Permeability of human red cells to a homologous series of aliphatic alcohols: Limitations of the continuous flow-tube method

Abstract

Human red cell permeability to the homologous series of methanol, ethanol, n-propanol, n-butanol, and n-hexanol was determined in tracer efflux experiments by the continuous flow tube method, whose time resolution is 2-3 ms. Control experiments showed that unstirred layers in the cell suspension were < 2 × 10-4 cm, and that permeabilities ≤ 10-2 cm s-1 can be determined with the method. Alcohol permeability varied with the chain length (25°C): pmeth 3.7 × 10-3 cm s-1, Peth 2.1 × 10-3 cm s-1, Pprop 6.5 × 10-3 cm s-1, Pbut ≤ 61 × 10-3 cm s-1, phex 8.7 × 10 -3 cm s-1. The permeability for methanol, ethanol, and n-propanol was concentration independent (1-500 mM). The permeability to n-butanol and n-hexanol, however, increased above the upper limit of determination at alcohol concentrations of 100 and 25 mM, respectively. The activation energies for the permeability to methanol, n-propanol, and n-hexanol were similar, 50-63 kJ mol-1. Methanol permeability was not reduced by pchloromercuribenzene sulfonate (PCMBS), thiourea, or phloretin, which inhibit transport of water or hydrophilic nonelectrolytes. It is concluded (a) that all the alcohols predominantly permeate the membrane lipid bilayer structure; (b) that both the distribution coefficient and the diffusion coefficient of the alcohols within the membrane determine the permeability, and (c) that the relative importance of the two factors varies with changes in the chain length. © 1983, Rockefeller University Press., All rights reserved.