Kinetics of erythrocyte penetration by aliphatic acids

The rate of entry of aliphatic acids into human red blood cells was studied using a new stopped-flow technique. Solutions of aliphatic acids and erythrocyte suspensions were rapidly mixed in a stopped-flow reaction apparatus. The change in extracellular pH resulting from entry of acid into the cell was monitored using chlorophenol red as an extracellular indicator. Extracellular pH of the mixture was linearly related to differential light transmission at 480 and 560 nm. Values of membrane permeability were fitted to data using a model of passive exchange with flux proportional to the transmembrane concentration gradient. At all temperatures the permeabilities were in the order: valeric acid>butyric acid>propionic acid. Permeabilities were highly temperature dependent and the activation energy (10.5 kcal/mol) remained constant between 7 and 37° C. Despite rapid rates of exchange, permeability measurements did not exhibit evidence of artifact secondary to an unstirred solvent layer on the cell surface. The data are compatible with the hypothesis that aliphatic acid transport across the erythrocyte membrane is passive in nature and proportional to the transmembrane concentration gradient.