Original Article

Transport of heptafluorostearate across model membranes. Membrane transport of long-chain fatty acid anions I

Correspondence to: G. Kurz

Heptafluorostearic acid, an isogeometric derivative of stearic acid, has a pK_a value of about 0.5. To evaluate the suitability of heptafluorostearate as model compound for anions of long-chain fatty acids in membrane transport, monolayer and liposome studies were performed with lipid mixtures containing phospholipids;–cholesterol–heptafluorostearate or stearate (100:40:20 molar ratios). Transfer of heptafluorostearate and stearate from liposomes to bovine serum albumin (BSA) was followed by measuring the intrinsic fluorescence of BSA. The percentage of heptafluorostearate, equivalent to the amount placed in their outer monolayer, transferred from liposomes (120;–130 nm diameter) to BSA was 55.7 ± 3.7% within 10 min at 25°C and 55 ± 2% within 5 min at 37°C. Slow transfer of 22.7 ± 2.5% of heptafluorostearate at 25°C followed with a half-life of 2.3 ± 0.4 h and of 20 ± 4% at 37°C with a half-life of 0.9 ± 0.1 h until the final equilibrium distributions between BSA and liposomes were reached, 79 ± 6% to 21 ± 5% at 25°C and 75 ± 5% to 25 ± 4% at 37°C. The pseudounimolecular rate constants for flip-flop of heptafluorostearate equal k_FF,25 = 0.24 ± 0.05 h^- and k_FF,37 = 0.6 ± 0.1 h^-, respectively. By comparison, transfer of stearate required only 3 min to reach equilibrium distribution.

The difference between heptafluorostearate and stearate may be explained by a rapid flip-flop movement of the un-ionized fatty acids which exist in different concentrations in accordance with their pK_a values. Half-life of flip-flop of heptafluorostearate makes it suitable to study mediated membrane transport of long-chain fatty acid anions.—Schmider, W., A. Fahr, H. E. Blum, and G. Kurz. Transport of heptafluorostearate across model membranes. Membrane transport of long-chain fatty acid anions. I. J. Lipid Res. 2000. 41: 775;–787.

Supplementary key words: flip-flop mechanism, intrinsic fluorescence, liposome, long-chain fatty acid, monolayer, transbilayer movement

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