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

Abstract

Mechanisms of proton conductance (GH) were investigated in phospholipid bilayer membranes containing long-chain fatty acids (lauric, myristic, palmitic, oleic or phytanic). Membranes were formed from diphytanoyl phosphatidylcholine in decane plus chlorodecane (usually 30% vol/vol). Fatty acids were added either to the aqueous phase or to the membrane-forming solution. Proton conductance was calculated from the steady-state total conductance and the H+ diffusion potential produced by a transmembrane pH gradient. Fatty acids caused GH to increase in proportion to the first power of the fatty acid concentration. The GH induced by fatty acids was inhibited by phloretin, low pH and serum albumin. GH was increased by chlorodecane, and the voltage dependence of GH was superlinear. The results suggest that fatty acids act as simple (A- type) proton carriers. The membrane: water partition coefficient (Kp) and adsorption coefficient (beta) were estimated by finding the membrane and aqueous fatty acid concentrations which gave identical values of GH. For palmitic and oleic acids Kp was about 10(5) and beta was about 10(-2) cm. The A- translocation or "flip-flop" rate (ka) was estimated from the value of GH and the fatty acid concentration in the membrane, assuming that A- translocation was the rate limiting step in H+ transport. The kA's were about 10(-4) sec-1, slower than classical weak-acid uncouplers by a factor of 10(5). Although long-chain fatty acids are relatively inefficient H+ carriers, they may cause significant biological H- conductance when present in the membrane at high concentrations, e.g., in ischemia, hypoxia, hormonally induced lipolysis, or certain hereditary disorders, e.g., Refsum's (phytanic acid storage) disease.

Proton leak and CFTR in regulation of Golgi pH in respiratory epithelial cells

“…Similar to the data presented for P H ϩ of the Golgi, artificial lipid bilayers also have pH-dependent P H ϩ (8,21). It therefore seems possible that P H ϩ of the Golgi might be solely due to simple H ϩ diffusion through membranes, and variations in P H ϩ could therefore be due to differences in fatty acids (which act as weak acid shuttles) and/or lipids (which can form water "wires") (8,20,21) in the Golgi of different cells. In addition, H ϩ channels may also provide leak pathways for H ϩ in the Golgi (43).…”

Membrane growth can generate a transmembrane pH gradient in fatty acid vesicles

“…However, a perceived disadvantage of pure fatty acid membranes is that they are highly permeable to protons and are therefore incapable of maintaining pH gradients. Indeed, the addition of a small amount of oleic acid to phospholipid vesicles results in the dissipation of preestablished pH gradients within several seconds (16)(17)(18)(19).…”

“…The mechanism of pH gradient decay in phospholipid vesicles doped with fatty acid is believed to involve incorporation of fatty acid into the membrane, followed by flip-flop of protonated fatty acid molecules and release of protons, thereby equilibrating the pH across the membrane (16,20). The change of pH inside vesicles can also be used as a surrogate measurement for the change in cation concentration, in situations in which proton flux is electrically counterbalanced by cation flux (21).…”

Control of respiration and ATP synthesis in mammalian mitochondria and cells

“…Free fatty acids increase the proton permeability of isolated mitochondria, whether added externally or generated internally (Borst et al, 1962), and act as natural protonophores and uncouplers (Gutknecht, 1988). High levels of free fatty acids can uncouple cells and perfused tissues (Soboll et al, 1984a), and such uncoupling may occur in some pathological states, such as ischaemia.…”