Nonelectrolyte diffusion across lipid bilayer systems.

Abstract

A B S T R A C T The permeability coefficients of a homologous series of amides from formamide through valeramide have been measured in spherical bilayers prepared by the method described by Jung. They do not depend directly on the water:ether partition coefficient which increases regularly with chain length. Instead there is a minimum at acetamide. This has been ascribed to the effect of steric hindrance on diffusion within the bilayer which increases with solute molar volume. This factor is of the same magnitude, though opposite in sign to the effect of lipid solubility, thus accounting for the minimum. The resistance to passage across the interface has been compared to the resistance to diffusion within the membrane. As the solute chain length increases the interface resistance becomes more important, until for valeramide it comprises about 90% of the total resistance. Interface resistance is also important in urea permeation, causing urea to permeate much more slowly than an amide of comparable size, after allowance is made for the difference in the water:ether partition coefficient. Amide permeation coefficients have been compared with relative liposome permeation data measured by the rate of liposome swelling. The ratios of the two measures of permeation vary between 3 and 16 for the homologous amides~ The apparent enthaipy of liposome permeation has been measured and found to be in the neighborhood of 12 kcal mo1-1 essentially independent of chain length. Comparison of the bilayer permeability coefficients with those of red cells shows that red cell permeation by the lipophilic solutes resembles that of the bilayers, whereas permeation by the hydrophilic solutes differs significantly.Nonelectrolyte diffusion across the red cell membrane provides information about membrane physical characteristics (1), which has been interpreted in the light of comparisons with model lipid bilayer systems such as liposomes (2). The multilamellar structure of the liposomes makes it difficult to obtain absolute permeability coefficients from the initial rate of swelling, the usual measure of liposome permeation. However, absolute permeability coefficients can be obtained from measurements of diffusion across bilayers. Though Andreoli et al. (3) report that the permeability coefficients of small nonelectrolytes are generally too small to measure across planar bilayers, such measurements have been made by Vreeman (4), Gallucci et al. (5,6), Lippe et al. (7,8), and Jung (9). We have used spherical lipid bilayers, prepared by the method of Jung (9), to obtain permeability coefficients across single spherical bilayers for a homologous series of monoamides and urea. These permeability coefficients are qualitatively,

Separative pathways for urea and water, and for chloride in chicken erythrocytes.

“…Kaplan, Hays & Hays (1974) concluded that a significant decrease of urea transport by phloretin indicates the presence of a saturable facilitated transport system for urea, whereas phloretin does not inhibit urea transport by simple diffusion. This is in agreement with the observations that phloretin did not reduce urea transport in chicken red cells (Table 5), where there is no evidence of a specialized urea transport system, whereas phloretin inhibition of the facilitated urea transport in human red cells has been reported repeatedly Wieth et al 1974; Kaplan et al 1974). Poznansky et al (1976 reported a doubling of the urea permeability of lipid bilayers (red cell lipid liposomes) by 0-25 mM phloretin, and ascribed the permeability increase to a liquefying effect of phloretin on the lipids.…”

“…2) is somewhat higher than that found in human red cells of 11 kcal/mole (Galey, Owen & Solomon, 1973). In bimolecular phospholipid membranes values of 12-14 kcal/mole have been found by Poznansky et al (1976).…”

“…2) is a little less than 10-6 cm/sec. In lecithin bilayers Vreeman (1966) obtained a value of 4-2 x 10-6 cm/sec at 200 C, Galucci, Micelli & Lippe (1971) found a permeability of 3-7 x 10-6 cm/sec at 280 C, and Poznansky et al (1976) found an urea permeability across egg lecithin spherical bilayers of 4-1 x 10-6 cm/sec at 250 C. The work of Galucci et al (1971) also shows that the permeability to thiourea (4.3 x 10-6 cm/see) was only slightly higher than the urea permeability, in spite of the fact that the partition coefficient of thiourea between olive oil and water (1.2 x 10-3, Collander & Barlund, 1933) is almost tenfold higher than that of urea (1-5 x 10-4). This finding stresses the observation of Poznansky et al (1976) that the permeation of 742 TRANSPORT OF UREA, WATER AND CHLORIDE 743 small amides is not just a simple function of partitioning, but depends also on the rate coefficient for entrance into the membrane from the aqueous environment.…”

Short‐ and Long‐distance Transport of Boric Acid in Plants

“…wt (60-06) to boric acid, 5 rather than 6 potential H-bonds according to the anah'sis of Stein (1967) and a much lower ether-water partition coefficient (0-00047; Collander, 1949). P^^,^ in phospholipid bilayers at 20 to 25 X is 2-3 to 4-1 x lO-ĉ ms^i (Poznansky et al, 1976;Finkelstein, 1976). While the difference in potential H-bonding groups should increase Py^ea compared with Pi3(on)., this should be more than outweighed by the difference in ether-water partition coefficient, sô BroH).…”

Co-evolution of proteins and solutions: protein adaptation versus cytoprotective micromolecules and their roles in marine organisms

“…Firstly, while most osmolytes are much higher inside cells, partly as a result of low membrane permeability, urea concentrations are about equal in cells and ECFs (Fig. 2) because it equilibrates readily across most membranes via facilitated urea transporters (McDonald et al, 2006) or by simple diffusion through the phospholipids depending on composition (Poznansky, et al, 1976). Rapid equilibration may explain why urea osmotic shock does Bowlus and Somero (1979 not cause DNA breaks in mammalian cells (Kültz and Chakravarty, 2001;Burg et al, 2007), and may be one benefit of using it as both an intra-and extracellular osmolyte.…”