although such behavior is consistent with protein mechanisms of general anesthesia (5), it presents a real problem for lipid theories. It has, however, been argued (4, 6, 7) that the observed cutoff in anesthetic potency is due to a corresponding cutoff in the ability of long-chain n-alcohols to partition into lipid bilayer portions of nerve membranes.Presently available data on the partitioning of radiolabeled long-chain alcohols into biological membranes (8) and lipid bilayers (6) do in fact support the hypothesis (4, 6, 7) that the higher n-alcohols are inactive simply because they cannot attain high enough concentrations in lipid bilayers. However, labeling techniques are notoriously unreliable when measuring large membrane/buffer partition coefficients, since the presence of very small amounts of water-soluble impurities can produce severe underestimates. We have now reinvestigated this problem by developing a technique, based on the inhibition of firefly luciferase light output by anesthetics (5, 9) in the presence of membranes, which can reliably measure membrane/buffer partition coefficients having values up to at least 10 million (when expressed as ratios of molar concentrations). PRINCIPL...

Anaesthetic potencies of primary alkanols: implications for the molecular dimensions of the anaesthetic site

“…One previous explanation for cut-off, derived from lipid based theories of anaesthesia, was based on the reported decrease in the membrane/buffer partition coefficient for tetradecanol, such that the maximum membrane concentration obtained was less than that necessary for anaesthesia (Pringle et al, 1981). However, recent data obtained in cholesterolcontaining bilayers (Franks & Lieb, 1986) revealed that partition coefficients continue to increase beyond dodecanol, making it unlikely that cut-off is a consequence of inability to achieve a sufficient membrane concentration. Therefore, either biomembranes behave differently in this respect from lipid bilayers, which seems unlikely, or the alternative explanation, that cut-off may arise from a loss in the intrinsic pharmacological efficacy must be considered (Pringle et al, 1981;Janoff & Miller, 1982).…”

“…A priori, this might arise from increasing lipophilicity and surface activity coupled with decreasing aqueous solubility. Simple calculations show that uptake into the fatty tissues of the tadpoles would account for much of the depletion within the range of error of the published lipidbuffer partition coefficients (Sallee, 1978;Franks & Lieb, 1986).…”

Sjögren-Larsson syndrome. Deficient activity of the fatty aldehyde dehydrogenase component of fatty alcohol:NAD+ oxidoreductase in cultured fibroblasts.

“…(17). Long-chain alcohols have been shown to partition into artificial lipid bilayers (17,18) and synaptic vesicles (19). If fatty alcohol accumulates in the skin of SLS patients, it could modify the epidermal water barrier, which is critically dependent on the lipid composition ofthe stratum corneum (20), and lead to increased transepidermal water loss and ichthyosis.…”

Naturally Occurring Antifungal Agents and Their Modes of Action

“…Similar to 2E-alkenals, the short chain alkanols enter the cell by passive diffusion across the plasma membrane and/or through porin channels (Schulz, 1996), and the long chain alkanols enter in part into the lipid bilayers (Franks and Lieb, 1986). The amount of alkanols entering into the cytosol or lipid bilayer is dependent on the length of the alkyl chain.…”

“…The inner and outer surfaces of the membrane are hydrophilic while the interior is hydrophobic, so the increased lipophilicity of 2E-alkenals should affect their movement further into the membrane lipid bilayer portions. It should be logical to assume that most of the lipophilic 2E-alkenal molecules being dissolved in the medium are partially incorporated into the lipid bilayers (Franks and Lieb, 1986) in which they may react with biologically important substances. The amount of 2E-alkenals entering into the cytosol or lipid bilayer is dependent on the length of the alkyl chain.…”