How fatty acids of different chain length enter and leave cells by free diffusion

https://doi.org/10.1016/j.plefa.2006.05.003Get rights and content

Abstract

Opposing views exist as to how unesterified fatty acids (FA) enter and leave cells. It is commonly believed that for short- and medium-chain FA free diffusion suffices whereas it is questioned whether proteins are required to facilitate transport of long-chain fatty acid (LCFA). Furthermore, it is unclear whether these proteins facilitate binding to the plasma membrane, trans-membrane movement, dissociation into the cytosol and/or transport in the cytosol. In this mini-review we approach the controversy from a different point of view by focusing on the membrane permeability constant (P) of FA with different chain length. We compare experimentally derived values of the P of short and medium-chain FA with values of apparent permeability coefficients for LCFA calculated from their dissociation rate constant (koff), flip-flop rate constant (kflip) and partition coefficient (Kp) in phospholipid bilayers. It was found that Overton's rule is valid as long as kflipkoff. With increasing chain length, the permeability increases according to increasing Kp and reaches a maximum for LCFA with chain length of 18 carbons or longer. For fast flip-flop (e.g. kflip=15 s−1), the apparent permeability constant for palmitic acid is very high (Papp=1.61 cm/s). Even for a slow flip-flop rate constant (e.g. kflip=0.3 s−1), the permeability constant of LCFA is still several orders of magnitude larger than the P of water and other small non-electrolytes. Since polyunsaturated FA have basically the same physico-chemical properties as LCFA, they have similar membrane permeabilities. The implications for theories involving proteins to facilitate uptake of FA are discussed.

Introduction

The ongoing debate about the mechanisms by which long-chain fatty acids (LCFA) cross cell membranes is fuelled in part by the fundamental assumption that transport must be tightly controlled to achieve regulation of uptake and metabolism. Thus, a simple mechanism such as free diffusion for transport of basic metabolites like LCFA does not seem to be a plausible one. Moreover, several putative fatty acid transporters in the plasma membrane have been identified over the last decade. As stated by Zakim, “The two opposing views about the mechanism for cellular uptake of fatty acids reflect, I believe, the biases and tension between the idea that all events in cells have complex biological basis versus the idea that this not need be so.” [1]. A more provocative general statement in biophysics was made by Sackmann: “Nature would not be stupid, not using simple mechanisms” [2]. There are several compelling reasons to argue that free diffusion is the main, if not sole, mechanism for membrane transport of LCFA. Since polyunsaturated fatty acids (PUFA) have similar biophysical properties as other LCFA, the arguments discussed below apply also for them. More extensive recent reviews have been published elsewhere by us [3], [4], [5] and others [6], [7], [8], [9], [10].

Section snippets

Argument 1: Overton rule

The first argument is a 100-year-old principle of the membrane permeability of any molecule: the more hydrophobic, the higher its membrane permeability [11]. This so-called “Overton rule” is illustrated in Fig. 1A. A concentration gradient of molecular species X dissolved in water exists across a membrane. The species binds at the water-membrane interface with partition equilibrium constant Kp=Xb/X=kon/koff, where Xb and X represent the concentrations of the bound and unbound species. kon and k

Discussion

Facilitated transport through biological membranes is a necessary mechanism for the large family of ions, charged metabolites like amino acids, lactate and pyruvate as well as large hydrophilic non-electrolytes (glucose). The values of the permeability constants for few selected molecules in Table 1 clearly demonstrate why. In the case of water, the discovery of aquaporin as transporter in specific tissues where diffusion does not suffice was originally considered revolutionary but is nowadays

Conclusion

We have argued that FA cross protein-free phospholipid bilayers very rapidly (t1/2<1 s). They also cross the plasma membrane of cells rapidly both in intact cells and in isolated membrane vesicles [35]. In the context of the lipid phase of biological membranes, where FA bind with high affinity and rapid kinetics, and where the energy barrier for translocation of the uncharged carboxyl is low [75], putative LCFA transport proteins do not fulfill classic requirements for transport proteins.

If

Acknowledgment

We thank Christian Haass, David Zakim, Frits Muskiet, Berthold Koletzko, Reinhart Heinrich and Klaus Beyer for stimulating discussions.

References (86)

  • J.B. Massey et al.

    Spontaneous transfer of monoacyl amphiphiles between lipid and protein surfaces

    Biophys. J.

    (1997)
  • N. Latruffe

    Transport of D-beta-hydroxybutyrate across rat liver mitochondrial membranes

    Comp. Biochem. Physiol. B

    (1987)
  • J.A. Hamilton

    Fatty acid interactions with proteins: what X-ray crystal and NMR solution structures tell us

    Prog. Lipid Res.

    (2004)
  • A. Stahl et al.

    Insulin causes fatty acid transport protein translocation and enhanced fatty acid uptake in adipocytes

    Dev. Cell

    (2002)
  • B.A. Ek et al.

    Fatty acid binding proteins reduce 15-lipoxygenase-induced oxygenation of linoleic acid and arachidonic acid

    Biochim. Biophys. Acta

    (1997)
  • P. Pohl et al.

    The size of the unstirred layer as a function of the solute diffusion coefficient

    Biophys. J.

    (1998)
  • Y.N. Antonenko et al.

    Weak acid transport across bilayer lipid membrane in the presence of buffers. Theoretical and experimental pH profiles in the unstirred layers

    Biophys. J.

    (1993)
  • I. Petitpas et al.

    Crystal structures of human serum albumin complexed with monounsaturated and polyunsaturated fatty acids

    J. Mol. Biol.

    (2001)
  • P. Haggarty et al.

    Long-chain polyunsaturated fatty acid transport across the perfused human placenta

    Placenta

    (1997)
  • E. Larque et al.

    In vivo investigation of the placental transfer of (13)C-labeled fatty acids in humans

    J. Lipid Res.

    (2003)
  • J.E. Schaffer et al.

    Expression cloning and characterization of a novel adipocyte long chain fatty acid transport protein

    Cell

    (1994)
  • P.A. Watkins et al.

    Human very long-chain acyl-CoA synthetase and two human homologs: initial characterization and relationship to fatty acid transport protein

    Prostaglandins Leukot. Essent. Fatty Acids

    (1999)
  • Z. Pei et al.

    Mouse very long-chain Acyl-CoA synthetase 3/fatty acid transport protein 3 catalyzes fatty acid activation but not fatty acid transport in MA-10 cells

    J. Biol. Chem.

    (2004)
  • D.W. Deamer et al.

    Permeability of lipid bilayers to water and ionic solutes

    Chem. Phys. Lipids

    (1986)
  • J.R. Alger et al.

    Nuclear magnetic resonance study of acetic acid permeation of large unilamellar vesicle membranes

    Biophys. J.

    (1979)
  • D. Zakim

    Fatty acids enter cells by simple diffusion

    Proc. Soc. Exp. Biol. Med.

    (1996)
  • J. Nardi

    Non-equilibria phenomena of free and bound vesicles: modelling cell adhesion and vesicle transport, Ph.D. Thesis

    (1999)
  • J. Hamilton

    Fast flip-flop of cholesterol and fatty acids in membranes: implications for membrane transport proteins

    Curr. Opin. Lipidol.

    (2003)
  • J.A. Hamilton et al.

    How are free fatty acids transported in membranes? Is it by proteins or by free diffusion through the lipids?

    Diabetes

    (1999)
  • J.A. Hamilton et al.

    Fatty acid transport. The diffusion mechanism in model and biological membranes

    J. Mol. Neurosci.

    (2001)
  • I. Bojesen

    Studies of membrane transport mechanism of long-chain fatty acids in human erythrocytes

    Rec. Res. Devel. Membr. Biol.

    (2002)
  • A.M. Kleinfeld

    Lipid phase fatty acid flip-flop, is it fast enough for cellular transport?

    J. Membr. Biol.

    (2000)
  • D. Zakim

    Thermodynamics of fatty acid transfer

    J. Membr. Biol.

    (2000)
  • J.F. Glatz et al.

    Unravelling the significance of cellular fatty acid-binding proteins

    Curr. Opin. Lipidol.

    (2001)
  • G.J. van der Vusse et al.

    Critical steps in cellular fatty acid uptake and utilization

    Mol. Cell Biochem.

    (2002)
  • Q. Al-Awqati

    One hundred years of membrane permeability: does Overton still rule?

    Nat. Cell Biol.

    (1999)
  • W. Stein

    Transport and Diffusion Across Cell Membranes

    (1986)
  • J.A. Hamilton et al.

    Transfer of oleic acid between albumin and phospholipid vesicles

    Proc. Natl. Acad. Sci. USA

    (1986)
  • S.G. McLaughlin et al.

    Transport of protons across membranes by weak acids

    Physiol. Rev.

    (1980)
  • J. Gutknecht

    Proton conductance caused by long-chain fatty acids in phospholipid bilayer membranes

    J. Membr. Biol.

    (1988)
  • J.M. Dietschy

    Lipid movement across biological membranes

  • H. Westergaard et al.

    Delineation of the dimensions and permeability characteristics of the two major diffusion barriers to passive mucosal uptake in the rabbit intestine

    J. Clin. Invest.

    (1974)
  • H. Rosen et al.

    Diffusion of weak acids across the toad bladder

    J. Gen. Physiol.

    (1964)
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