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

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 kflip≪koff. 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.